Our infectious diseases physician asked for information on the incidence of community-associated MRSA (CA-MRSA) isolates encountered in our laboratory. Can CA-MRSA be identified by antimicrobial susceptibility profiles? Are there other ways to determine if an MRSA isolate is CA-MRSA versus a healthcare-associated MRSA (HA-MRSA)?

Answer

The answer to both questions is no. CA-MRSA cannot be distinguished from HA-MRSA by antibiograms alone. Most strains of CA-MRSA are resistant to erythromycin and beta-lactams but susceptible to clindamycin (although inducible resistance may be present), aminoglycosides, fluoroquinolones, rifampin, tetracyclines, and trimethoprim-sulfamethoxazole, whioe HA-MRSA are typically multi-resistant. However, CA-MRSA isolates in the San Francisco area are often resistant to fluoroquinolones and tetracycline in addition to erythromycin. Suggestions for identifying CA-MRSA can be found at http://www.cdc.gov/ncidod/hip/ARESIST/mrsa.htm. Criteria for categorizing an isolate as a CA-MRSA strain are primarily based on patient history rather than characteristics of the organism, such as no medical history of MRSA and no hospitalization or exposure to a healthcare facility within the previous year.

Important points to remember about CA-MRSA from a laboratory perspective are:

1. CA-MRSA are typically not multiply resistant.

2. CA-MRSA often require a clindamycin induction test since they are resistant to erythromycin but susceptible to clindamycin. A clindamycin susceptible (S) result should not be automatically changed to resistant (R) in erythromycin-R, clindamycin-S staphylococci.

3. Commercial methods for detecting oxacillin resistance have been reliable for most CA-MRSA examined to date. However, there have been rare reports of CA-MRSA strains from certain geographic areas that demonstrate oxacillin-susceptible results by commercial MIC methods and by the NCCLS broth microdilution and agar dilution reference methods. These strains are oxacillin resistant by disk diffusion, oxacillin salt agar screen plate (6 µg/ml oxacillin + 4% salt), and PBP2a assay. Many CA-MRSA produce toxins, such as Panton -Valentine Leukocidin (PVL), that contribute to the organism’s virulence.

4. Some public health departments are soliciting information on CA-MRSA isolates. Make certain that you are aware of any requirements to report such isolates in your area.

5. CA-MRSA can spread from infected skin to healthy skin and from contaminated objects to healthy skin. Follow your laboratory’s hand washing and other safety precautions!