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Third case of VRSA appears in the United States
Lab test methods may miss emerging pathogen
The Centers for Disease Control and Prevention (CDC) reports the third documented clinical isolate of vancomycin-resistant Staphylococcus aureus (VRSA) from a patient in the United States. There are also troubling indications that emerging VRSA may not be detected by commonly used automated antimicrobial susceptibility testing, CDC warns.1
On March 17, a urine culture obtained from a resident of a long-term care facility yielded S. aureus. The isolate was tested for antimicrobial susceptibility by using Microscan overnight panels (Dade Behring, Deerfield, IL); vancomycin MIC was 4 µg/mL. Further testing by Etest (AB Biodisk North America Inc., Piscataway, NJ) indicated that the isolate was resistant to vancomycin (MIC >256 µg/mL).
After notification and subsequent analysis by the New York State Department of Health, the isolate was forwarded to CDC, where it was confirmed to be VRSA (vancomycin MIC = 64 µg/mL, using the National Committee for Clinical Laboratory Standards broth microdilution reference method). The isolate contained both the mecA and vanA genes mediating oxacillin and vancomycin resistance, respectively. The isolate was susceptible to chloramphenicol, linezolid, minocycline, quinupristin-dalfopristin, rifampin, and trimethoprim-sulfamethoxazole.
The patient remains in a long-term care facility, and state health officials are investigating the case. This VRSA isolate appears to be unrelated epidemiologically to the VRSA isolate identified previously in Michigan and Pennsylvania. Although the New York isolate contained the vanA resistance gene, the vancomycin MIC of the isolate appeared low when tested initially by an automated method. Additional testing at CDC indicated that Microscan and Vitek (bioMerieux, Hazelwood, MO) testing panels and cards available in the United States did not detect vancomycin resistance in this VRSA isolate.
Consequently, additional VRSA infections might have occurred but were undetected by laboratories using automated methods.
Potential VRSA isolates should be saved for confirmatory testing, and clinical microbiology laboratories must ensure that they are using susceptibility testing methods that will detect VRSA, CDC recommends.
The most accurate form of vancomycin susceptibility testing for staphylococci is a nonautomated MIC method (e.g., broth microdilution, agar dilution, or agar-gradient diffusion) in which the organisms are incubated for a full 24 hours before reading results. Therefore, when performing automated susceptibility testing of S. aureus strains, particularly methicillin-resistant S. aureus, laboratories should include a vancomycin-agar screening plate containing 6 µg/mL of vancomycin and examine the plate for growth after a 24-hour incubation. Consequently, additional VRSA infections might have occurred but were undetected by laboratories using automated methods, CDC notes.
Use of proper infection-control practices and appropriate antimicrobial agent management can help limit the emergence and spread of antimicrobial-resistant microorganisms, including VRSA. The CDC recommends contact precautions when caring for patients with these infections, including:
1. Centers for Disease Control and Prevention. Brief report: Vancomycin-resistant Staphylococcus aureus — New York, 2004. MMWR 2004; 53(15):322-323.