PanOph

Bacterial Keratitis

Cornea

1. Clinical Analogy & Conceptual Framework

The Battlefield Analogy

Exam pearl
  • Think of the cornea as a fortress wall - normally impenetrable due to its epithelial barrier, tear film antimicrobials (lysozyme, lactoferrin, IgA), and constant mechanical flushing. Bacterial keratitis occurs when this fortress wall is BREACHED - through trauma, contact lens wear, dry eye, or prior surgery - allowing bacteria to storm in and release proteases, toxins, and recruit neutrophils. The resulting 'battlefield' is the suppurative stromal infiltrate we see clinically. Your therapeutic goal: reinforce the wall (epithelial healing), eliminate the invaders (antibiotics), and neutralize collateral damage (judicious steroids, anti-proteases). The size and depth of the infiltrate = how deep the army has advanced into the stroma.

Definition

  • Definition: Bacterial keratitis is a bacterial corneal infection that can threaten vision and commonly presents with stromal infiltration and an overlying epithelial defect.
  • Anterior chamber inflammation and suppuration may accompany these findings but are not universal.
  • It is an ophthalmic emergency requiring urgent assessment and prompt initiation of appropriate treatment.

2. Epidemiology & Risk Factors

Epidemiology

  • Incidence: The global incidence of bacterial keratitis varies substantially by region.
  • Infectious keratitis is an important cause of corneal blindness, particularly in low- and middle-income settings.
  • In the United States and United Kingdom: Contact lens wear is a major predisposing factor for bacterial keratitis; its proportion varies by setting and case mix.
  • In many agricultural and low-resource settings: Ocular trauma, often involving soil or vegetative material, is a common predisposing factor; the distribution of risk factors varies by region and organism.
  • Bimodal age distribution: Younger patients commonly have contact lens use as a leading risk factor; older patients commonly have immunosuppression and predisposing ocular surface conditions.
  • Sex distribution varies by population and risk-factor mix; male predominance may occur in some trauma-associated cohorts, but a consistent sex predominance in contact lens-related bacterial keratitis has not been established.
Risk Factor Classification
Risk Factor Classification
CategorySpecific Risk FactorsMechanism
Contact Lens Use• Overnight or extended wear • Inadequate disinfection • Poor case hygiene • Storing or rinsing lenses in water• Epithelial microtrauma • Hypoxia • Biofilm formation • Altered tear film
Ocular Surface Disease• Dry eye • Exposure keratopathy • Neurotrophic keratopathy• Impaired mucosal immunity • Tear film instability • Epithelial fragility
Trauma• Ocular trauma• Direct inoculation through epithelial breach
Prior Ocular Surgery• Post-laser-assisted in situ keratomileusis (LASIK) • Penetrating keratoplasty (PKP) • Deep anterior lamellar keratoplasty (DALK) • Cataract surgery • Pterygium excision• Disrupted epithelial barrier • Suture tracks (nidus)
Systemic Conditions• Diabetes mellitus • Malnutrition • Immunodeficiency • Systemic immunosuppressive treatment• Impaired neutrophil function • Reduced tear immunoglobulin A • Neuropathy
Topical Medications• Prolonged topical steroids • Topical anesthetic abuse • Preservatives• Immunosuppression • Epithelial toxicity • Altered flora
Eyelid Abnormalities• Blepharitis • Eyelid abnormalities• Repeated epithelial trauma • Altered microbiome proximity
Environmental• Environmental and occupational patterns vary geographically • In this South India study, bacterial keratitis occurred more commonly in nonagricultural workers • Agricultural work and vegetative injury were associated with fungal keratitis• Higher organism load • Delayed presentation • Limited access
EXAM TRAP
  • TRAP: In laser-assisted in situ keratomileusis (LASIK)-related infectious keratitis, Mycobacterium species were the most prevalent isolates in a systematic review; the most common organism is NOT Pseudomonas or Staphylococcus epidermidis.
  • Consider nontuberculous mycobacteria (NTM), including Mycobacterium chelonae, in delayed or indolent post-refractive keratitis; obtain appropriate corneal or interface specimens for stains and cultures when suspected.
  • Nontuberculous mycobacteria (NTM) keratitis after laser-assisted in situ keratomileusis (LASIK) is often delayed and indolent; average onset is about 3–4 weeks, although slow-growing species may present later.
  • A characteristic sign is radiating corneal infiltrate projections producing a “cracked windshield” appearance. Infectious crystalline keratopathy has also been described in some cases.

3. Microbiology & Pathophysiology

Causative Organisms by Geography & Setting
Causative Organisms by Geography & Setting
SettingCommon OrganismsNotes
Developed nations (contact lens)• Pseudomonas aeruginosa is commonly the most frequently isolated organism. • Staphylococcus species, including coagulase-negative staphylococci and Staphylococcus aureus, are also common.• Pseudomonas aeruginosa is associated with severe clinical manifestations. • The supplied evidence does not establish biofilm formation as the cause of rapid progression. • The supplied evidence does not establish toxin-mediated disease as a general characterization of Staphylococcus aureus keratitis.
Developed nations (non-contact-lens)• Staphylococci species are commonly implicated. • Streptococci species are also identified among causative isolates. • Moraxella is also identified among causative isolates.• The supplied evidence does not verify a specific serpiginous-ulcer association for Streptococcus pneumoniae. • The supplied evidence does not verify a specific association between Moraxella keratitis, alcohol use, and immunocompromise.
India / developing (trauma)• An Eastern India study identified Pseudomonas species among bacterial keratitis isolates. • Klebsiella species were identified among bacterial keratitis isolates. • Streptococcus pneumoniae was identified among bacterial keratitis isolates. • Staphylococcus aureus was identified among bacterial keratitis isolates.• Agricultural trauma is a risk factor for infectious keratitis. • In India, fungal keratitis associated with agricultural trauma and debris is an important consideration. • The supplied evidence does not support mixed flora or Gram-negative predominance as a general rule.
Post-operative• Nontuberculous mycobacteria are among the commonly reported pathogens causing infectious keratitis after laser in situ keratomileusis. • The supplied evidence does not establish coagulase-negative staphylococci as the most common cause of bacterial keratitis specifically after surgery.• Nontuberculous mycobacterial keratitis after laser in situ keratomileusis is typically indolent. • Mean onset is approximately 3.4 weeks and may be later with slow-growing mycobacteria. • Diagnosis is often delayed.
Neonates• Neisseria gonorrhoeae is an important cause of ophthalmia neonatorum and may progress to microbial keratitis. • Chlamydia trachomatis is an important cause of ophthalmia neonatorum, usually presenting as conjunctivitis rather than corneal keratitis.• Gonococcal ophthalmia neonatorum is typically acute, presenting about 2–5 days after birth. • It may progress to corneal ulceration, perforation, and blindness if untreated.
Immunocompromised• Less-common bacterial causes may include Nocardia species and Listeria monocytogenes. • Moraxella catarrhalis and Pseudomonas aeruginosa are also recognized causes of bacterial keratitis. • Polymicrobial infection may occur. • The organism profile should not be presented as specific to immunocompromise without direct supporting data.• Bacterial keratitis in immunocompromised patients may be severe or atypical. • Obtain corneal cultures and appropriate ancillary testing when disease is severe, atypical, postoperative, progressive, or unresponsive to empiric therapy.
Pathophysiology - Step-by-Step Cascade
Pathophysiology - Step-by-Step Cascade
StepDescription
STEP 1: EPITHELIAL BREACH• The intact corneal epithelium with its tight junctions, surface glycocalyx, and mucosal immunoglobulin A (IgA) represents an effective barrier. • Disruption - via trauma, contact lens hypoxia, or prior disease - exposes the basement membrane and anterior stroma.
STEP 2: BACTERIAL ADHESION & COLONIZATION• Bacteria use organism-specific surface adhesins to attach to exposed corneal epithelial or stromal extracellular-matrix components, including collagen and fibronectin. • Staphylococcus aureus uses collagen- and fibronectin-binding adhesins, while Pseudomonas aeruginosa uses type IV pili and other surface factors to interact with corneal receptors. • Pseudomonas aeruginosa uses type IV pili and flagella. • Biofilm formation, particularly by Pseudomonas aeruginosa, can increase antimicrobial tolerance and may limit penetration or sequester selected antibiotics, contributing to persistent infection and treatment difficulty.
STEP 3: TOXIN & PROTEASE RELEASE• Bacteria release exotoxins that can disrupt epithelium directly. • Pseudomonas aeruginosa releases exotoxin A, which inhibits protein synthesis. • Pseudomonas aeruginosa proteases, including elastase, can degrade corneal collagen and host-defense proteins such as immunoglobulins and complement. • Pseudomonas aeruginosa also produces alkaline protease and phospholipase C. • Lipopolysaccharide and peptidoglycan trigger innate immune activation via Toll-like receptor 4 (TLR-4) and Toll-like receptor 2 (TLR-2).
STEP 4: INNATE IMMUNE RESPONSE & NEUTROPHIL RECRUITMENT• Corneal epithelial cells and other resident corneal and immune cells contribute to the inflammatory response; epithelial cells produce interleukin-1 beta, interleukin-6, and interleukin-8. • Staphylococcus aureus exoproducts and peptidoglycan can also induce epithelial interleukin-6, interleukin-8, and tumor necrosis factor alpha. • Cytokines and chemokines recruit neutrophils to the cornea during the innate immune response to bacterial keratitis. • Neutrophils are the primary cells in corneal infiltrates. • Neutrophil products, including enzymes and reactive oxygen species, contribute to stromal damage; host corneal cells also upregulate matrix metalloproteinases, including matrix metalloproteinase-9 (MMP-9), which can amplify stromal degradation. • The ulcer may enlarge even after bacteria are killed because the inflammatory cascade persists.
STEP 5: STROMAL MELTING & PERFORATION RISK• Bacterial proteases and host-derived collagenases degrade stromal collagen → progressive stromal thinning → descemetocele formation → perforation. • Stromal melting is mediated by bacterial enzymes and host inflammatory and stromal-cell matrix metalloproteinase activity, including neutrophil-mediated tissue injury. • This explains why adjunctive anti-protease therapy, for example acetylcysteine, and cautious corticosteroid use have a role.
STEP 6: ENDOPHTHALMITIS• Corneal perforation can be associated with progression of bacterial keratitis to endophthalmitis; vigilance is warranted in severe Pseudomonas aeruginosa keratitis. • Bacterial keratitis may be associated with hypopyon; corneal perforation and a larger hypopyon are risk factors for progression to keratitis-related endophthalmitis. • A sterile hypopyon from a toxin-mediated anterior-chamber reaction is far more common and does not indicate intraocular infection per se.
VIVA PEARL
  • Pseudomonas aeruginosa has two overlapping mechanisms of corneal damage:
  • Extracellular toxins and proteases cause direct tissue destruction.
  • Lipopolysaccharide (LPS)-mediated host inflammation causes neutrophil-mediated bystander injury.
  • Effective antibiotics suppress viable bacterial growth, but preformed bacterial products and the established host inflammatory response may continue to cause corneal injury after bacterial killing.
  • Inflammation may therefore persist after microbiological control.
  • Adjunctive topical corticosteroids may be considered cautiously after effective antibiotic therapy has been established. The Steroids for Corneal Ulcers Trial required at least 48 hours of topical moxifloxacin before treatment allocation, but showed no overall benefit and did not establish a universal more-than-48-hour rule or require an improving infiltrate.

