Bacterial keratitis — overview & distribution
The bacteria most often responsible are staphylococci, streptococci, Pseudomonas, and enterobacteria (notably Klebsiella, Enterobacter, Serratia, Proteus).
- Gram-positive cocci are the most frequently isolated in industrialized countries;
- Gram-negatives (Pseudomonas ++) predominate in contact-lens-related infections; they are more virulent than Gram-positives, with more extensive clinical damage;
- Virulence varies by strain: Pseudomonas may produce exotoxin or proteases, with very different clinical consequences.
Source of the organisms: periocular flora (mostly Gram-positive), oropharyngeal or gut flora (mostly Gram-negative); post-traumatic or postoperative infections may be exogenous (foreign body, environment).
The distribution of organisms and their susceptibility vary widely by region and host [4,6]. The ARMOR (United States) and BOSS (Australia) surveillance programs report a concerning rise in resistance to fluoroquinolones, particularly among staphylococci, requiring regular reassessment of empiric antibiotic protocols [5].
Gram-positive cocci
Classic presentation: central location, round or oval shape, well-demarcated infiltrate with a sharp border, cream or gray-white in color.
6.1 Staphylococci
The most frequent cause of infectious abscess; an organism present on the skin and periocular adnexa. S. aureus is more virulent than coagulase-negative staphylococci (including S. epidermidis) and produces more severe infiltrates with more marked necrosis.
- Risk factors (S. aureus)
- Diseased cornea (herpes, bullous dystrophy, rosacea, dryness, allergy), contact lenses, diabetes, age > 50 years.
- Course
- Small infiltrates, cornea relatively spared between lesions; rapid progression; flare.
6.2 Streptococcus — pneumococcus
- Risk factors
- Corneal trauma, dacryocystitis, filtering surgery.
- Course
- Very rapidly progressive; perilesional epithelium markedly disrupted and irregular; extension into the deep stroma; marked anterior chamber reaction, hypopyon, synechiae; radial Descemet folds, endothelial fibrin deposit.
6.3 Viridans — (micro)crystalline keratitis
White, dry stromal infiltration with crystalline borders and spicules, without major inflammatory reaction.
- Risk factors
- Chronic infection, steroids, corneal graft, corneal herpes, contact lens wear.
- Organisms in crystalline keratitis
- Streptococcus viridans/mitis, staphylococcus, Haemophilus, mycobacteria, among others.
Bacilli, Neisseria, mycobacteria & Nocardia
7.1 Gram-positive bacilli
Rarely responsible for corneal abscesses.
- Bacillus cereus: severe infection with a fulminant course; circumferential epithelial involvement with a microcystic appearance that can progress to perforation within hours.
- Listeria: flagellated bacillus, immune ring, anterior chamber reaction, bullous keratopathy with gas.
- Cutibacterium acnes (formerly Propionibacterium acnes): chronic keratitis, often late and indolent.
7.2 Gram-negative bacilli — Pseudomonas
Diffuse location, marked perilesional stromal edema, very rapid course. Pseudomonas aeruginosa is the most frequent organism in this category, especially in the contact lens wearer: it adheres to the damaged epithelium and rapidly invades the stroma.
| Form | Features |
|---|---|
| Protease-producing | Extension mainly into the depth; grayish "creamy" central ulcer, rapidly necrotic; acute purulent stromal melt, descemetocele or perforation; adherent yellow-green mucopurulent secretions; hypopyon. |
| Exotoxin-producing | Hyperacute, less necrotizing infection; diffuse "ground-glass" epithelial involvement; Wessely ring possible around 72-96 h. |
Other Gram-negatives: Serratia, Klebsiella, Proteus, Escherichia coli, Moraxella, Haemophilus influenzae.
7.3 Gram-negative cocci — Neisseria
Neisseria gonorrhoeae: rare infection, occurring during sexually transmitted infections and in the newborn. It is one of the few bacteria able to cross an intact corneal epithelium, carrying a high risk of perforation.
7.4 Mycobacteria & Nocardia
- Mycobacteria — risk factors
- Trauma, refractive surgery (often unilateral, several weeks after the procedure).
- Appearance
- Irregular infiltrate with blurred margins ("feathery," cotton-wool appearance), immune ring, satellite lesions, crystalline keratopathy.
- Nocardia — risk factors
- Trauma, refractive surgery.
- Appearance
- Yellow-white anterior stromal infiltrates, "pinhead" lesions arranged in a wreath; scleritis possible.
Faced with an atypical, indolent keratitis after refractive surgery, consider mycobacteria and Nocardia — slow-growing organisms that standard culture may miss.