Until the 1990s, MRSA rarely caused infections among community members without exposure to the health care setting (one exception is injection drug users). An outbreak of CA-MRSA infections occurred between 1989 and 1991 among indigenous Australians in western Australia without health care contact [58
]. CA-MRSA infections were also reported in people from neighboring regions [59
]. In the late 1990s, several cases of aggressive MRSA infection also occurred among individuals in the United States without established risk factors for MRSA. Four children died of CA-MRSA infections in Minnesota and North Dakota from 1997 to 1999. All the cases were rapidly fatal and were associated with necrotizing pneumonia or pulmonary abscesses and sepsis [60
]. The strain responsible for these infections was ST1 and PFGE type USA400 (also known as the MW2 strain) [52
]. Subsequently, clonal outbreaks of skin and soft-tissue infection caused by CA-MRSA were also reported among prison inmates, men who have sex with men, soldiers, and athletes, particularly football players [61
]. The strain responsible for these infections was ST8 and PFGE type USA300 [53
]. Cases of CA-MRSA skin infection and necrotizing pneumonia were reported internationally as well [65
In addition to causing necrotizing pneumonia, CA-MRSA has recently been reported to cause infections or infectious complications in situations in which S. aureus
or MRSA is an unusual pathogen. These have included cases of necrotizing fasciitis caused by PFGE type USA300 [67
], as well as cases of pyomyositis [68
], purpura fulminans with toxic shock syndrome [70
], and Waterhouse-Friderichsen syndrome [71
The number of CA-MRSA infections appears to be increasing, and the strains responsible for these infections have now entered the health care setting, blurring the line between “community” and “hospital” strains [72
]. The strains that cause these virulent infections carry SCCmec
IV (sometimes SCCmec
V), the smallest of the SCCs that confer methicillin resistance, and are generally susceptible to several non–β
-lactam antibiotics. This is in contrast to the multidrug-resistant nosocomial MRSA strains that carry larger SCCmec
]. CA-MRSA strains may also have a growth advantage over HA-MRSA strains [27
IV has appeared in several different genetic backgrounds [55
], PFGE types USA300 (ST8) and USA400 (ST1)—both agr
type III—accounted for the vast majority of CA-MRSA infections in individuals without the usual MRSA risk factors or health care contact in the United States [52
]. USA300 is now the predominant strain. Of interest, some of these USA300 isolates that cause infections are PVL positive but methicillin susceptible [78
Worldwide, there are other prevalent CA-MRSA strains, such as ST80 (France-Switzerland), ST30 (SWP clone), and ST93 (Australia Queensland clone) [65
]. Said-Salim et al. [77
] identified additional “community-acquired strains” (CA-MRSA strains defined as containing SCCmec
IV); however, these were in individuals with MRSA risk factors or health care contact.
The basis for the apparent increased virulence of CA-MRSA strains is incompletely understood. Numerous factors have been proposed, such as increased fitness, improved evasion of the host immune system, and unique toxin production. The genes and mechanisms by which CA-MRSA strains may cause aggressive disease are discussed in the sections that follow. Because these strains usually contain PVL, which is usually absent in HA-MRSA strains, some researchers postulate that this protein, with leukocytolytic and dermonecrotic activity, is responsible.
The role of PVL versus other virulence determinants
There is a strong epidemiological association between PVL and the emergence of CA-MRSA infections. PVL is uncommonly found in MSSA and HA-MRSA isolates [79
]. In a study of 593 S. aureus
isolates in France, PVL was absent in HA-MRSA isolates but was associated with all CA-MRSA strains [83
]. In another study, PVL was ubiquitous in a large sample of CA-MRSA isolates collected from across the globe [65
]. It is usually present in USA300 and USA400 [27
] and is often harbored by other SCCmec
IV-containing strains [77
]. The outbreaks of skin and soft-tissue infections and necrotizing pneumonia mentioned above were caused by PVL-positive strains.