4. Classification of Bacterial Keratitis

A. By Severity (Ulcer Size & Depth - Most Clinically Useful)
A. By Severity (Ulcer Size & Depth - Most Clinically Useful)
FeatureMild/PeripheralModerate/CentralSevere
Infiltrate Size• No universal threshold defines mild disease• No universal threshold defines moderate disease• Infiltrate measuring at least 2 mm in greatest linear dimension is an immediately sight-threatening criterion
Location• No consensus three-tier location classification• No consensus three-tier location classification• Infiltrate edge within 3 mm of the corneal centre is an immediately sight-threatening criterion
Depth• No universal anterior one-third classification• No universal middle-stroma classification• Historical severity systems have used inner-third corneal involvement or more than 50% maximal thinning as severe features
Hypopyon• No consensus hypopyon-height classification• No consensus hypopyon-height classification• Anterior chamber cells of at least 1+ are an immediately sight-threatening criterion
Visual Acuity• No universal near-normal threshold• No universal moderate-reduction threshold• Fixed visual-acuity thresholds should not be presented as universal severity criteria
Initial treatment• Small peripheral or low-risk ulcers may receive topical fluoroquinolone monotherapy• Treatment is guided by clinical risk rather than a fixed severity tier• Vision-threatening features warrant corneal scraping or culture when indicated • Use intensive topical therapy with fortified antibiotics or a fourth-generation fluoroquinolone • Hospitalization is individualized
Surgical consideration• No universal surgical tier• Monitor for progressive thinning and impending perforation• Consider tectonic measures or therapeutic keratoplasty for impending or frank perforation • Consider therapeutic keratoplasty when medical treatment cannot preserve the globe
A. By Severity (Ulcer Size & Depth - Most Clinically Useful)
A. By Severity (Ulcer Size & Depth - Most Clinically Useful)
FeatureMild/PeripheralModerate/CentralSevere
Infiltrate Size< 2 mm2–6 mm> 6 mm or > 1/3 corneal area
LocationPeripheral (>2mm from centre)Paracentral or centralCentral or full corneal involvement
DepthAnterior 1/3 stromaMiddle stromaDeep stroma / near Descemet's
HypopyonAbsentSmall (<2mm)Present (≥2mm) or progressive
VANear normal or mild reductionModerate reductionSevere reduction (< 6/60)
Initial treatmentTopical antibiotics q1–2hHourly fortified drops + cultureFortified + consider hospitalization
Surgical considerationNone typicallyMonitor Descemet's closelyTherapeutic PKP if perforation
B. By Clinical Pattern
B. By Clinical Pattern
PatternOrganism(s)Hallmark Feature
Suppurative/Purulent UlcerPseudomonas aeruginosa, Staphylococcus aureus (S. aureus), Neisseria gonorrhoeae (GC)Dense white-yellow infiltrate • Rapid progression • Ring infiltrate may occur
Serpiginous (Hypopyon) UlcerStreptococcus pneumoniae (S. pneumoniae), MoraxellaUndermined edges • Intense hypopyon • Geographic spread
Indolent UlcerStaphylococcus epidermidis (S. epidermidis), MoraxellaSlow progression • Minimal reaction
Ring AbscessPseudomonas aeruginosaCircumferential stromal ring • May represent an infectious ring abscess • A separate immune ring may occur
Crystalline KeratopathyViridans streptococci • Nontuberculous mycobacteria (NTM) reported rarelyNeedle-like or branching stromal crystals • Minimal inflammation
Marginal UlcerStaphylococcus aureus (S. aureus)Peripheral ulcer with clear zone • Toxin-mediated • Immune-mediated
EXAM TRAP
  • Do not equate a Pseudomonas ring abscess with a sterile Wessely immune ring.
  • A Pseudomonas ring abscess is an infectious process associated with living bacteria, bacterial virulence factors, and dense peripheral polymorphonuclear leukocyte accumulation with cellular and lamellar debris.
  • A true Wessely immune ring is sterile and may result from antigen-antibody-mediated complement activation; early Pseudomonas-associated immune rings may also involve properdin-mediated alternative-pathway complement activation.
  • Do not state categorically that bacteria are confined to the center; the peripheral ring contains dense inflammatory-cell accumulation and debris, and the process remains associated with living bacterial infection.
  • Treat an infectious ring abscess with topical antibiotics. Do not add corticosteroids solely for the ring; any adjunctive corticosteroid use requires appropriate clinical selection after infection is controlled.

