Secretory Phospholipase A(2) Is the Principal Bactericide for Staphylococci and Other Gram-Positive Bacteria in Human Tears

We examined human tears for molecules that killed gram-positive bacteria. The principal mediator of bactericidal activity against staphylococci proved to be a calcium-dependent enzyme, secretory phospholipase A(2). Whereas the concentration of secretory phospholipase A(2) in the normal tear film exceeded 30 μg/ml, only 1.1 ng (<0.1 nM) of the enzyme per ml sufficed to kill Listeria monocytogenes and 15 to 80 ng/ml killed Staphylococcus aureus. Despite its efficacy against gram-positive bacteria, secretory phospholipase A(2) lacked bactericidal activity against gram-negative organisms (Escherichia coli, Salmonella typhimurium, and Pseudomonas aeruginosa) when tested in the ionic environment of tears. Given the presence of secretory phospholipase A(2) in tears, intestinal secretions, and leukocytes, this enzyme may play a substantial role in innate mucosal and systemic bactericidal defenses against gram-positive bacteria.