Pseudomonas aeruginosa
The answer is A. Pseudomonas aeruginosa is the most critical organism to consider in contact lens-related microbial keratitis. This organism is consistently identified as the most common gram-negative pathogen in contact lens wearers and is associated with the most severe disease outcomes, including worse visual acuity and higher rates of corneal transplantation 1, 2, 3.
Why Pseudomonas aeruginosa is the Priority
Contact lens wear is the most common risk factor for bacterial keratitis in developed countries 1. Among contact lens wearers specifically:
P. aeruginosa dominates the microbial landscape: It accounts for 22.7% of bacterial keratitis cases overall 1, but this percentage increases dramatically in contact lens-associated cases, where it represents 55% of positive cultures compared to 0-23% for other organisms 4
Unique virulence characteristics: P. aeruginosa has specific virulence factors that allow it to thrive in the contact lens/storage case/ocular environment 5, making it particularly adapted to this niche
Severity of disease: Infections with P. aeruginosa are associated with significantly worse visual outcomes 6 and more severe keratitis at presentation 4
Why the Other Options Are Less Important
Corynebacterium diphtheriae (Option B): This is not a relevant corneal pathogen. While Corynebacterium species can be isolated (6.21% of bacterial keratitis) 7, these are typically non-diphtherial species and are far less clinically significant than Pseudomonas.
Epstein-Barr virus (Option C): While viruses including EBV can produce immunologically mediated corneal infiltrates 1, they are not the primary concern in contact lens-related keratitis. Viral keratitis typically occurs in different clinical contexts.
Candida albicans (Option D): Fungal keratitis represents only 7.31% of microbial keratitis cases 7. Among fungi, Fusarium species (35.42%) are more common than Candida in keratitis 7. Fungal infections are more associated with trauma than contact lens wear 7.
Clinical Implications
The 2024 Bacterial Keratitis Preferred Practice Pattern emphasizes that P. aeruginosa is the most common gram-negative organism in bacterial keratitis 1. Multiple international studies confirm this pattern:
- Turkey: P. aeruginosa was the most common pathogen in contact lens keratitis 6
- Spain: P. aeruginosa presence was associated with more severe sequelae and greater need for corneal transplantation 3
- Global systematic review: P. aeruginosa was the most common causative organism worldwide in contact lens-related bacterial keratitis 2
Treatment consideration: P. aeruginosa shows excellent sensitivity to fluoroquinolones (especially ciprofloxacin) and aminoglycosides 2, with only 2% resistance to ciprofloxacin in some series 4. This informs empiric antibiotic selection in contact lens wearers presenting with keratitis.