5. Clinical Features & Slit-Lamp Examination

Symptoms
  • Pain - often severe and increases with time; may be mild in neurotrophic keratopathy because of reduced corneal sensation
  • Photophobia - due to ciliary spasm from anterior chamber involvement
  • Lacrimation & mucopurulent discharge
  • Blurring of vision - central ulcers cause earlier, more severe visual acuity loss
  • Foreign body sensation
  • Eyelid swelling may occur
Slit-Lamp Examination - Systematic Approach
Slit-Lamp Examination - Systematic Approach
StructureFindingSignificance
Eyelids/Adnexa• Blepharitis • Trichiasis • Entropion • Lagophthalmos• Predisposing cause • Address to prevent recurrence
Conjunctiva• Circumlimbal injection (ciliary flush) • Chemosis • Mucopurulent discharge• Severity indicator • Neisseria gonorrhoeae (GC) → hyperacute conjunctivitis
Epithelium• Epithelial defect - stain with fluorescein • Measure size (millimetres)• Extent of breach • Use Rose Bengal for devitalized epithelial cells and areas of mucin deficiency
Infiltrate• Density • Color (white/grey/yellow) • Border (fluffy versus well-defined) • Depth • Satellite lesions• Central/large = worse prognosis • Fluffy borders = suggestive of fungal infection
Stroma• Stromal edema • Thinning • Melting • Descemetocele• Thinning <10% residual = emergency penetrating keratoplasty (PKP) consideration
Descemet's Membrane• Folds • Descemetocele (protruding grey membrane)• Imminent perforation • Urgent surgical decision
Anterior Chamber• Cells and flare • Hypopyon level (measure in millimetres) • Fibrin• Hypopyon = severe anterior chamber inflammation (usually sterile) • Fibrin = anterior chamber inflammation
Iris• Posterior synechiae • Small sluggish pupil• Chronic inflammation • Risk of secondary glaucoma
Lens• Cataract in severe cases• Long-term complication
Intraocular pressure (IOP)• May be elevated • May be low with iridocyclitis• Elevated intraocular pressure (IOP) worsens prognosis • Requires management
Organism-Specific Slit-Lamp Clues
Organism-Specific Slit-Lamp Clues
OrganismSlit-Lamp Clues
Pseudomonas aeruginosa• Rapid progression (hours!) • Blue-green pigment in ulcer bed • Liquefactive necrosis • Ring abscess • Early perforation risk • Hypopyon
Streptococcus pneumoniae (Pneumococcus)• Classic serpiginous ulcer • Creeps across cornea with overhanging (undermined) edges • Dense hypopyon • Usually central
Staphylococcus aureus• Oval, densely white infiltrate • Well-defined edges • Associated blepharitis • Marginal ulcer variant • Can cause crystalline keratopathy
Moraxella• Indolent or subacute corneal ulcer • Often round or oval • Hypopyon • Associated with local ocular disease • May occur in patients with chronic alcoholism or debility
Neisseria gonorrhoeae• Hyperacute (symptoms develop within 12–48 hours after exposure) • Associated with copious purulent conjunctivitis • Rapid corneal ulceration and perforation risk
Neisseria meningitidis• Similar to gonococcal keratitis • Less common • Systemic meningococcemia possible
Nocardia• Feathery or wreath-like infiltrate • Associated with soil or plant-matter injury • Slow progression • May resemble fungal keratitis • Corticosteroids may worsen outcomes
VIVA PEARL - N. gonorrhoeae
  • Neisseria gonorrhoeae can penetrate an intact corneal epithelium through virulence mechanisms involving surface structures such as pili and opacity/invasin proteins; it is not the only bacterium reported to penetrate intact corneal epithelium.
  • Gonococcal keratitis may occur without preceding traumatic epithelial disruption.
  • Hyperacute conjunctivitis with copious hyperpurulent discharge requires immediate microbiologic evaluation, including conjunctival Gram staining and culture when gonococcal infection is suspected; perform corneal scraping when corneal involvement or keratitis is present.
  • If untreated, gonococcal keratoconjunctivitis can cause rapid corneal destruction and perforation, with blindness possible within hours.

6. Microbiological Investigations

When to Culture - Indications

When to Culture - Indications
  • ALL moderate-to-severe ulcers with a central, large infiltrate (>2 mm) and/or significant stromal involvement or melting; culture is also indicated when there are >1+ anterior-chamber cells
  • Chronic or atypical presentations
  • Failure to respond to initial broad-spectrum antibiotic therapy
  • Post-surgical keratitis, including post-refractive surgery keratitis
  • Suspected unusual organisms, including fungal, Acanthamoeba, or mycobacterial keratitis
  • Immunocompromised patient
  • Peripheral ulcers that may be infective versus immune-mediated

Corneal Scraping - Step-by-Step Protocol

Corneal Scraping - Step by Step Protocol
  1. TOPICAL ANESTHETIC - Use preservative-free proparacaine 0.5% if available; avoid tetracaine (amethocaine) because of its greater bacteriostatic effect
  2. STOP ANTIBIOTICS - If possible, withdraw antimicrobial agents for 24 hours before corneal sampling
  3. INSTRUMENTS - Use an appropriate sterile collection device, such as a Kimura platinum spatula, surgical blade, forceps, or fine-gauge needle. Use a fresh sterile instrument or re-sterilize the spatula between specimens according to local protocol
  4. TECHNIQUE - Scrape the advancing edge of the infiltrate and, when clinically appropriate, the ulcer base; avoid relying only on central necrotic material
  5. MULTIPLE PASSES - Obtain multiple scrapings from representative areas, including the advancing edge and base when appropriate; use a fresh sterile instrument or re-sterilize the spatula between specimens, and inoculate the indicated media directly at the bedside
  6. INOCULATE DIRECTLY - Inoculate appropriate media directly at the bedside; apply material to solid media in small C-shaped streaks
  7. SMEARS - Prepare a Gram-stained slide for bacteria and fungal elements; use Giemsa for Acanthamoeba and potassium hydroxide (KOH), with calcofluor white when available, for fungal elements
  8. CONFOCAL MICROSCOPY - Non-invasive adjunct; can demonstrate characteristic double-walled Acanthamoeba cysts and hyper-reflective fungal hyphae; does not replace corneal scraping and culture when microbiological diagnosis is required
Culture Media - High-Yield Table
Culture Media - High-Yield Table
MediumPrimary UseIncubationWhat Grows
Blood Agar (5% sheep blood)• General-purpose enriched medium for aerobic bacteria • Supports haemolysis-pattern identification• 35–37°C, 48–72 hours • Extend incubation only when slow-growing organisms are suspected• Most aerobic bacteria • Haemolysis pattern identification
Chocolate Agar• Fastidious organisms, including Haemophilus, Neisseria, and Moraxella• 35–37°C in 5% carbon dioxide • Examine at 24 and 48 hours• Neisseria • Haemophilus • Moraxella
MacConkey's Agar• Gram-negative bacilli, especially Enterobacteriaceae and non-fermenters including Pseudomonas• 35–37°C in air • Typically 18–24 hours; examine again at 48 hours when indicated• Enterobacteriaceae • Pseudomonas as a non-fermenter
Thioglycolate Broth• Backup enrichment medium for low-inoculum specimens • Supports selected aerobic, microaerophilic, and anaerobic bacteria• 35–37°C • Incubate according to the organism and laboratory protocol • Prolonged incubation up to 14 days when slow-growing anaerobes are suspected• Anaerobic and microaerophilic bacteria • Enrichment medium
Sabouraud Dextrose Agar• Fungi when fungal infection is suspected• 25–30°C • Incubate and read according to local laboratory protocol• Fungi, including Fusarium, Aspergillus, and Candida
Non-nutrient agar with Escherichia coli overlay• Acanthamoeba• 30°C for 1–2 weeks• Acanthamoeba cysts and trophozoites
Löwenstein–Jensen (LJ)• Mycobacterium, including Mycobacterium tuberculosis and nontuberculous mycobacteria such as Mycobacterium chelonae and Mycobacterium abscessus• Approximately 36–37°C • Incubation and final-read duration according to laboratory protocol • Often 6–8 weeks for mycobacterial culture• Mycobacterium tuberculosis • Mycobacterium chelonae • Mycobacterium abscessus
Brain Heart Infusion (BHI) broth• Backup enrichment medium• 33–37°C • Use the appropriate atmosphere and incubation duration for the organism and laboratory protocol• Various fastidious and nonfastidious microorganisms
Gram Stain Interpretation - Quick Guide
Gram Stain Interpretation - Quick Guide
Gram Stain FindingLikely Organism
• Gram-positive cocci in clusters• Staphylococcus species
• Gram-positive cocci in pairs/chains• Streptococcus species / pneumococcus
• Gram-negative diplococci (intracellular)• Neisseria gonorrhoeae (within polymorphonuclear leukocytes)
• Gram-negative rods (long, slender)• Pseudomonas aeruginosa
• Gram-negative rods (motility variable)• Enterobacteriaceae (Klebsiella species are non-motile)
• Gram-positive branching filaments (beaded)• Nocardia (weakly acid-fast too)
• Septate / non-septate hyphae• Fungi (not bacteria - guide to potassium hydroxide/calcofluor)
• Negative Gram stain / potassium hydroxide preparation• Consider Acanthamoeba or viral keratitis; obtain targeted testing or culture as clinically indicated

ADVANCED INVESTIGATION: In Vivo Confocal Microscopy

  • In vivo confocal microscopy (IVCM): Provides non-invasive, real-time imaging of the cornea at cellular level.
  • Key role in:
  • Acanthamoeba: Double-walled cysts (10–25 micrometres).
  • Fungal keratitis: Hyphae.
  • Nontuberculous mycobacteria (NTM): Acid-fast filaments (less consistent).
  • In bacterial keratitis, in vivo confocal microscopy (IVCM) may show hyperreflective inflammatory cells, including neutrophils and dendritic cells.
  • These findings are nonspecific and do not reliably identify the causative organism.
  • In vivo confocal microscopy (IVCM) may be useful as an adjunct when cultures are negative or the diagnosis remains uncertain, particularly when Acanthamoeba or fungal keratitis is suspected.
  • In vivo confocal microscopy (IVCM) does not replace corneal scraping, staining, or culture.

