Pathogenesis of Staphylococcus in the rabbit anterior chamber

PURPOSE. To investigate the host defense against Staphylococcus in the rabbit anterior chamber. METHODS. The bactericidal activity of rabbit aqueous humor was investigated in vitro. Rabbit anterior chambers were injected with viable Staphylococcus aureus or Staphylococcus epidermidis ( 1,000 or 500,000 colony-forming units [CFU]), killed bacteria, culture supernatants of either organism, or purified S. aureus alpha-toxin. CFU as well as phospholipase (PLA(2)) and myeloperoxidase (MPO) activities of aqueous humor were determined up to 25 hours postinfection ( PI). RESULTS. The number of viable S. aureus or S. epidermidis was significantly reduced when incubated with aqueous humor for 30 minutes ( P <= 0.0001). Rabbits challenged with either S. aureus or S. epidermidis demonstrated a significant reduction in CFU in aqueous humor by 1 hour PI ( P <= 0.0044). Eyes infected with either S. aureus or S. epidermidis demonstrated a significant increase in MPO activity beginning at 1 hour PI ( P <= 0.0455), but only S. aureus caused an increase in PLA(2) activity at 20 and 25 hours PI ( P <= 0.0002). No significant increases in PLA(2) activity were observed after injection of killed bacteria into the aqueous humor at any time point; however, injection of S. aureus supernatant or alpha-toxin into the anterior chamber significantly increased PLA(2) activity ( P <= 0.0210). Injection of alpha-toxin also resulted in significant increases in MPO activity beginning at 10 hours after injection ( P <= 0.001). CONCLUSIONS. This study demonstrates that aqueous humor has a potent host defense capability and that S. aureus, but not S. epidermidis, triggers a PLA(2) response in the rabbit anterior chamber that appears to be due to alpha-toxin.