During our initial investigations of clusters of MRSA infections in U.S. prisons, we were surprised that MRSA isolates with indistinguishable PFGE patterns were recovered from prisoners in Mississippi, Texas, and Georgia (7
). We were further surprised to see additional MRSA isolates with the same PFGE pattern isolated from a variety of athletes and sports teams (9
), military recruits (72
), children from Tennessee (5
) and Texas (13
), and a large county hospital in Atlanta (34
). Clearly, this MRSA strain is widely disseminated within the United States and appears to be a major cause of community-associated infections.
USA300-0114 is ST8 by MLST, which differs by one or two loci from previously described representatives of the archaic (ST250) and Iberian (ST247) clones (24
). The USA300-0114 isolates also share a common spa
type motif (MBQBLO) and most likely evolved from a common ancestor with a genotype of the early archaic strains of the 1960s (55
). All four SCCmec
types have been identified among MRSA of ST8 (23
), although USA300-0114 is exclusively type IVa. Most USA300-0114 isolates are resistant only to β-lactams and macrolides, although plasmid-mediated resistance markers, such as tetracycline and clindamycin resistance, mediated by tet
(K) and ermC
, respectively, are starting to appear.
Microarray analysis confirms that, in general, USA300 isolates are highly related to USA500 isolates. However, USA500 isolates are typically multiply resistant and are more likely to cause health care-associated infections than infections in community settings. When we compared the profiles of genes present within USA300 but missing from USA500, the community-associated lineage USA400, and the major health care-associated lineage USA100, several differences became apparent. First, USA300 harbors sequences from (i) a number of bacteriophages, including phi PVL and phi N315, (ii) the SaPIn2 pathogenicity island, (iii) the SaPIbov pathogenicity island, and (iv) the genes encoding a number of fibronectin-binding proteins that are missing either totally or in part from the isolates of the other PFTs surveyed. These factors may bekey contributors to USA300's ability to cause infection in diverse patient populations. For instance, despite the high degree of relatedness between USA300 and USA500, the latter strain lacks members of the bacteriophage phi PVL and phi N315 gene sets. These factors are also absent from the major hospital PFT, USA100, suggesting that they may be essential for pathogenesis in the community setting. Indeed, phi PVL (lukF-PV
) genes are thought to distinguish community from health care isolates (2
). Moreover, a comparison of the two community-associated MRSA PFTs, USA300 and USA400, indicates that all of the USA400 isolates examined are missing the fibronectin-binding proteins, fnbA, fnbB
, and ebh
, as well as SaPIn2 and several but not all phi N315 genes, suggesting that these components may contribute to the virulence of the USA300-0114 isolates. These results also indicate that although both community-associated MRSA lineages harbor the PVL toxin genes (which health care-associated lineages typically do not), other factors distinguish these lineages from health care-associated isolates. Based on these comparisons, 20 genes or hypothetical genes unique to USA300-0114 isolates have been identified. Other PFTs contain some but not all of these 20 genes. It is likely that USA300 isolates contain additional virulence loci that are not represented on the Saur2a chip and that are yet to be elucidated.
The microarray-based procedure used here monitored the presence or absence of both well-studied virulence determinants (Table ) and genes and ORFs that have not previously been shown to influence pathogenesis directly (see the supplemental material). It is likely that there are unrecognized virulence and other factors (51
) in addition to latter collection of determinants. The genes that were identified in our analysis, including many bacteriophage-encoded proteins, may represent a series of new virulence factors which collectively facilitate the organism's ability to both circulate and cause infection within diverse environmental conditions and in diverse populations. Although 20 USA300-0114-specific and hypothetical genes have been identified in this study, it is difficult to know whether these genes are actively transcribed, translated, and functional or are in fact silent. These factors may be expressed only in specific environmental settings, such as those that are encountered during the course of infection. Further characterization of these genes and their protein products is expected to facilitate our understanding of the pathogenic potential of USA300-0114 and may lead to strategies that attenuate the strain.
In conclusion, USA300-0114 is a highly stable strain of S. aureus that is primarily responsible for community-associated infections in the United States. This strain continues to evolve its antimicrobial resistance profile through plasmid acquisition. Its natural reservoir remains an open question.