7. Differential Diagnosis

ConditionKey Differentiating FeaturesDiagnostic Clue
Fungal Keratitis• Feathery/satellite infiltrates • Dry texture • Elevated edges • Slow progression • History of vegetative trauma• Potassium hydroxide (KOH) / Calcofluor white: hyphae • Gram stain may demonstrate fungal elements and bacteria
Acanthamoeba Keratitis• Severe pain disproportionate to signs • Radial perineuritis (classically associated) • Pseudo-dendrites • Ring infiltrate later • Contact lens wear• In vivo confocal microscopy (IVCM): double-walled cysts • Non-nutrient agar with Escherichia coli overlay
Herpetic Stromal Keratitis (Disciform/Necrotizing)• History of dendritic ulcer • Reduced corneal sensation • Disc-shaped edema • Wessely immune ring • Minimal discharge• No mucopurulent discharge • Corneal sensation reduced • Polymerase chain reaction (PCR) for herpes simplex virus (HSV)
Mooren's Ulcer• Peripheral, painful ulcer with overhanging edge • Bilateral involvement sometimes • No infection• Progressive from limbus centrally • Idiopathic (autoimmune) • No hypopyon • Cultures negative
Marginal Keratitis (Staphylococcal immune)• Peripheral marginal keratitis associated with blepharitis • Immune reaction to staphylococcal antigens• Lid hygiene • Topical antibiotic therapy • Topical corticosteroids when immunologic inflammation is present
Peripheral Ulcerative Keratitis (PUK)• Associated with systemic autoimmune disease • Especially rheumatoid arthritis (RA) • Also granulomatosis with polyangiitis (GPA) and systemic lupus erythematosus (SLE) • Scleral involvement• Systemic evaluation is warranted • Infectious causes must be considered before immunosuppression
Neurotrophic Keratitis• Reduced or absent corneal sensation • Epithelial breakdown or ulceration • Associated with trigeminal-nerve pathology• Corneal sensitivity testing is essential • Cenegermin is indicated for neurotrophic keratitis
Thygeson's Superficial Punctate Keratitis (SPK)• Coarse central superficial epithelial lesions • No primary stromal involvement • Recurrent course • Marked photophobia• Lesions may stain with Rose Bengal • Topical corticosteroids can improve symptomatic episodes
EXAM TRAP
  • TRAP: Acanthamoeba keratitis is frequently MISDIAGNOSED as bacterial or herpetic keratitis in early stages.
  • The classic teaching clue is RADIAL PERINEURITIS - inflammation along corneal nerve trunks.
  • It appears as linear white infiltrates radiating from the centre of the cornea.
  • Radial perineuritis is a highly suggestive, sometimes described as pathognomonic, early sign; it is not a late sign.
  • Ring infiltrates are associated with later-stage disease.
  • Early: pseudo-dendrites and epitheliopathy.
  • Always ask about contact lens wear and tap-water or swimming-pool exposure.
  • Antibacterial drops alone do not treat suspected Acanthamoeba keratitis; arrange urgent ophthalmic assessment and initiate antiamoebic therapy.
  • Antibacterial drops may be used only as adjunctive treatment when clinically indicated.
  • Common initial antiamoebic regimens include topical polyhexamethylene biguanide or chlorhexidine with propamidine or hexamidine.
  • Treatment should be guided by a corneal specialist.

8. Management - Medical & Surgical

Step 1 - Initial Assessment & Triage

  1. Measure Visual acuity (VA)
  2. Perform slit-lamp with full documentation of ulcer size/depth/hypopyon
  3. Perform corneal scraping for smears and cultures before initiating antibiotics when indicated and feasible
  4. Assess for perforation risk (thinning, descemetocele)
  5. Classify severity to guide intensity of therapy

Step 2 - Antibiotic Selection

  • Initial antibiotic therapy is usually started empirically after indicated corneal scraping, without waiting for culture results.
  • Culture and susceptibility testing are particularly important for severe, central, large, atypical, chronic, postoperative, or treatment-unresponsive ulcers.
  • Modify therapy when the clinical response is inadequate.
  • Select therapy that is active against the likely pathogen.
  • Ensure adequate corneal penetration and tissue concentration.
  • Use appropriate empiric coverage of likely Gram-positive and Gram-negative organisms when the pathogen is unknown.
  • Choose the lowest practical ocular toxicity.
  • Narrow therapy according to microbiological results and clinical response.
AgentCoverageConcentration (Fortified)Key Notes
Moxifloxacin 0.5% (commercial)• Broad-spectrum Gram-positive and Gram-negative coverage, including limited Pseudomonas coverage• Commercial; no fortification needed• Best corneal penetration of all commercial drops • Highly lipophilic • First-line for mild-to-moderate
Ciprofloxacin 0.3%• Excellent Gram-negative coverage, with strong Pseudomonas activity • Moderate Gram-positive coverage• Commercial ciprofloxacin ophthalmic solution: 3 mg/mL (0.3%) • The reviewed source does not support describing 3 mg/mL as a separately fortified preparation• Indicated for susceptible Pseudomonas aeruginosa corneal ulcers • White crystalline precipitates may occur with frequent use but did not adversely affect the clinical course or visual outcome in the cited studies
Tobramycin (Fortified)• Gram-negative coverage, with strong Pseudomonas activity • Moderate Gram-positive coverage• 13.6 mg/mL (from parenteral formulation)• Used with cefazolin as a classic combination
Cefazolin (Fortified)• Gram-positive coverage: Streptococcus and Staphylococcus • Not methicillin-resistant Staphylococcus aureus• 50 mg/mL (from parenteral formulation)• Combined with tobramycin as a classic fortified combination • Covers the Gram-positive gap
Vancomycin (Fortified)• Gram-positive coverage, including methicillin-resistant Staphylococcus aureus • Covers susceptible streptococci• 50 mg/mL• Reserve for methicillin-resistant Staphylococcus aureus • Risk of resistance • Do not use empirically as monotherapy
Amikacin (Fortified)• Gram-negative coverage, including Pseudomonas, Nocardia, and nontuberculous mycobacteria• 20–40 mg/mL• First-line for Nocardia and nontuberculous mycobacterial keratitis • Use with clarithromycin for nontuberculous mycobacterial keratitis
Ceftazidime (Fortified)• Gram-negative coverage, with strong Pseudomonas activity• 50 mg/mL• Alternative to tobramycin for Pseudomonas • Less available
Gentamicin (Fortified)• Gram-negative coverage • Some Gram-positive coverage• 14 mg/mL• May cause ocular irritation and conjunctival epithelial defects • Use cautiously
Dosing Regimen by Severity
Dosing Regimen by Severity
SeverityRegimenLoading PhaseMaintenance
Mild (peripheral, <2 millimetres [mm]) • Without significant anterior chamber inflammation • Without visual-axis involvement• Commercial fluoroquinolone • Moxifloxacin or ciprofloxacinEvery 1–2 hours while awake initially• Reduce frequency stepwise as the ulcer and symptoms improve • Arrange early clinical review
Moderate (approximately 2–6 millimetres [mm]) • Classify using size, location relative to the corneal center, stromal involvement, and anterior chamber inflammation• Fortified tobramycin plus fortified cefazolin • Alternating drops when combination therapy is selected • Or moxifloxacin monotherapy• Begin intensive topical therapy, commonly every hour initially • Adjust frequency according to severity and clinical response• Step down according to epithelial healing, infiltrate stability, inflammation, and clinical response • Arrange early reassessment
Severe • Large or central infiltrate • Marked anterior chamber inflammation or hypopyon • Deep stromal involvement or thinning • Risk of perforation• Intensive topical therapy • Fortified tobramycin plus fortified cefazolin is an acceptable regimen • Tailor treatment to smear, culture, susceptibility, local resistance patterns, and clinical response • Consider hospital admission when frequent dosing or close observation cannot be safely achieved as an outpatient• Loading dose, such as every 5–15 minutes • Then every hour • Tailor to clinical severity and response• Continue intensive treatment, often every hour including overnight initially • Reduce frequency according to clinical response • Closely monitor infiltrate progression, stromal thinning, and perforation risk
Pseudomonas (suspected)• Intensive topical therapy with ciprofloxacin, tobramycin, or another appropriate antipseudomonal agent • Use combination therapy when clinically indicated • Tailor treatment to culture and susceptibility resultsEvery hour initially• Continue until clinical response • Direct treatment by culture and susceptibility results
Methicillin-resistant Staphylococcus aureus (MRSA) (confirmed)• Fortified topical vancomycin • Tailor treatment to susceptibility results • Consider systemic therapy only when infection extends beyond the cornea or another systemic indication is present• For severe disease, intensive topical vancomycin initially • Loading dose followed by every-hour applications • Adjust according to clinical response and susceptibility results• Tailor treatment to susceptibility results • Do not routinely add rifampicin eye drops • Polymyxin B/trimethoprim plus rifampin remains investigational for bacterial keratitis

