USA300 is the leading cause of community-associated bacterial infections in the United States [24
]. In addition, USA300, which is epidemic in the United States, appears to have enhanced virulence compared to traditional hospital-associated MRSA strains [1
]. Although the pathogen can cause severe or fatal invasive disease [26
], the vast majority of USA300 infections are those of skin and soft tissue [28
]. There has been an intense effort to better understand mechanisms of USA300 virulence and transmission (reviewed in refs. [30
). Recent work indicates Hla is produced by USA300 at relatively high levels in vitro [34
], and studies in animal models revealed a prominent role for this secreted toxin in the pathogenesis of USA300 pneumonia [6
]. However, the contribution of Hla to the severity of USA300 skin infections had not been evaluated until now.
Herein we demonstrate that Hla contributes significantly to the severity of USA300 skin infections in a mouse model. Compared with strain Newman, the contribution of Hla to disease severity was more pronounced in mice infected with USA300 (LAC) ( and ). LAC is a widely used clinical isolate representative of the USA300 epidemic clone [12
], whereas Newman is a methicillin-susceptible S. aureus
strain originally isolated from a case of secondarily infected osteomyelitis in 1952 [35
]. Newman expresses less Hla compared to LAC (ref. 
and data not shown), and consistent with this observation, deletion of Hla from Newman minimally changed the size of skin lesions (). These data are in accordance with the recent studies of Li et al. [1
], which indicate that differential expression of virulence molecules rather than presence or absence in the core genome dictates (at least in part) differences in virulence among S. aureus
strains. Thus, the findings here suggest factors other than Hla in strain Newman play a more prominent role in the pathogenesis of skin infections, whereas the toxin is a major determinant of severity in USA300 skin infections. It is also evident that multiple S. aureus
molecules contribute to skin infections in general, since deletion or neutralization of Hla did not completely ablate formation of abscesses.
Previous studies have not attempted to correlate expression of Hla with severity of skin infections. However, production of Hla was recently shown to correlate with refractory S. aureus
skin colonization in patients with atopic dermatitis [36
]. Inasmuch as hla
is present in the genome of many S. aureus
strains, it is likely the toxin contributes to severity of human S. aureus
infection, a notion that has been confirmed in animal infection models [6
]. Although the mouse skin infection model cannot mimic all of the features of human skin infection, such as the size of inoculum, the primary readouts of our mouse skin infection model (skin necrosis and abscess size) are two parameters used to classify severe human skin and soft tissue infections [38
]. Moreover, USA300 is known to cause necrotizing skin and soft tissue infections in humans [26
]. Therefore, it is reasonable to conclude that the mouse skin infection model described herein can be used as a general or rudimentary approximation of S. aureus
skin infection in humans.
There is also little information on anti-Hla antibody titers in human skin infections. In 1962, Lack and Towers reported that only 48% of patients with proven S. aureus
infection had relatively high anti-Hla antibody titers [39
]. More notably, anti-Hla antibody titers did not consistently increase in patients with S. aureus
infections, and when they did increase following acute infection, antibody levels then decreased rapidly over time [39
]. These early observations suggest humans may lack or have limited protection against Hla, albeit the anamnestic antibody response to Hla should be robust in individuals re-infected with Hla-producing S. aureus
. The passive and active immunization data presented here suggest Hla is a potential target for vaccines or therapeutics designed to moderate the severity of S. aureus
skin infections. Inasmuch as Hla is a core genome-encoded toxin present in virtually all CA-MRSA strains, a therapeutic approach directed at Hla would be relatively broad in scope.