Ans. b. Pseudomonas (Ref: Khurana 5/e p96)Pseudomonas infection is highly virulent and may cause corneal ulcer perforation within 48 hours.Pseudomonas KeratitisPseudomonas aeruginosa is the MC gram-negative organism isolated from corneal ulcers and is a frequent cause of contact lens-associated keratitisQ.Source of Infection:These aerobic bacilli are found in moist environmentsQFrequently contaminate inadequately chlorinated swimming pools and hot tubs, ventilators, nebulizer and vaporizer solutions, and ophthalmic solution bottlesQ.Pathophysiology:The organism readily adheres to damaged epithelium, and stromal invasion is rapidQ.Pseudomonas keratitis tends to progress rapidly if inadequately treated.Most commonly, the organism produces destructive enzymes such as protease, lipase, elastase, and exotoxin, which results in necrotic, lipase, elastase, and exotoxin, which results in necrotic, soupy ulceration.The ulcer often extends peripherally and deeply within hours and can rapidly involve the entire cornea.Ring ulcers can develop, and the corneal epithelium peripheral to the primary ulcer typically develops a diffuse gray, ground-glass appearance.The corneal stroma appears to dissolve into a greenish yellow mucous discharge that fluoresces under ultraviolet (but not under cobalt blue) light.The suppurative ulcer frequently thins to a discemetocele that perforatesQ.The ulcer is often associated with a marked anterior chamber reaction and hypopyon formation.Extensive keratitis can extend to the limbus and produce an infectious sclertis.