Step 3 - Adjunctive Therapy

  1. Cycloplegics - May be used for pain from ciliary spasm and to prevent posterior synechiae when bacterial keratitis is associated with anterior-chamber inflammation.
  2. Topical Steroids - Controversial. Use ONLY after: (a) 48–72 hours of effective antibiotic therapy, (b) documented clinical improvement (decreasing infiltrate), (c) negative culture OR known organism responding to treatment. Rationale: Reduce neutrophil-mediated collagenolysis and scarring. NEVER initiate steroids before antibiotic control. NEVER use in suspected fungal/Acanthamoeba keratitis.
  3. Anti-proteases - Matrix metalloproteinase inhibitors, including N-acetylcysteine, have been proposed to limit stromal destruction; evidence for efficacy in human bacterial keratitis is insufficient.
  4. Intraocular Pressure Management - If intraocular pressure (IOP) is elevated: Timolol, Brimonidine, carbonic anhydrase inhibitors (CAIs) (Dorzolamide). AVOID prostaglandin analogues in active anterior inflammation (increase breakdown of the blood-aqueous barrier (BAB), worsen inflammation).
  5. Systemic Antibiotics - Indicated for suspected gonococcal keratitis, corneal perforation, scleral involvement, or endophthalmitis. Use specialist- and guideline-directed ceftriaxone therapy; the regimen depends on the ocular and systemic presentation.
  6. Bandage Contact Lens - May provide pain relief for selected sterile or noninfected epithelial defects under close ophthalmic supervision. Avoid routine placement over active bacterial keratitis; if used for a specific therapeutic indication, provide antimicrobial cover and close monitoring because microbial keratitis risk remains.
  7. Lubricants - Preservative-free artificial tears to supplement the tear film and reduce ocular-surface toxicity from frequent preserved drops; they are supportive and do not treat the infection itself.
  8. Vitamin C (ascorbate) - Limited retrospective evidence suggests that systemic vitamin C supplementation may reduce corneal opacity during infectious keratitis, but its role in severe stromal melting and its collagen-synthesis mechanism remain unproven; it is not universal practice.

Step 4 - Monitoring & Response Assessment

  • Reassess at 48 hours to assess culture results and response to treatment.
  • Subsequent review should depend on severity and clinical improvement.

Signs of improvement

  1. Decreasing infiltrate extent or size, with stabilization and no progressive stromal loss
  2. Epithelial healing at margins
  3. Reduction in discharge and inflammation
  4. Decreasing hypopyon, or a stable hypopyon with concurrent improvement in the infiltrate, epithelial defect, pain, and anterior-chamber inflammation
  5. Patient reports pain reduction

Signs of failure or alternative diagnosis - reassess and consider change

  1. No improvement or worsening at 48 hours
  2. Progressive thinning
  3. Progression of the corneal infiltrate
  4. New satellite lesions - reassess for an alternative diagnosis
  5. Increasing hypopyon
CRITICAL MANAGEMENT PRINCIPLE
  • When to Change Antibiotics: Do NOT change antibiotics based solely on culture sensitivity report if the patient is clinically improving - this is a common exam trap and real-world error. Culture sensitivity is a GUIDE. The clinical response at 48–72 hours is the primary decision driver. Change antibiotics if: 1. Clinical deterioration at 48 hours, 2. Organism resistant to current regimen AND not improving, 3. Culture reveals an organism requiring specific therapy (e.g., Methicillin-resistant Staphylococcus aureus (MRSA) → vancomycin; Nontuberculous mycobacteria (NTM) → amikacin + clarithromycin).
Step 5 - Surgical Management
Step 5 - Surgical Management
ProcedureIndicationKey Operative Pearls
Therapeutic Penetrating Keratoplasty• Actual corneal perforation • Descemetocele or another imminent threat to globe integrity • Infectious keratitis that fails to respond despite maximal medical therapy, particularly when structural integrity is threatened• Use a donor graft approximately 0.5–1 millimeter larger than the infiltrate or recipient bed. • Remove all diseased or infected host tissue. • Obtain a vitreous sample for microbiological testing when endophthalmitis is suspected, coordinated with the surgical management plan. • Graft failure is common in active infection and with larger grafts; reported rates vary by cohort and clinical factors. • Primary purpose: tectonic support and infection control; optical rehabilitation is secondary. • Continue postoperative antimicrobial therapy. • Introduce topical corticosteroids after a brief period of quiescence when there are no infectious foci or graft infection; timing should be individualized to the clinical course.
Tissue Adhesive (Cyanoacrylate Glue)• Small corneal perforation, generally 3 millimeters or smaller • Impending perforation or thin descemetocele • Temporary bridge to definitive penetrating keratoplasty• Apply to a dry ocular surface; technique and applicator gauge vary. • A bandage contact lens may be placed over the glue to protect the eyelid. • Direct cyanoacrylate glue alone is generally unsuitable for large perforations or cases with iris prolapse or a collapsed anterior chamber; consider reformation of the anterior chamber or a modified tectonic technique. • Histoacryl is an N-butyl-2-cyanoacrylate formulation; several cyanoacrylate adhesives are used clinically.
Amniotic Membrane Transplantation• Persistent epithelial defect or neurotrophic ulcer component • Adjunct to antimicrobial therapy in bacterial keratitis, including moderate-to-severe disease• Use only as an adjunct to appropriate antimicrobial therapy in active infection, with close clinical monitoring. • Promotes epithelialization and has anti-inflammatory effects. • PROKERA, a self-retained cryopreserved amniotic-membrane device, has been used for severe bacterial keratitis and persistent epithelial defects; specific usefulness for smaller defects remains uncertain.
Conjunctival Flap (Gunderson)• Chronic indolent ulcer in an eye with poor visual potential • Alternative to penetrating keratoplasty in elderly or medically unfit patients • Salvage after failed medical therapy• Provides vascularized coverage, tectonic support, and pain relief. • Limits visualization of the cornea and may limit visual rehabilitation. • Variants include a thin pedicle flap and a total Gundersen flap.
Evisceration / Enucleation• Panophthalmitis with no visual potential • Perforated ulcer with suspected endophthalmitis in a painful blind eye• Evisceration preserves the scleral shell and extraocular muscles and may better preserve orbital volume and implant motility than enucleation. • Enucleation removes the globe and is selected when indicated by the clinical context, including severe infection or a painful blind eye. • Submit the removed globe or ocular contents for histopathologic examination, particularly when the diagnosis, history, examination, or surgical findings are unclear or malignancy is possible.
SURGICAL PEARL
  • Therapeutic penetrating keratoplasty (TPK): The donor graft should be at least 0.5 mm larger than the ulcer to permit complete excision of infected tissue.
  • Trephination is planned to encompass the ulcer with an approximately 0.5–1 mm margin of uninvolved cornea; it is usually centered over the lesion but may be decentered according to ulcer location.
  • When endophthalmitis is suspected, assess for posterior segment involvement; vitreous sampling and pars plana vitrectomy with intravitreal antimicrobials should be considered according to the intraocular findings and surgical judgment.
  • The optical outcome is secondary to the tectonic goal in therapeutic penetrating keratoplasty; visual prognosis is guarded because graft rejection is frequent in inflamed, vascularized recipient beds.

