Phospholipase A(2) in rabbit tears: A host defense against staphylococcus aureus

PURPOSE. This study analyzed rabbit tears for anti-staphylococcal activity, the role of phospholipase A(2) (PLA2) in this reaction, and the ability of enzyme inhibitors to promote bacterial survival. METHODS. Contact lenses with Staphylococcus aureus were applied to scarifie d rabbit eyes. The colony-forming units (CFU) per cornea or lens were deter mined and pathology was scored by slit-lamp examination (SLE). The bacteric idal activity was measured by incubating bacteria with rabbit tears or PLA2 at 33 degrees or 37 degreesC. Radiolabeled S. aureus was incubated with PL A2 or tears to quantify the release of a membrane component that was identi fied by thin-layer chromatography. Inhibitors of these reactions were also analyzed. RESULTS. Application of Staphylococcus, on contact lenses, to rabbit cornea s resulted in bacterial killing and limited inflammation. Incubation of tea rs and bacteria (1:1; v/v) in tryptic soy broth at 33 degreesC decreased CF U approximately 4 logs. Tears (greater than or equal to 30 mul) or PLA2 (gr eater than or equal to 30 U) incubated with bacteria in phosphate-buffered saline were bactericidal. PLA2 (greater than or equal to0.2 U) or tears (gr eater than or equal to2 Al) cleaved bacterial membranes, liberating arachid onic acid. Spermidine or tetracaine inhibited cleavage of bacterial membran es by tears or PLA2 and spermidine promoted bacterial survival and growth i n tears. Tears (60 mul) killed > 99% of the bacterial inoculum, whereas bac teria incubated in tears plus spermidine approximately doubled in number. CONCLUSIONS. PLA2 in rabbit tears kills Staphylococcus by hydrolyzing bacte rial membranes to release arachidonic acid. Spermidine and tetracaine inhib ited PLA2 activity and spermidine protected Staphylococcus from PLA2 in rab bit tears.