Antimicrobial susceptibility and staphylococcal chromosomal cassette mec type in community- and hospital-associated methicillin-resistant Staphylococcus aureus

Study Objective. To differentiate the characteristics of community-associated methicillin-resistant Staphylococcus aureus (MRSA) and hospital-associated MRSA isolates on the basis of their susceptibility profiles, induction of clindamycin resistance, and staphylococcal chromosomal cassette (SCC) mec types. Design. In vitro molecular and susceptibility study of isolates obtained from December 2004-January 2006 as part of a large, ongoing clinical study Setting. Level I trauma center in Detroit, Michigan. Bacterial Strains. Three hundred eight MRSA isolates randomly collected from patients; 130 were classified as community-associated, and 178 were classified hospital-associated by using definitions from the Centers for Disease Control and Prevention (CDC). Intervention. Minimum inhibitory concentrations were tested on the basis of current guidelines from the Clinical and Laboratory Standards Institute. Measurements and Main Results. All tested MRSA isolates were susceptible to daptomycin, linezolid, and vancomycin. In addition, community-associated MRSA isolates were significantly (all p <= 0.05) more susceptible to trimethoprim-sulfamethoxazole (99%), clindamycin (96%), and a fluoroquinolone (76%) than hospital-associated MRSA isolates. Inducible resistance to clindamycin was demonstrated in 8.4% of community-associated MRSA isolates versus 50% of hospital-associated MRSA isolates (p <= 0.001). Of interest, 35% of the MRSA isolates collected from hospitalized patients (> 48 hrs after admission and according to the CDC definition) possessed SCCmec type IV. Conclusion. Overall, inducible clindamycin resistance occurred at significantly higher rates in the hospital-associated MRSA isolates, susceptibility differed significantly between community- and hospital-associated MRSA, and most of the hospital isolates contained SCCmec type IV.