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Pseudomonas aeruginosa PAO1 deficient in LasA protease was reported to be ocularly avirulent. However, the avirulence of this mutant could not attributed to the loss of LasA protease. The purpose of this study was to define the mechanism for such a mutant's inability to cause corneal disease.A LasA protease–deficient mutant of P. aeruginosa PAO1 was constructed by allelic exchange. Virulence of this mutant in mouse and rabbit models of keratitis was assessed by scoring for ocular disease and quantitating viable bacteria from infected corneas. Adherence to scarified mouse corneal tissue was determined with an organ culture assay.In the mouse eye, the LasA protease–deficient mutant was not virulent, despite being as adherent as its parent strain. Virulence of the mutant was also significantly reduced in the rabbit eye. Complementation with las A did not restore virulence in either model of infection. Neither the mutant nor the mutant complemented with las A grew well in ocular tissue. An analysis of the mutant showed that it was auxotrophic for leucine.These data show that the mutant's avirulence in the eye is caused by poor growth in the ocular environment and not the loss of a functional las A gene.