Infection of traumatized cornea by Pseudomonas aeruginosa is usually rapid and self-limiting and often results in impaired vision due to corneal scarring (22). The hallmark of this infection is a liquefactive necrosis associated with extensive ulceration and corneal perforation (1). Years ago, Fisher and Allen (7) suggested that the characteristic damage observed with P. aeruginosa eye infections may be attributable to the action of bacterial proteases on the corneal proteoglycan. Several groups, including Kawaharajo et al. (16), Hirao and Homma (12), Kreger and Gray (20) and Kreger and Griffin (21) have since provided data to support this hypothesis. Recently, Ohman et al. (27) initiated a genetic study to examine the role of bacterial toxin and protease in corneal infection. They examined the virulence of an elastolysis-deficient mutant of P. aeruginosa and concluded that elastolysis was not required to establish an active infection. They also concluded that if extracellular protease is required for virulence in Pseudomonas eye infections, then alkaline protease produced by the mutant strain they examined may have been sufficient to cause corneal damage. Isolation of mutants specifically deficient in the production of alkaline protease would facilitate definitive studies addressing the role of alkaline protease in P. aeruginosa infections. This approach has been complicated by the fact that most strains of P. aeruginosa produce at least two distinct extracellular proteases (25) and that alkaline protease does not possess a unique or stringent substrate specificity (26). We have been unable to specifically inhibit the activity of interfering protease (elastase) to facilitate detection of alkaline protease mutants on skim milk agar plates (unpublished data). To circumvent these problems, we chose to isolate alkaline protease mutants in strain PA103, which produces alkaline protease (4) but no detectable elastase (28). Despite the fact that strain PA103 is serum sensitive (41), it is virulent in a mouse eye model of corneal infection (17, 27). We have isolated alkaline protease-deficient mutants of strain PA103 after ethyl methanesulfonate (EMS) mutagenesis and screening of mutagenized colonies on skim milk agar plates. After in vitro characterization of these
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mutants, mutants specifically deficient in alkaline protease production were identified, and the virulence of these mutants was compared with that of the wild-type strain in a mouse eye model.