9. Organism-Specific Keratitis Profiles

Pseudomonas aeruginosa
Pseudomonas aeruginosa
FeatureDetails
Gram Stain• Gram-negative rod
Risk Factors• Contact lens • Burns • Physically damaged eyes
Clinical Features• RAPID FULMINANT course (hours) • Blue-green pigment (pyocyanin) visible in ulcer bed • Stromal necrosis and melt • Ring abscess • Hypopyon • Early perforation risk
Virulence Factors• Exotoxin A (protein synthesis inhibitor) • Alkaline protease • Elastase (disrupts corneal extracellular matrix proteins) • Lipopolysaccharide • Pyocyanin (oxidative damage) • Biofilm formation (exopolysaccharide matrix; Psl is prominent in ocular biofilms)
Treatment• Ciprofloxacin 0.3% hourly as fluoroquinolone monotherapy, or fortified antibiotics selected according to severity and susceptibility • Severe ulcers may require a fortified beta-lactam or vancomycin plus tobramycin 9–14 milligrams per millilitre hourly after a loading dose • Ciprofloxacin 0.3% is an accepted topical fluoroquinolone option for Pseudomonas keratitis; tailor therapy to severity, culture and susceptibility results, and local resistance patterns • Consider systemic therapy with scleral involvement or threatened perforation
Notes
  • PEARL: • A ring infiltrate may represent an immune (Wessely) ring; a ring abscess is an infectious suppurative ring. • In a Wessely ring, antigen-antibody complexes deposit in a sterile immune ring; an infectious ring abscess contains microbial material.
Staphylococcus aureus
Staphylococcus aureus
FieldDetails
Gram Stain• Gram-positive coccus (clusters)
Risk Factors• Blepharitis • atopic disease • corticosteroid use • post-cataract setting
Clinical Features• Dense white-yellow oval infiltrate • Well-defined edges • Hypopyon possible • Associated blepharitis/meibomian gland disease • Methicillin-resistant Staphylococcus aureus (MRSA) epidemiology is region- and time-dependent; some surveillance studies report emerging MRSA clones and increasing multidrug resistance.
Virulence Factors• Protein A (immune evasion) • Panton-Valentine leukocidin (polymorphonuclear leukocyte destruction) • Alpha toxin • Toxic shock syndrome toxin 1 • Exfoliative toxins • Coagulase
Treatment• Moxifloxacin may be used as empiric monotherapy for selected bacterial keratitis cases; choose initial therapy according to ulcer severity, risk factors, and local susceptibility patterns. • Fortified cefazolin if methicillin-susceptible Staphylococcus aureus (MSSA) is confirmed. • Fortified vancomycin (50 milligrams per milliliter) for methicillin-resistant Staphylococcus aureus (MRSA) or other resistant Staphylococcus species; if used empirically, combine it with gram-negative coverage. • Consider oral doxycycline for selected posterior blepharitis or meibomian gland dysfunction; it is not routine treatment for Staphylococcus aureus keratitis.
Notes
  • PEARL Methicillin-resistant Staphylococcus aureus (MRSA) keratitis epidemiology is region- and time-dependent; some surveillance studies report emerging MRSA clones and increasing multidrug resistance. Always culture in non-responding ulcers. Do not use fortified vancomycin as the sole initial empiric therapy because it lacks gram-negative activity; reserve it for resistant Staphylococcus species or when empiric resistant gram-positive coverage is clinically indicated, and combine it with gram-negative coverage when used empirically.
Streptococcus pneumoniae
Streptococcus pneumoniae
FieldDetails
Gram Stain• Gram-positive diplococci (lancet-shaped)
Risk Factors• Dacryocystitis • Nasolacrimal duct obstruction
Clinical Features• Serpiginous ulcer with an advancing border and active infiltrate • Undermined edges; trailing edge may show healing • Hypopyon • Associated dacryocystitis • Can lead to perforation
Virulence Factors• Capsule (anti-phagocytic) • Pneumolysin • Immunoglobulin A (IgA) protease • Autolysin
Treatment• Moxifloxacin or Penicillin G (fortified) • Cefazolin fortified • MUST treat underlying dacryocystitis • Dacryocystorhinostomy (DCR) after acute phase resolves
PEARL
  • Assess the patency of the lacrimal drainage system in Streptococcus pneumoniae keratitis, particularly when dacryocystitis or nasolacrimal obstruction is suspected.
  • Mucopurulent regurgitation on pressure over the lacrimal sac is consistent with dacryocystitis.
  • Treat coexisting dacryocystitis and underlying nasolacrimal obstruction; consider dacryocystorhinostomy according to the clinical setting and timing of infection control.
Moraxella species
Moraxella species
FeatureMoraxella species
Gram Stain• Gram-negative short rods, coccobacilli, or cocci, depending on species
Risk Factors• Alcoholism • Malnutrition • Debility • Immunosuppression • Dry eye
Clinical Features• Inferior or inferocentral/paracentral cornea • Indolent, slow-progressive ulcer • Oval, localized ulcer with an undermined necrotising edge • May be associated with a hypopyon • May have an undermined necrotising edge, with the organism persisting in the depth of the ulcer crater • Minimal pain
Virulence Factors• Pili contribute to adhesion • Proteases and endotoxin may contribute to corneal pathogenicity • Surface proteins of Moraxella catarrhalis can mediate adhesion and other host interactions; relevance is species-dependent
Treatment• Topical ciprofloxacin or moxifloxacin • Begin treatment intensively, then taper or modify according to clinical response and bacterial susceptibility • Severe or nonhealing disease may require adjunctive procedures • Assess for and manage relevant systemic contributors, including malnutrition and alcohol use, as clinically appropriate
PEARL
  • Moraxella keratitis is associated with ocular-surface disease and systemic debility, including alcoholism, diabetes, malnutrition, and immunodeficiency.
  • Human immunodeficiency virus (HIV) testing is not specifically indicated solely because of Moraxella keratitis or young age; assess for immunodeficiency when clinically suspected.
  • An indolent, usually peripheral or paracentral, often oval ulcer may progress deep into the stroma over days or weeks; these features can help distinguish it from more fulminant bacterial keratitides but are not pathognomonic.
Neisseria gonorrhoeae
Neisseria gonorrhoeae
CategoryDetails
Gram Stain• Gram-negative intracellular diplococci
Risk Factors• Neonates: birth-canal exposure • Sexually active adults with gonococcal conjunctivitis • Penile or vaginal contact with the eyes
Clinical Features• Hyperacute presentation • Copious purulent discharge • Chemosis • Lid edema • Can rapidly progress to corneal perforation if untreated • Penetrates intact corneal epithelium
Virulence Factors• Pili: adhesion • Immunoglobulin A protease • PorB • Opacity-associated proteins • Lipooligosaccharide • Penetrates intact corneal epithelium through multiple gonococcal adherence and invasion factors, including type IV pili
Treatment• Systemic therapy: ceftriaxone 1 gram intramuscularly as a single dose • Topical antibiotic therapy alone is inadequate • Consider one-time saline lavage • Evaluate and treat sex partners • Neonates: erythromycin 0.5% ophthalmic ointment in each eye as a single application at birth
PEARL
  • Neisseria gonorrhoeae is one of the rare bacteria that can penetrate an intact corneal epithelium. This is a guaranteed viva question. Mechanism: pilus-mediated adherence to intact corneal epithelium, followed by epithelial-cell uptake.
Nocardia asteroides
Nocardia asteroides
FeatureDetails
Gram Stain• Gram-positive, beaded branching filaments • Weakly acid-fast on modified Ziehl–Neelsen or Kinyoun stain
Risk Factors• Soil or sand trauma • Post-laser-assisted in situ keratomileusis • Topical steroid use
Clinical Features• WREATH-LIKE stromal infiltrate with satellite lesions • Slow progression • Fluffy or feathery margins may resemble fungal keratitis • Moderate anterior-chamber reaction or hypopyon may occur
Virulence Factors• Catalase-positive (evades polymorphonuclear leukocyte killing) • Cord factor • Sulpholipids
Treatment• Topical amikacin 20 to 25 milligrams per milliliter is first-line; initially dose hourly, then taper according to clinical response over approximately 4 to 6 weeks • Consider systemic trimethoprim-sulfamethoxazole for deep stromal involvement or scleral extension • Use susceptibility testing to guide alternatives, including imipenem for amikacin-resistant isolates
PEARL
  • Nocardia is WEAKLY ACID-FAST - modified Ziehl-Neelsen stain shows pink/red filaments.
  • This differentiates it from Actinomyces (not acid-fast) and Mycobacterium (strongly acid-fast).
  • Corneal scrapings should include a modified Ziehl-Neelsen stain when Nocardia is suspected.

