, 2010; Kreisel et al., 2011). USA300-related strains were also more prone to spread from the initial infection site and caused more severe infections than HA-MRSA in patients suffering from Selleck Z VAD FMK pneumonia with pulmonary emboli (Ganga

et al., 2009; Hota et al., 2011). However, other reports describe better clinical outcomes associated with USA300 infections (Lalani et al., 2008; Moore et al., 2009). Although some studies that reported more positive clinical outcomes with CA-MRSA also describe hypervirulent CA-MRSA trends that merely lack full statistical significance, such as increased risk of being admitted into intensive care (OR = 1.8, P = 0.09) (Popovich et al., 2008). Selleck GSK3 inhibitor Additionally, effective treatment, which is easier to achieve when treating CA-MRSA infections given their inherent susceptibility to clindamycin, tetracyclines, rifampicin and trimethoprim/sulfonamide, can reduce

the severity of CA-MRSA disease outcomes in population-based studies (Bassetti et al., 2011). Unfortunately, this trend of increased antibiotic susceptibility may be diminishing as new reports show increased antibiotic resistance among USA300 isolates, possibly through direct acquisition of resistance determinants from multidrug-resistant HA-MRSA strains (McDougal et al., 2010). Thus, the future clinical outlook appears grim with respect to USA300 infections given their increased prevalence in both hospital- and community-acquired infections, their propensity

to acquire new antibiotic resistance determinants, and the steady decline in positive clinical outcomes associated with USA300 infections. Given the recent impact of USA300 on human health, significant research effort has been exerted to elucidate the source of USA300 success. Here, we review these findings and broadly categorize them into three main classes: (1) newly acquired genes that promote virulence and/or fitness, (2) altered regulation of GNAT2 core genes resulting in elevated virulence and/or fitness, and (3) nonsynonymous mutations in core genes that enhance virulence and/or fitness. Many different lineages of CA-MRSA (USA400, USA1000, and USA1100) cause outbreaks and invasive infections, but in North America, none are as prevalent as epidemic USA300. These clones have acquired many genes in the form of MGEs that may confer a selective advantage over other CA-MRSA strains. Several groups have investigated many of these MGEs with the goal of elucidating factors (if any) that have contributed to the overwhelming success of USA300. USA300 CA-MRSA isolates contain genes encoding enterotoxins K and Q (sek2 and seq2) in a unique pathogenicity island SaPI5 (Diep et al., 2006a). Sek2 and Seq2 are thought to contribute to pathogenesis by stimulating T-cells through binding of the Vβ chain of αβ T-cell receptors.