For instance, α-toxin or α-hemolysin (Hla) is a potent heptameric pore-forming toxin known to be critical for virulence in nearly every tested disease model from skin lesions and endocarditis to murine mastitis (Jonsson et al., 1985; O’Reilly et al., 1986; Bayer et al., 1997). Upon interacting with susceptible cells, which include leukocytes, keratinocytes, platelets, and endothelial cells, it forms a 100 Å deep pore in the plasma membrane Ivacaftor nmr resulting

in rapid cell lysis (Song et al., 1996; Gouaux, 1998). Recently, a number of reports have shown that Hla expression is highly elevated in USA300 clones compared with other S. aureus isolates (Montgomery et al., 2008; Li et al.,

2009, 2010; Cheung et al., 2011). Moreover, deletion of hla abrogates USA300 virulence in murine and rabbit skin lesion models as well as pneumonia (Bubeck Wardenburg et al., 2007a; Kennedy et al., 2008, 2010). However, it should be noted that hla mutants in almost any S. aureus background are attenuated (O’Reilly et al., 1986; Patel et al., 1987; Idasanutlin in vitro Bramley et al., 1989; McElroy et al., 1999; Bubeck Wardenburg et al., 2007b); thus, the loss of virulence in USA300 ∆hla mutants is consistent with α-toxin in general being a critical pathogenicity factor to S. aureus. δ-toxin (encoded by hld) and related α-type PSMs (αPSMs) are amphipathic α-helical peptides with potent leukocidal and chemotactic properties (Wang et al., 2007). They have been shown to be overproduced by CA-MRSA clones with respect

to most HA-MRSA isolates (Wang et al., 2007; Li et al., 2009, 2010). Their abundant production is essential for full virulence in murine and rabbit skin models of infection as well as murine sepsis (Wang et al., 2007; Kobayashi et al., 2011). Montelukast Sodium Interestingly, they have recently been shown to exert potent antimicrobial activity against multiple Gram-positive bacterial species (Joo et al., 2011). This property may prove critical for efficient colonization of nonsterile sites such as skin and nasal passages, thereby providing CA-MRSA with a selective advantage during transmission. Finally, S. aureus expresses a number of secreted proteases that, while antagonistic to in vitro biofilm formation, likely mediate the breakdown of host fibrotic tissue synthesized to confine S. aureus-containing lesions thereby promoting bacterial dissemination and disease progression. As with α-toxin and αPSMs, USA300 clones are also known to excrete proteases in excess, potentially limiting the host’s ability to control minor skin and soft tissue infections (Lauderdale et al., 2009). Thus, several groups have consistently reported the robust expression of numerous virulence determinants in USA300 compared with other clinical isolates.