Molecular Characterization of Methicillin-Susceptible Staphylococcus aureus Clinical Isolates in the United States, 2004 to 2010

While much is known about the geographic distribution of different clonal types of methicillin-resistant Staphylococcus aureus (MRSA), few studies have assessed the molecular epidemiology of methicillin-susceptible S. aureus (MSSA), despite its continued clinical importance. In each U. S. Census region, reference laboratories collected successive MSSA isolates from patients with invasive or superficial staphylococcal infections for use in the Tigecycline Evaluation and Surveillance Trial. All isolates from the periods of 2004 to 2005 and 2009 to 2010 underwent antimicrobial susceptibility testing and characterization of their staphylococcal protein A (spa) type. Of the 708 isolates analyzed, 274 spa types were identified and divided into 15 genetic clusters. The most common clones were spa t002 (n = 63, 8.9%) and t008 (n = 56, 7.9%). While the distribution of the predominant spa types did not differ by U. S. Census region or time period, spa t008 was nearly twice as common in community skin and soft tissue infections than in nosocomial bloodstream infections (11.1% versus 5.6%, respectively; P = 0.008). Despite such differences, both community and nosocomial settings had diverse staphylococcal clonal types representing all major spa clusters. In contrast to those of MRSA, MSSA infectious isolates show wide genetic diversity without clear geographical or temporal clustering. Notably, the prevalent MSSA strains (spa t002 and spa t008) are analogous to the predominant MRSA clones, further demonstrating the importance of both lineages.