Panton-Valentine leukocidin (PVL) has long been considered a critical toxin in severe Staphylococcus aureus infection. PVL presumably breaches the body's defense system by lysing human polymorphonuclear cells (PMNs). Recently, however, bioactive peptides--phenol-soluble modulins (PSMs)--have been proposed as the main player in the lysis of PMNs, rather than PVL. This study aimed to resolve uncertainty concerning the cause of the lysis of human PMNs by using recombinant PVL toxins and PVL-neutralizing monoclonal antibodies. The recombinant PVL toxins showed strong lytic activity against human but not murine neutrophils. Moreover, the lytic activity of culture supernatants of strains USA400 MW2 and USA300 FPR3757 were completely neutralized by anti-PVL monoclonal antibodies. In contrast, phenol-soluble modulin alpha 3--the most potent PSM peptide--failed to lyse human PMNs at the concentrations contained in the culture supernatants. Phenol-soluble modulin alpha 3 did, however, enhance PVL-mediated lysis of human PMNs.

We determined the population genetic structures of feline and canine Staphylococcus aureus strains in Japan by multilocus sequence typing (MLST). Ecological analyses suggested that multiple feline-related S. aureus clones, including ST133, naturally occur as commensals and can cause endogenous infections in felines. In contrast, S. aureus populations do not likely include any clone that exhibits tropism in domestic dogs. Even if S. aureus infections occur in dogs, the pathologies are likely exogenous infections.