No abstract

Methicillin-resistant Staphylococcus aureus (MRSA) remains a major problem in hospitals, and it is now spreading in the community. A single toxin, Panton-Valentine leukocidin (PVL), has been linked by epidemiological studies to community-associated MRSA (CA-MRSA) disease. However, the role that PVL plays in the pathogenesis of CA-MRSA has not been tested directly. To that end, we used mouse infection models to compare the virulence of PVL-positive with that of PVL-negative CA-MRSA representing the leading disease-causing strains. Unexpectedly, strains lacking PVL were as virulent in mouse sepsis and abscess models as those containing the leukotoxin. Isogenic PVL-negative (lukS/F-PV knockout) strains of USA300 and USA400 were as lethal as wild-type strains in a sepsis model, and they caused comparable skin disease. Moreover, lysis of human neutrophils and pathogen survival after phagocytosis were similar between wild-type and mutant strains. Although the toxin may be a highly linked epidemiological marker for CA-MRSA strains, we conclude that PVL is not the major virulence determinant of CA-MRSA.

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is epidemic in the United States, even rivaling HIV/AIDS in its public health impact. The pandemic clone USA300, like other CA-MRSA strains, expresses Panton-Valentine leukocidin (PVL), a pore-forming toxin that targets polymorphonuclear leukocytes (PMNs). PVL is thought to play a key role in the pathogenesis of necrotizing pneumonia, but data from rodent infection models are inconclusive. Rodent PMNs are less susceptible than human PMNs to PVL-induced cytolysis, whereas rabbit PMNs, like those of humans, are highly susceptible to PVL-induced cytolysis. This difference in target cell susceptibility could affect results of experimental models. Therefore, we developed a rabbit model of necrotizing pneumonia to compare the virulence of a USA300 wild-type strain with that of isogenic PVL-deletion mutant and -complemented strains. PVL enhanced the capacity of USA300 to cause severe lung necrosis, pulmonary edema, alveolar hemorrhage, hemoptysis, and death, hallmark clinical features of fatal human necrotizing pneumonia. Purified PVL instilled directly into the lung caused lung inflammation and injury by recruiting and lysing PMNs, which damage the lung by releasing cytotoxic granule contents. These findings provide insights into the mechanism of PVL-induced lung injury and inflammation and demonstrate the utility of the rabbit for studying PVL-mediated pathogenesis.

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections are predominantly those affecting skin and soft tissues. Although progress has been made, our knowledge of the molecules that contribute to the pathogenesis of CA-MRSA skin infections is incomplete. Here we tested the hypothesis that alpha-hemolysin (Hla) contributes to severity of USA300 skin infections in mice and determined whether vaccination against Hla reduces disease severity. Compared with wild-type USA300 and Newman strains, isogenic hla-negative (Δhla) strains caused significantly smaller skin lesions in a mouse infection model. Moreover, infection with wild-type strains produced dermonecrotic skin lesions, whereas there was little or no dermonecrosis in mice infected with Δhla strains. Passive immunization with Hla-specific antisera or active immunization with a non-toxigenic form of Hla significantly reduced the size of skin lesions caused by USA300 and prevented dermonecrosis. We conclude Hla is a potential target for therapeutics or vaccines designed to moderate severe S. aureus skin infections.