Phospholipase A as a potent virulence factor of Vibrio vulnificus

Vibrio vulnificus infection has attracted special interest because of its high mortality rate. However, the identification of its major pathogenic determinant still remains obscure. In this study, a cytolysin-negative mutant strain of V. vulnificus CVD707 was used to determine the role of phospholipase A (PLA) in the pathogenesis of this bacterial infection. The mutant strain caused the lysis of erythrocytes in vitro and elevated plasma hemoglobin during the infection in mice. Both the hemolytic and PLA activities were dependent on calcium. Inhibition of hemolysis by PLA inhibitors including tetracyclin and the PLA substrate phosphatidylcholine also supports the possibility of membranous PLA as a major hemolytic factor in the cytolysin-deficient mutant. To identify the role of PLA in the pathogenesis of V. vulnificus infection, the effects of tetracycline on bacteria-induced macrophage cytotoxicity and lethality were compared with those of penicillin, an antibiotic with no inhibitory effect on PLA. Both the macrophage cytotoxicity and the lethality of V. vulnificus CVD707 to mice were significantly attenuated by tetracycline, but not by penicillin. However, bacterial counts in culture medium and mouse blood revealed that penicillin was more effective than tetracycline in killing bacteria under our experimental conditions. These results indicate that PLA activity is important in V. vulnificus-induced cytotoxicity and lethality, suggesting a crucial role for PLA in the pathogenesis of V. vulnificus infection.