Clinical and laboratory features of community-associated methicillin-resistant Staphylococcus aureus: is it really new?

Objective: To review the epidemiologic and molecular characteristics of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in Detroit, Michigan, to assess the risk factors for infection and the response to therapy.

Design: Prospective clinical and laboratory study of 2003-2004 CA-MRSA isolates. Molecular features were compared with CA-MRSA isolates from 1980.

Setting: A 600-bed urban academic medical center.

Patients: Twenty-three patients with CA-MRSA infections from 2003-2004 were evaluated. In addition, laboratory analysis was performed on 13 CA-MRSA isolates from 1980.

Main Outcome Measures: Laboratory analysis of isolates included antimicrobial susceptibility testing, pulsed-field genotyping, testing for Panton-Valentine leukocidin (PVL) genes, and staphylococcal cassette chromosome mec typing.

Results: Patients were predominantly young African American males and presented with skin and soft-tissue infections. All isolates were resistant to erythromycin and highly susceptible to other agents. Patients were generally treated successfully with combination incision and drainage and systemic antibiotics. Among the 23 isolates, 20 (87%) were the same strain. This strain carried the staphylococcal cassette chromosome mec type IV and PVL genes and is genetically identical to USA 300. Thirteen isolates of patients from our community who presented with CA-MRSA infections in 1980 represented a single clone that is unique compared with the 2003-2004 isolates. This strain carried staphylococcal cassette chromosome mec type IVA but did not carry the PVL genes.

Conclusions: In our community, CA-MRSA is largely due to a single clone with a type IV mec gene and PVL gene. The type IV staphylococcal cassette chromosome mec type can be demonstrated in CA-MRSA isolates from a remote period, suggesting that earlier outbreaks were not related to healthcare exposure.