Which gram-negative bacterium is most commonly associated with corneal ulcers in contact lens wearers?

• A. Pseudomonas aeruginosa
• B. Haemophilus influenzae
• C. Neisseria meningitidis
• D. Klebsiella pneumoniae

Answer: (A)

The main idea is that contact lens–related corneal ulcers are most strongly associated with Pseudomonas aeruginosa because this organism thrives in moist environments, readily forms biofilms on contact lenses and lens cases, and possesses virulence factors that allow rapid invasion and destruction of corneal tissue.

Pseudomonas aeruginosa is a gram-negative rod that is commonly found in water and aqueous solutions, making contaminated lenses or lens cases a frequent source. Its ability to form biofilms protects it from disinfection and antibiotics, so infections can progress quickly, leading to a serious, sight-threatening ulcer. The bacterium also produces enzymes and toxins that degrade corneal tissue, amplifying damage if treatment is delayed. Clinically, it tends to cause a sudden, painful, purulent keratitis with rapid progression, which is characteristic in wearers of contact lenses.

In the lab, this organism is typically identified as oxidase-positive and a non-lactose fermenter, features that help distinguish it from other gram-negative bacteria. While other bacteria can cause ocular infections, they are not as strongly linked to contact lens–associated keratitis as Pseudomonas aeruginosa, making it the most likely culprit in this scenario.