10. Complications & Prognostic Factors

ComplicationMechanismManagement
• Corneal perforation• Corneal stromal melt from bacterial proteases and host inflammatory collagenolysis • Pseudomonas aeruginosa is highly destructive • Gonococcal infection can progress rapidly to perforation• Cyanoacrylate glue for small defects; the exact less-than-3-millimetre threshold remains unverified • Therapeutic penetrating keratoplasty for larger defects or iris prolapse; the exact greater-than-3-millimetre threshold remains unverified
• Endophthalmitis• Perforation permits direct bacterial entry into the eye • Pseudomonas aeruginosa and gonococcal infection may cause aggressive disease; comparative ranking remains unverified• Vitreous sampling and intravitreal antibiotics when bacterial endophthalmitis is suspected • Pars plana vitrectomy if there is inadequate response; the exact keratitis-associated threshold remains context-dependent
• Corneal scarring• Stromal collagen is replaced by irregular scar tissue after ulceration • Replacement collagen is produced during the injury response by keratocyte-derived fibroblasts and myofibroblasts• Optical penetrating keratoplasty after adequate quiescence; the fixed 12-month minimum remains unverified • Contact lens correction for irregular astigmatism
• Secondary glaucoma• Trabeculitis • Peripheral anterior synechiae • Corticosteroid-induced pressure elevation • Pupil block • The complete mechanism list is context-dependent in bacterial keratitis• Intraocular-pressure-lowering drops; blanket avoidance of prostaglandin analogues remains unverified • Goniotomy or trabeculectomy if refractory; the specific role remains context-dependent • Reassess corticosteroid use if intraocular pressure rises
• Complicated cataract• Direct lens toxicity in perforation • Prolonged inflammation • Corticosteroid exposure • The exact contribution of each mechanism remains incompletely verified• Phacoemulsification after quiescence; the specific timing remains unverified • Combined penetrating keratoplasty with extracapsular cataract extraction when both are required
• Corneal vascularization• Chronic inflammation promotes corneal angiogenesis • Corneal vascularization worsens corneal graft prognosis• Topical corticosteroids • Subconjunctival bevacizumab; this is off-label and the supporting evidence concerns corneal neovascularization generally • Fine-needle diathermy
• Phthisis bulbi• End-stage ocular atrophy and shrinkage after severe infection, perforation, or intraocular inflammation • Hypotony and structural disorganization of the globe• Enucleation or evisceration may be used for hopeless, unmanageable keratitis or keratitis with endophthalmitis • Prosthesis fitting may follow definitive globe removal
• Recurrent keratitis• Persistent predisposing factors, including contact-lens use and adjacent blepharitis or dacryocystitis • Recurrent Staphylococcus aureus infection may reflect ocular-surface, lid-margin, or nasopharyngeal colonization• Identify and treat the source of recurrence • Consider Staphylococcus aureus decolonization in selected recurrent cases • Obtain repeat culture when clinical response is unfavorable • Tailor antimicrobial therapy to microbiological results
Prognostic Factors
Prognostic Factors
Poor Prognostic FactorsGood Prognostic Factors
• Central location (involving visual axis) • Large infiltrate size (>3 mm) • Deep stromal involvement (>50% depth) • Presence of hypopyon (marker of severe disease) • Threatened or actual corneal perforation (marker of severe disease and poor corneal healing outcome) • Virulent organisms (Pseudomonas, gonococcal infection, Nocardia) • Symptom duration ≥7 days (associated with worse visual outcome on univariable analysis; not independently significant in multivariable analysis) • Systemic immunosuppression (risk factor for bacterial keratitis; not independently validated as a prognostic factor in this cohort) • Failure to respond to initial antibiotic treatment (marker of severe or refractory disease; prompts escalation) • Prior corneal surgery, including corneal graft surgery (risk factor for bacterial keratitis; independent prognostic effect not established in this cohort)• Small infiltrate size (≤3 mm) and peripheral location (features associated with less severe disease; independent favorable prognostic effect of the combined category not established) • Early assessment and treatment (clinically important for prevention of progression; independent favorable prognostic effect not established in this cohort) • Lower antibiotic minimum inhibitory concentration or demonstrated susceptibility (associated with smaller infiltrate or scar size in one study; independent favorable prognostic effect not established) • Good early clinical response within 48–72 hours (favorable treatment-response marker; independent prognostic effect of epithelial healing alone not established) • Absence of pre-existing corneal vascularization • Absence of systemic immunosuppression (lower-risk host characteristic; independent favorable prognostic effect not established in this cohort) • Organism-specific prognosis varies; Moraxella can cause severe vision-threatening keratitis

11. Landmark Trials & Evidence Base

Trial / StudyDesignKey FindingClinical Implication
Steroids for Corneal Ulcers Trial (SCUT), National Eye Institute (NEI), 2012- Randomized controlled trial (RCT); 500 patients; multicenter - Topical prednisolone was started after at least 48 hours of topical moxifloxacin- No significant difference in best-corrected visual acuity (BCVA) at 3 months overall - Subgroup: completely central ulcers had better visual acuity with steroids- Steroids are not universally beneficial - In SCUT, steroids were started after at least 48 hours of topical moxifloxacin - A subgroup benefit was reported for completely central ulcers
Moxifloxacin Monotherapy for Bacterial Keratitis- Randomized controlled equivalence trial - Compared moxifloxacin 0.5% monotherapy with fortified cefazolin 5% plus tobramycin 1.3% - Microbiologically proven, moderate, nonperforated bacterial corneal ulcers- Moxifloxacin 0.5% produced equivalent 3-month ulcer healing compared with fortified cefazolin 5% plus tobramycin 1.3%- Moxifloxacin monotherapy is an effective option for microbiologically proven, moderate, nonperforated bacterial corneal ulcers - Applicability to severe or perforated disease is not established by this trial
Fluoroquinolone Resistance in Bacterial Keratitis- Surveillance studies from the United States, India, and United Kingdom - Regional and organism-specific fluoroquinolone resistance or reduced susceptibility has been reported - Trends are heterogeneous rather than uniformly increasing- Fluoroquinolone resistance has been reported - Prior fluoroquinolone exposure may be associated with higher minimum inhibitory concentrations - Susceptibility patterns vary by location, population, and organism- Fluoroquinolone resistance is an emerging concern - Obtain cultures and susceptibility testing for clinically significant, atypical, or nonresponding keratitis - Consider fortified combination therapy when indicated by severity or treatment response
Therapeutic Penetrating Keratoplasty Outcomes (various corneal centers)- Retrospective case series from tertiary corneal centers - Evidence includes a 14-year series of culture-proven microbial keratitis- Infection eradication and anatomical success were assessed - Graft clarity and survival are variable - Visual outcome may remain limited by corneal scarring and vascularization- Therapeutic penetrating keratoplasty (TPK) can eradicate infection and preserve anatomical integrity - Graft clarity and visual outcomes are variable - Realistic counseling is essential
Povidone-Iodine for Bacterial Keratitis (emerging evidence)- Evidence includes a randomized controlled trial (RCT) and additional small clinical studies - A randomized controlled trial evaluated topical povidone-iodine 1.25% in 172 patients with bacterial keratitis- Topical povidone-iodine may have antimicrobial activity - In the trial, it did not differ significantly from commonly available topical antibiotics - It may be useful when effective antibiotic treatment is not practical- Topical povidone-iodine may be considered when effective antibiotic treatment is not practical, particularly in resource-limited settings - Its role as a routine adjunct is not established
Bacteriophage Therapy for Bacterial Keratitis- Evidence includes preclinical studies and limited human ocular experience - The reviewed evidence does not establish that the entire 2020–2024 literature is limited to case reports and animal models- Bacteriophage-based treatments have shown activity against multidrug-resistant Pseudomonas aeruginosa in preclinical keratitis models - Clinical effectiveness in refractory keratitis is not established- Bacteriophage therapy remains investigational for bacterial keratitis - Published human ocular experience includes compassionate-use treatment - Preclinical studies support further clinical evaluation

