Antibiotic Resistance in Community-Associated MRSA

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is an important cause of disease worldwide. Previous studies have shown that >80% of CA-MRSA in the U.S. is susceptible to clindamycin and tetracycline. Now, investigators report increasing resistance to these antibiotics in CA-MRSA isolates collected in Boston.

Pulsed-field gel electrophoresis (PFGE) and antibiotic susceptibility testing were performed on all MRSA isolates collected at a single Boston ambulatory care center from May 2004 through November 2005. Of the 123 isolates examined, 102 had PFGE patterns consistent with CA-MRSA strain type USA300. Among these USA300 isolates, resistance rates were high to erythromycin (96%), levofloxacin (85%), clindamycin (57%), and tetracycline (30%). Eighteen isolates (18%) were resistant to all four of these oral antibiotics. Twelve of the 18 were tested further and found to have high-level mupirocin resistance. Two major USA300 subtypes were represented (0114 and 0247), and most of the antimicrobial resistance was found among the USA300-0247 isolates. The resistance genes detected among multidrug-resistant isolates (tetK, ermC, and mupA) are typically found on plasmids. All isolates were susceptible to trimethoprim-sulfamethoxazole, linezolid, doxycycline, and rifampin.