Population Dynamics of Nasal Strains of Methicillin‐ResistantStaphylococcus aureus—and Their Relation to Community‐Associated Disease Activity

Abstract

Comparison of MRSA strains from asymptomatic carriers versus concurrently collected community-associated clinical strains from patients treated at local health-care facilities allowed for the identification of 3 population dynamics of nasal strains of MRSA: (1) endemic clones--for example, ST8:C and ST59:P--sustained asymptomatic carriage and infection over prolonged periods; (2) an epidemic clone, ST8:S, demonstrated enhanced capacity for rapid transmission and widespread infections; and (3) an outbreak clone, ST30:Z (USA1100), was highly infectious but exhibited poor asymptomatic transmission.

The role of virulence determinants in community-associated MRSA pathogenesis

“…Genome sequencing of two CA-MRSA strains, the Midwest Clone MW2 (USA400) and the pandemic clone USA300 [7,[9][10][11]25,26], revealed that ~20% of the unique genomic contents of CA-MRSA strains are due to the horizontal acquisition of multiple mobile genetic elements, including prophages and pathogenicity islands, which are absent from the traditional hospitalassociated MRSA strains COL, N315 and MRSA252 ( Figure 2). The prophages and pathogenicity islands contained numerous specialized pathogenicity factors, such as enterotoxins and exoproteins that could allow CA-MRSA to evade or subvert host defenses [25,26].…”

“…Type I ACME has been postulated to contribute to growth and survival of USA300 within the host, and promote colonization of the human skin [26]. ACME is integrated downstream of type IV SCCmec (Figure 2), utilizing the same cassette chromosome recombinases A and B (ccrAB) contained with SCCmec for mobilization and transfer [15][16][17][18][19][20][21][22][23][24]34]. Type I ACME is a distinctive genetic feature of the USA300 pandemic clone [26,34,35], although ACME-like elements have been detected in sporadic MRSA [32][33][34].…”

“…For example, PVL-positive ST1 strains (MW Clone) are prevalent in the United States, but PVL-negative ST1 was earlier frequent in communities in Western Autralia [4]. Similarly, whereas PVL-positive ST59 are endemic in Taiwan and Vietnam, the vast majority of ST59 strains in the United States do not carry the PVL-harboring prophage [11][12][13]29]. Despite the prevalence of PVL-deficient CA-MRSA strains, PVL is widely regarded as the primary virulence determinant driving the epidemic spread of the major CA-MRSA clones worldwide [14,40,41].…”

“…• The second CA-MRSA strain-ST30 clonal lineage-is known as the Southwest Pacific/Oceania Clone, as it was implicated in localized community outbreaks in Australia, Greece, Mexico and United States [7][8][9][10][11].…”

High prevalence of ST121 in community-associated methicillin-susceptible Staphylococcus aureus lineages responsible for skin and soft tissue infections in Portuguese children

“…Moreover, single ST121 MRSA isolates were reported in China (http://saureus.mlst. net/) and the United States [28].…”