Steroids for Corneal Ulcers Trial - Exam Pearl

Steroids for Corneal Ulcers Trial Key Takeaway for Exams
  • Topical corticosteroids did not improve overall visual outcomes in bacterial keratitis.
  • In the Steroids for Corneal Ulcers Trial, topical corticosteroids were started after at least 48 hours of topical moxifloxacin treatment.
  • Patients with completely central ulcers had better 3-month visual acuity with topical corticosteroids, irrespective of Pseudomonas aeruginosa status.
  • In Pseudomonas aeruginosa keratitis, adjunctive topical corticosteroids showed no significant benefit or harm compared with placebo.
  • Bottom line: Individualize topical corticosteroid use; do not use routinely. Consider adjunctive topical corticosteroids on a case-by-case basis after at least 48 hours of topical antibiotic treatment.

12. Emerging Therapies & Future Directions

Bacteriophage TherapyDescription
Bacteriophage Therapy- Phages are viruses that infect bacteria; lytic phages replicate in susceptible bacteria and cause bacterial lysis. - Phages offer highly specific lysis of susceptible bacteria, but bacterial resistance to phages can develop. - Compassionate-use bacteriophage therapy has been reported for multidrug-resistant Pseudomonas aeruginosa infections; a specific keratitis case was not verified in the supplied findings. - Used in a reported case of recurrent antibiotic-resistant Staphylococcus aureus keratitis, including vancomycin-intermediate and vancomycin-resistant isolates. - PhagoBurn was a European Union-funded phase I–II trial of bacteriophage therapy for Escherichia coli and Pseudomonas aeruginosa burn-wound infections; it provides translational and regulatory evidence, not direct keratitis evidence, and was completed in 2017. - No additional ongoing bacteriophage trial specifically for bacterial keratitis was verified in the supplied findings. - Challenges include narrow host range, because phages infect particular susceptible bacterial hosts, and regulatory hurdles related to manufacturing and authorization.
Emerging TherapyDescription
Povidone-Iodine (1.25%) Topical- Broad antimicrobial activity against bacteria, fungi, protozoa including Acanthamoeba, and viruses. - Low cost; no reported resistance. - A randomized trial found no significant difference in outcome versus available topical antibiotics for bacterial keratitis. - May be considered where antibiotic treatment is impractical, including resource-limited settings. - Further investigation is needed regarding concentration and dosing frequency.
Antimicrobial Peptides (AMPs)- Defensins and cathelicidins are naturally occurring antimicrobial peptides with broad-spectrum activity. - Topical antimicrobial-peptide candidates remain investigational. - Resistance can develop; potential resistance evolution and transfer require evaluation. - Esculentin-1a(1-21)NH2 showed benefit in a murine Pseudomonas aeruginosa keratitis model.
Nanoparticle Drug Delivery- Nanoformulations may enhance corneal penetration and provide sustained drug release in preclinical studies. - Chitosan-based and lipid-based ciprofloxacin nanoformulations have been investigated in laboratory, ex vivo, and animal studies. - Clinical-trial status of these formulations remains unresolved.
Anti-virulence Strategies- Targeting bacterial quorum sensing (QS) may inhibit biofilm formation and virulence in experimental models. - Resistance or reduced activity can still emerge; quorum-sensing inhibition is not proven to prevent resistance selection. - Gallium compounds can disrupt Pseudomonas aeruginosa iron metabolism; benefit has been shown in preclinical keratitis models. - Oral doxycycline may be used as an adjunct to inhibit matrix metalloproteinases (MMPs) and counteract progressive corneal stromal thinning; clinical evidence is limited.
Collagen Cross-Linking (CXL) as Adjunct- Riboflavin activated by ultraviolet-A (UVA) light can damage microbial nucleic acids and generate reactive oxygen species. - Photoactivated chromophore for infectious keratitis–corneal cross-linking (PACK-CXL) may have antimicrobial and corneal-stabilizing effects. - Clinical evidence for bacterial keratitis remains limited and low certainty. - The Dresden protocol was developed for corneal ectasia. - Modified PACK-CXL protocols remain under investigation. - Not standard of care; remains investigational as an adjunct in bacterial keratitis.
Cenegermin (Oxervate) - Nerve Growth Factor (NGF) Eye Drops- Recombinant human nerve growth factor (NGF) indicated for neurotrophic keratitis. - Treat active ocular infection before initiating cenegermin. - Promotes corneal epithelial healing in neurotrophic keratitis. - Reduced reinfection risk in bacterial keratitis has not been established.

13. High-Yield MCQs with Explanations

MCQ

Q1. A 24-year-old contact lens wearer presents with a 12-hour history of a rapidly expanding corneal ulcer with ring-shaped stromal infiltrate, hypopyon, and blue-green tinge in the ulcer bed. Which statement best distinguishes an early infectious bacterial ring from a later sterile Wessely immune ring?

MCQ

Q2. A 35-year-old male presents with severe photophobia, pain, and a dendritic-looking lesion in the corneal epithelium following 6 months of daily contact lens use. He denies prior herpes. Corneal sensation is normal. In vivo confocal microscopy shows double-walled cysts of 15 µm. What is the MOST APPROPRIATE initial treatment?

MCQ

Q3. A 55-year-old diabetic patient with chronic epiphora presents with a central corneal ulcer with undermined edges spreading in a serpiginous pattern with dense hypopyon. Which investigation is MOST IMPORTANT in the immediate management of this patient?

MCQ

Q4. A post-laser-assisted in situ keratomileusis patient has keratitis. Smear shows acid-fast bacilli, and culture on Löwenstein–Jensen medium suggests a rapidly growing nontuberculous mycobacterium. What is the preferred regimen while species identification and susceptibility testing are obtained?

MCQ

Q5. The Steroids for Corneal Ulcers Trial (SCUT) demonstrated that adjuvant topical steroids in bacterial keratitis showed better 3-month best spectacle-corrected visual acuity in which specific subgroup?

MCQ

Question 6. Which antibiotic is associated with white crystalline precipitates in a corneal ulcer during treatment of bacterial keratitis?

MCQ

Question 7. A 6-day-old neonate presents with bilateral copious purulent discharge, severe lid edema, and a corneal ulcer in the right eye. Gram stain of discharge shows Gram-negative intracellular diplococci within polymorphonuclear leukocytes. What is the SINGLE MOST IMPORTANT treatment to initiate immediately?

14. Viva Questions & Consultant Extension

Consultant Viva Questions
Q1

What is the difference between a corneal ulcer and a corneal infiltrate?

Q2

I Why should you NOT use proparacaine drops repeatedly before corneal scraping?

Q3

A patient with bacterial keratitis has cultures positive for Strep viridans, showing crystalline deposits. How does this differ from infectious crystalline keratopathy?

Q4

What is the rationale for using N-acetylcysteine in bacterial keratitis?

Q5

When is systemic antibiotic therapy indicated in bacterial keratitis?

Q6

What is the role of collagen cross-linking (CXL) in bacterial keratitis?

15. Consultant-Level Extension Question

Consultant-level oral-exam question

A 42-year-old corneal graft recipient (post-PKP for keratoconus 3 years ago) on topical dexamethasone 0.1% TDS presents with a 3-week history of slowly progressive needle-like crystalline infiltrate in the mid-periphery of the graft near a 10-0 nylon suture, with almost no surrounding inflammation and VA 6/18. Cultures show polymicrobial growth. Discuss the full pathogenesis, likely organisms, why inflammation is minimal, and construct a complete management strategy including when and how to involve surgery. If Streptococcus viridans is the predominant organism, does this change management? Finally, what modification to the graft sutures would be appropriate and when?

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