Lina et al. [66
] determined the presence of lukS-PV
(the cotranscribed genes for PVL) in 172 S. aureus
strains collected from patients with a variety of clinical syndromes. PVL was significantly associated with community-acquired pneumonia (85% of strains), compared with hospital-acquired pneumonia (0%). PVL was also significantly associated with strains causing invasive skin infections such as furunculosis (93%) and cutaneous abscess (50%), compared with superficial folliculitis (0%). PVL was not observed in strains associated with infective endocarditis, urinary tract infections, toxic shock syndrome, or mediastinitis, although few strains were tested [66
]. Diep et al. [80
] reported a similar association of PVL and skin and soft-tissue infections caused by MRSA isolated from inpatients and outpatients from San Francisco General Hospital and inmates in county jails.
In addition to the epidemiological evidence suggesting that PVL may be a virulence factor in CA-MRSA, there is a scientific rationale for this association. Staphylococcal leukotoxins, including PVL, are secreted as bicomponent toxins consisting of S and F proteins [16
]. Depending on the combination of particular S and F proteins, a toxin is formed with varying leukocytolytic, erythrocytolytic, and dermonecrotic properties [84
]. PVL consists of LukS-PV and LukF-PV and 4 units of each form of octameric β
-barrel pores in leukocyte membranes in vitro, resulting in cell lysis [19
]. This may cause cells such as neutrophils to release inflammatory enzymes and cytokines (sublytic concentrations of PVL also appear to induce the release of these substances) [88
]. PVL also appears to induce apoptosis of neutrophils via a mitrochondrial pathway at lower concentrations, whereas, at higher concentrations, PVL induces necrosis [91
]. In vivo, PVL causes dermonecrosis when injected intradermally in rabbits [92
Given this evidence and the strong epidemiological association between PVL-containing CA-MRSA strains and necrotizing pneumonia and skin and soft-tissue infections, it is plausible that PVL is partly responsible for the enhanced virulence of CA-MRSA (other leukocidins may also play a role). However, recent studies comparing the virulence of PVL-positive and PVL-negative strains have had conflicting results.
Saïd-Salim et al. [77
] compared human polymorphonuclear cell lysis among PVL-positive and PVL-negative CA-MRSA strains with similar genetic backgrounds and found no difference in polymorphonuclear lysis. Voyich et al. [93
] compared PVL-positive strains and PVL-negative strains with similar genetic backgrounds in mouse sepsis and abscess models, as well as PVL knockouts created for the USA300 and USA400 strains. There was no difference in survival in the mouse sepsis model. In the abscess model, PVL-negative strains unexpectedly caused slightly larger abscesses than did the PVL-positive strains. Isogenic pvl
strains of USA300 and USA400 showed no difference in the ability to cause polymorphonuclear lysis in vitro. The authors concluded that the PVL “...toxin is not the major determinant of disease caused by these prominent CA-MRSA strains” [93
, p. 1769]. It is possible that the mouse models used in this study were not optimal to assess the in vivo effects of PVL, or, as the authors suggested, that PVL either is a marker for other virulence factors present in these strains or is one of many factors causing the enhanced virulence of particular CA-MRSA strains.
PVL was investigated in a mouse pneumonia model by Labandeira-Rey et al. [94
]. Mice were infected with isogenic PVL-positive and PVL-negative (non–CA-MRSA) strains. PVL-positive strains caused necrotizing pneumonia similar to that seen in humans, whereas PVL-negative strains showed only some leukocytic invasion. When PVL-negative mutants were complemented with plasmids containing the PVL operon, massive tissue damage and mortality resulted. In mice, exposure to LukS-PV and LukF-PV toxin was sufficient to cause lung damage, weight loss, and increased mortality in a concentration-dependent fashion [94
]. In these studies, however, a single non–CA-MRSA strain was used.
In contrast, Bubeck Wardenburg et al. [95
] recently reported conflicting results. They demonstrated that α
-hemolysin and not PVL was responsible for mortality in a mouse pneumonia model, using USA300 and USA400 CA-MRSA strains.
