ABSTRACT
BACKGROUND: It is now recognized that screening for methicillin-resistant Staphylococcus aureus (MRSA) in hospital is an effective infection control measure, and selective media-based methods have been commonly used. MRSASelect (MRSAS; Bio-Rad, Hercules, CA, USA) is MRSA selective agar incorporating chromogenic enzymatic substrates, and have been found to be more sensitive and specific than other selective media. The aim of present study was to evaluate MRSAS for discrimination of MRSA from other staphylococci by comparison with mannitol-salt agar with oxacillin (MSO) which is widely used as a MRSA selective medium. METHODS: Ninety-eight staphylococcal strains which were isolated from blood culture specimen, representing 16 MRSA, 6 methicillin-susceptible S. aureus, 59 methicillin-resistant coagulase- negative staphylococci (MRCNS), and 17 methicillin-susceptible coagulase-negative staphylococci were tested. The isolated colonies from pure culture were directly inoculated onto MSO and MRSAS respectively. On MRSAS any growth appearing pink after 24 hours incubation, and on MSO any growth appearing yellow after 48 hours incubation was interpreted as positive for the presence of MRSA. RESULTS: Sensitivities of MRSAS and MSO for MRSA detection were equal (93.8%). Specificities for MRSA discrimination from other staphylococci were 98.8% and 89.0%, and especially from MRCNS were 100% and 84.7%, for MRSAS and MSO, respectively. CONCLUSIONS: The MRSAS showed equal sensitivity compared with MSO for the detection of MRSA. MRSAS showed higher specificity than MSO in discrimination MRSA from MRCNS. It was suggested that the implementation of MRSAS in MRSA screening could decrease the work needed for MRCNS identification.
Subject(s)
Agar , Discrimination, Psychological , Infection Control , Mass Screening , Methicillin Resistance , Methicillin-Resistant Staphylococcus aureus , Oxacillin , Sensitivity and SpecificityABSTRACT
BACKGROUND: A rapid and sensitive surveillance culture has a pivotal role in infection control of methicillinresistant Staphylococcus aureus (MRSA). This study was aimed to compare the performance of MRSASelect (Bio-Rad, France) to that of mannitol salt agar containing 6 microgram/mL of oxacillin (MSA-OX) for detecting MRSA in surveillance cultures. METHOD: From May to June 2006, 86 nasal swabs and 21 sputum specimens were enrolled. All specimens were inoculated onto MRSASelect and MSA-OX, which were incubated for 2 days and 3 days, respectively, and colonies were read daily by a technologist. Pink colonies on MRSASelect and yellow colonies on MSA-OX were examined with Gram stain, Pastorex(R) Staph-plus (Bio-Rad) and mecA-PCR. After the final reading, both media were re-examined by a superviser. RESULTS: Of the 107 specimens cultured, 32 (29.9%) were positive for MRSA. Of these, 27 were detected by both media, one by MSA-OX only, and 4 by re-examination. The day-1 and day-2 sensitivities/specificities of MRSASelect were 78.1%/97.3% and 84.4%/97.3%, respectively, while those of MSA-OX were 53.1%/100% and 78.1%/92.1%, respectively. With MRSASelect, two more positives were detected at day 2, but their incubation was less than 18 hour at day 1. There were six false positive organisms detected: three Enterobacter spp., one Acinectobacter spp., and two coagulase-negative staphylococci (CNS). But, the two CNS grew on MSA-OX only. CONCLUSION: MRSASelect with 1-day incubation showed a sensitivity equivalent to and a specificity better than MSA-OX with 2-day incubation. MRSASelect should be a useful medium for MRSA surveillance when it is read after an incubation of 18-28 hours with the confirmatory Gram stain of screen-positives.