The data presented show that at least low levels of
extracellular protease are essential for maximum virulence
of P. aeruginosa in a mouse eye model. The strain chosen
for this study, PA103, produces no detectable elastase and
such low levels of alkaline protease that it is considered by
other investigators to be protease deficient (17, 32). Our
study and others (17, 27) have shown that this strain is
virulent in a mouse eye model. We found, however, that
alkaline protease-deficient mutants derived from strain
PA103 were avirulent in the mouse eye model. Virulence of
these mutants could be restored if subdamaging amounts of
purified Pseudomonas alkaline protease were added at the
time of infection and for a total of 8 consecutive days,
indicating that the loss of virulence in these mutant strains
was indeed due to a specific mutation affecting alkaline
protease and not some undetected secondary or pleiotropic
effect of the mutagen. Interestingly, we found that sub-damaging amounts of Pseudomonas elastase could substi-tute for alkaline protease and restore the virulence of the