These studies suggest that the association of PVL with enhanced S. aureus
virulence is complex and controversial and warrants further investigation. Furthermore, Wang et al. [20
] recently discovered that phenol-soluble modulins, a previously unrecognized class of secreted S. aureus
peptides, are up-regulated in CA-MRSA strains, compared with the level in HAMRSA strains; cause inflammation; destroy neutrophils; and are responsible for virulence in mouse abscess and bacteremia models. Other toxins, such as the enterotoxins, may also play an important role in these infections.
Virulence of USA400
USA400 (or MW2) is a highly virulent CA-MRSA strain. This is apparent not only in human disease but also in animal models [27
]. Initially, its only resistance genes were mec
which encodes penicillinase. Researchers sequenced USA400 and compared its sequence with the sequences of 5 other strains (N315, a Japanese MRSA; Mu50, a vancomycin-resistant MRSA; E-MRSA-16, an epidemic MRSA in the United Kingdom; COL, a MRSA strain; and NCTC8325, a widely used reference strain) to identify potential virulence factors associated with this strain. USA400 was the only strain to contain the PVL operon. In addition, it contained 16 unique superantigen genes, including 11 exotoxin genes and 5 enterotoxin genes. These genes had at least a 2% difference in their amino acids, compared with their homologues. One exception was staphylococcal enterotoxin H (seh
), which was unique to USA400 [27
] and can cause a toxic-shock–like syndrome [96
]. USA400 also contained a novel gene cluster dubbed “bacteriocin of S. aureus”
encodes a potential bacteriocin, or antibacterial agent. This bacteriocin could help USA400 compete with other colonizing flora and increase the chance of infection with this strain [27
]. These data suggest that there are several factors that may contribute to the virulence of USA400 and that these factors are ripe for future investigation.
Virulence of USA300
Like USA400, USA300 is associated with virulent disease [93
]; however, USA300 causes far more incident cases of CA-MRSA infection and is becoming resistant to several non–β
-lactam antibiotics [28
]. The genome of USA300 was sequenced by Diep et al. [28
] and compared with 10 previously sequenced S. aureus
strains as well as 4 coagulase-negative strains to identify factors potentially associated with its high virulence. Of interest, there were minimal differences between the core sequences of USA300 and COL, an early MRSA. In addition to harboring SCCmec
IV and the PVL operon, USA300 contained homologues closely related to staphylococcal enterotoxins Q and K, designated SEQ2 and SEK2. Like COL and USA400, USA300 also has a genome that includes a bacteriocin gene cluster. Most notably, USA300 contains a genomic island, termed “arginine catabolic mobile element” (ACME), which encodes an arginine deaminase pathway that converts l
-arginine to carbon dioxide, adenosine triphosphate, and ammonia. Arginine deaminase, a known virulence factor in other pathogens, may enhance the virulence of USA300 by enabling it to (1) survive more easily on acidic, human skin; (2) proliferate more easily in conditions low in oxygen, such as abscesses; and (3) evade host defenses by inhibiting production of nitric oxide and mononuclear cell proliferation as in Streptococcus pyogenes
]. Further investigation of ACME may help elucidate the remarkable success and virulence of the USA300 strain.
Colonization and CA-MRSA
As discussed above, the anterior nares are the classic reservoir for nosocomial S. aureus
infections, including HA-MRSA. However, data suggest that other sites of colonization or modes of transmission play an important and underappreciated role in the development of CA-MRSA infection. Heterosexual contact was recently identified as a mode of transmission of CA-MRSA. Most cases had genital CA-MRSA colonization without nasal colonization [98
]. In an outbreak investigation of CA-MRSA abscesses among St. Louis Rams football players, no MRSA was isolated from nasal or environmental samples. Perhaps other sites of colonization, shared items, or an unsampled environmental site played a role in transmission [64
]. Future epidemiological investigations of CA-MRSA should include sampling of several environmental and body sites in addition to the anterior nares.