Chelation in experimental Pseudomonas keratitis.

In cohtrast to its occasional occurrence during the first half of the 20th century (Joy, I942), Pseudomonas aeruginosa to-day ranks as a common, if not the most common, cause of bacterial corneal ulcer in the United States of America. Even with prompt laboratory diagnosis and use of adequate and specific antimicrobial therapy, the rapidly destructive lesion caused by this bacterium frequently results in severe visual impairment and often in corneal perforation with its attending complications. Such perforation is often predictable, in the experience of the author, when a spreading colliquative necrosis of the cornea continues to occur following conversion of daily cultures from positive to negative. It has been suggested that this rapid corneal destruction may be the result ofan extremely potent bacterial proteolytic enzyme (Fisher and Allen, I958a, b; Fisher, I958). This has been described as being intracellularly synthesized by Pseudomonas aeruginosa, released and extracellularly activated by available calcium cations (Morihara, 1963). The ability to split certain synthetic hexapeptides in a highly specific manner, plus its apparent action on the corneal stroma which is largely collagen, suggests further that this metallo-enzyme is in fact a collagenase (Schoellmann and Fisher, I966). Apart from Pseudomonas aeruginosa, only three groups of microbial organisms have been shown to produce such an enzyme (MacLennan, Mandl, and Howes, 1953; Rippon, I 968a, I968b) (Table I).