Emergence of Resistance among USA300 Methicillin-Resistant Staphylococcus aureus Isolates Causing Invasive Disease in the United States

In the United States, a single pulsed-field type (PFT), USA300 (multilocus sequence type [MLST] 8), is the predominant cause of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections (26, 35). A subset of isolates with the pulsed-field gel electrophoresis (PFGE) pattern USA300-0114 and other closely related strains have disseminated clonally throughout the United States (25, 35) and typically carry Panton-Valentine leukocidin (PVL) toxin genes, the arginine catabolic mobile element (ACME), and staphylococcal cassette chromosome mec (SCCmec) type IVa (13). Although PFT USA100 (MLST 5) is the most common PFT isolated in cases of health care-associated MRSA infections in the United States (23), USA300 isolates have now emerged as a cause of health care-associated infections (22, 23, 32). Whereas traditional health care-associated MRSA strains are usually resistant to multiple antimicrobial classes, USA300 isolates are typically susceptible to most classes of antimicrobial agents and are resistant only to oxacillin and erythromycin (12, 17, 35). However, resistance to other antimicrobial agents has been sporadically reported in USA300 isolates (i.e., resistance to fluoroquinolones, tetracycline, clindamycin, and mupirocin) (10, 16, 35). Treatment options for MRSA skin and soft-tissue infections include trimethoprim-sulfamethoxazole (TMP-SMZ), clindamycin, and tetracyclines (6, 9, 14); the emergence of resistance to these agents in USA300 strains will pose a challenge for treating both community- and health care-associated S. aureus infections. Since 2001, when USA300 isolates were first recognized (25), these isolates have become increasingly resistant to the fluoroquinolones; 54% of the Active Bacterial Core surveillance (ABCs) isolates from 2005 to 2006 were resistant to levofloxacin (23). Fluoroquinolone resistance in S. aureus arises via chromosomal mutations in the DNA gyrase gene gyrA (18). Most other antimicrobial resistance in staphylococci emerges by acquisition of resistance determinants residing on plasmids and is often associated with transposons or insertion sequences (33). In previous analyses of USA300 isolates, several plasmids were identified. Among isolates with a typical susceptibility pattern (resistance to penicillin, oxacillin, and erythromycin), two plasmids were described: a small, 3.1-kb cryptic plasmid and a 27-kb mosaic plasmid that harbors several resistance determinants, including resistance to erythromycin (msrA) and penicillin (blaZ) (17, 35). Among USA300 isolates with resistance to tetracycline but susceptibility to doxycycline and minocycline, a 4.4-kb plasmid with the tetK determinant was reported (11, 30, 35), and isolates with clindamycin resistance carried a small, 2.6-kb plasmid harboring ermC (35). For USA300 isolates with both clindamycin and high-level mupirocin resistance, Diep et al. (10, 11) described a large conjugative plasmid, pUSA03, which carried two resistance determinants, ermC and mupA. The pUSA03 plasmid is related to a plasmid family that was first detected in gentamicin-resistant staphylococcal strains isolated in the mid-1970s (1, 2) and includes pGO1, pSK41, and pLW1043 (4, 5, 36). These plasmids are defined in this study as “pSK41-like” plasmids. They contain a highly conserved, transfer-associated region consisting of 15 tra genes. The plasmids also typically have multiple copies of the insertion element IS257, which act as preferred sites for integration of mobile elements that can carry resistance genes (4, 5). Antimicrobial resistance determinants described on pSK41-like plasmids include aac6′-aph2″, which confers resistance to aminoglycosides (4, 5, 36, 37); dfrA, which confers high-level trimethoprim resistance (5); mupA, which confers high-level mupirocin resistance (11, 27); and vanA, which confers vancomycin resistance (36). Previously, reports of USA300 isolates with plasmid-mediated resistance have been single events or restricted to specific geographical areas (10, 16). Here we describe USA300 isolates with unusual plasmid-mediated antimicrobial resistance patterns, including resistance to clindamycin, mupirocin (high level), gentamicin, trimethoprim (high level), and/or doxycycline. Outside of the ABCs collection (23), gentamicin and trimethoprim resistance have not been previously reported for USA300 isolates.