Spectra MRSA, a New Chromogenic Agar Medium To Screen for Methicillin-ResistantStaphylococcus aureus

Abstract

A novel chromogenic medium, Spectra MRSA (Remel, Lenexa, KS), was designed to detect methicillinresistant Staphylococcus aureus (MRSA) rapidly and more efficiently than traditional media (i.e., tryptic soy agar with 5% sheep blood [SBA] and mannitol salt agar [MSA]). A multicenter study (including four clinical trial sites and the Medical College of Wisconsin [MCW]Milwaukee, WI) compared the performance characteristics of Spectra MRSA to those of the traditional media for the detection of MRSA. For this study, 767 nasal swab specimens from the multicenter study (traditional medium used, SBA) and 667 nasal swab specimens from MCW (traditional medium used, MSA) were plated on each test medium and examined after 24 and 48 h of incubation. At 24 h, the sensitivity and the specificity of each medium were as follows: in the multicenter study, 95.4% and 99.7%, respectively, for Spectra MRSA and 93.6% and 100%, respectively, for SBA; at MCW, 95.2% and 99.5%, respectively, for Spectra MRSA and 88.7% and 94.0%, respectively, for MSA. The positive predictive values of each medium at 24 h were as follows: in the multicenter study, 98.1% for Spectra MRSA and 100% for SBA; at MCW, 95.2% for Spectra MRSA and 60.4% for MSA. In our evaluation, we found that Spectra MRSA was able to rapidly identify and differentiate methicillin-resistant S. aureus from methicillinsusceptible S. aureus on the basis of the utilization of chromogens that result in denim blue colonies, thus eliminating the need for biochemical analysis and antimicrobial susceptibility testing. Extending the incubation beyond 24 h did not significantly improve the recovery of MRSA and resulted in decreased specificity.A significant effort has been put forth to determine effective infection control practices that may be used to limit the spread of methicillin-resistant Staphylococcus aureus (MRSA) and minimize its impact on patient care and hospital budgets in response to the increasing rates of occurrence of MRSA in health care settings. Although most hospitals adhere to a policy of contact isolation and attempt to limit inappropriate antimicrobial usage, there is strong evidence that active surveillance cultures (ASCs) for patients at risk for MRSA colonization can increase the chance of identifying occult MRSA reservoirs and further limit the nosocomial spread of the organism (16). Recent reports by Salgado and Farr (17) and Lucet et al. (13) have shown that the organisms in positive cultures of clinical specimens routinely submitted from patients represent only a small fraction of the reservoir of antibiotic-resistant pathogens, and the largest source for nosocomial spread was attributed to asymptomatic, colonized patients who went unrecognized and unisolated in the absence of ASCs. Additionally, consensus suggests that a restricted formulary alone is unlikely to prevent the emergence and persistence of MRSA and that the use of contact precautions is important to prevent the spread of MRSA from colonized patients (5).While the anterior nares are the most common ...

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