Pseudomonas aeruginosa is a leading cause of bacterial keratitis, especially in users of contact lenses. These infections are characterized by extensive degradation of the corneal tissue mediated by Pseudomonas protease activities, including both Pseudomonas protease IV (PIV) and the P. aeruginosa small protease (PASP). The virulence role of PIV was determined by the reduced virulence of a PIV-deficient mutant relative to its parent strain and the mutant after genetic complementation (rescue). Additionally, the non-ocular pathogen Pseudomonas putida acquired corneal virulence when it produced active PIV from a plasmid-borne piv gene. The virulence of PIV is not limited to the mammalian cornea, as evidenced by its destruction of respiratory surfactant proteins and the cytokine interleukin-22 (IL-22), the key inducer of anti-bacterial peptides. Furthermore, PIV contributes to the P. aeruginosa infection of both insects and plants. A possible limitation of PIV is its inefficient digestion of collagens; however, PASP, in addition to cleaving multiple soluble proteins, is able to efficiently cleave collagens. A PASP-deficient mutant lacks the corneal virulence of its parent or rescue strain evidencing its contribution to corneal damage, especially epithelial erosion. Pseudomonas-secreted proteases contribute importantly to infections of the cornea, mammalian lung, insects, and plants.

Staphylococcus aureus is a major cause of corneal infections that can cause reduced vision, even blindness. Secreted toxins cause tissue damage and inflammation resulting in scars that lead to vision loss. Identifying tissue damaging proteins is a prerequisite to limiting these harmful reactions. The present study characterized a previously unrecognized S. aureus toxin. This secreted toxin was purified from strain Newman ΔhlaΔhlg, the N-terminal sequence determined, the gene cloned, and the purified recombinant protein was tested in the rabbit cornea. The virulence of a toxin deletion mutant was compared to its parent and the mutant after gene restoration (rescue strain). The toxin (23 kDa) had an N-terminal sequence matching the Newman superantigen-like protein SSL1. An SSL1 homodimer (46 kDa) had proteolytic activity as demonstrated by zymography and cleavage of a synthetic substrate, collagens, and cytokines (IL-17A, IFN-γ, and IL-8); the protease was susceptible to serine protease inhibitors. As compared to the parent and rescue strains, the ssl1 mutant had significantly reduced virulence, but not reduced bacterial growth, in vivo. The ocular isolates tested had the ssl1 gene, with allele type 2 being the predominant type. SSL1 is a protease with corneal virulence and activity on host defense and structural proteins.

Conclusions Methicillin-resistant S aureus inoculation beneath a rabbit corneal flap produced an infection that was useful for quantitative microbiological studies. A significant advantage in using moxifloxacin relative to gatifloxacin was observed in prophylaxis of keratitis ( P = .0001).