Evaluation of Four Selective Agars and Two Enrichment Broths in Screening for Methicillin-Resistant Staphylococcus aureus

Abstract

To evaluate methicillin-resistant Staphylococcus aureus detection, we tested in vitro four selective agars and two enrichment broths apart and in combination. Tryptone soya broth with salt, aztreonam, and cefoxitin appeared to be the most sensitive medium. This broth was superior to a phenol red mannitol broth with aztreonam and ceftizoxime.Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of morbidity and mortality worldwide. Detection of MRSA in screening samples is an important part of the control of MRSA, but due to annex flora, the low level of MRSA often found in screening samples, and the emergence of heteroresistant MRSA strains (4,8,9,12), this is laborious and often difficult. Further cocolonization with other staphylococcal species might lead to false-positive results, especially when using PCR-based methods (1). The use of an enrichment broth for MRSA detection is known to increase sensitivity but requires an extra day of incubation (13,14,16). Several brands of chromogenic agars supplemented with antibiotics have recently become available and have performed well by direct inoculation and 24-h incubation (2,13,14). However, studies that compare chromogenic agars show conflicting results (2). Most of the published evaluations were performed on samples with the traditional "health care-associated" MRSA strains and with no information on the clonal diversity of the investigated isolates. Clonal differences are known to be one of the reasons for discrepancies when MRSA selective media are studied (7).We evaluated the performance in vitro of four different selective agars and two enrichment broths, separately and in combination, using low inocula of MRSA isolates.Ninety-six well-characterized MRSA strains, representing 13 clonal complexes and all six main SCCmec types (6), including low-level-resistant isolates; 52 methicillin-sensitive Staphylococcus aureus (MSSA) isolates; and 49 methicillin-resistant coagulase-negative staphylococcus (MRCNS) isolates were tested. The agars tested were ChromID MRSA (bioMérieux, Marcy l'Etoile, France); MRSA Select (Bio-Rad, Hercules, CA); a chromagar plate manufactured at Statens Serum Institut (SSI) on license from CHROMagar, Paris, France (MRSA SSI); a mannitol salt agar with 4 mg/liter cefoxitin (MSA); and a 5% blood agar (BA). Two enrichment broths were tested, (i) phenol red mannitol broth with 0.5% salt, 75 mg/liter aztreonam, and 5 mg/liter ceftizoxime (PHMB) (16) and (ii) tryptone soya broth with 2.5% salt, 20 mg/liter aztreonam, and 3.5 mg/liter cefoxitin (TSB) from SSI. The concentrations included in TSB were based on pilot investigations (data not shown). These tests showed that 3 mg/liter cefoxitin allowed breakthrough growth of MSSA and that 4 mg/liter cefoxitin and Ͼ20 mg/liter aztreonam resulted in loss of sensitivity. Approximately 20 CFU (20 l of 10 3 CFU/ml vortexed suspension in 0.9% NaCl) of each strain taken from the same suspension, made from fresh overnight-incubated isolates, were inoculated directly onto agar plates and into th...

Performance of a New Chromogenic Medium, BBL CHROMagar MRSA II (BD), for Detection of Methicillin-Resistant Staphylococcus aureus in Screening Samples

“…We did not study the specific effects of enrichment, as no direct inoculation was performed. In a previous study, Böcher et al (2) showed no increase in yield after selective enrichment, whereas other studies showed that an enrichment step in combination with chromogenic agars led to increased sensitivities of 14 to 26% (11) and 12% (16), respectively.…”

Comparison of MRSA Select Agar, CHROMagar Methicillin-Resistant Staphylococcus aureus (MRSA) Medium, and Xpert MRSA PCR for Detection of MRSA in Nares: Diagnostic Accuracy for Surveillance Samples with Various Bacterial Densities

“…Because of their resistance to antimicrobials in the agar, MRSA colonies grow and subsequently produce distinct color changes caused by the cleavage of a chromogenic substrate by a specific enzyme of S. aureus. Results can be achieved in as little as 18 to 24 h of incubation, depending on the agar used (1,4,5,15,27 (2,9,16,21); however, enrichment delays results and adds to workload and costs. Thus, a discussion of the advantages and limitations of each methodological approach become critical.…”

Better Detection of Staphylococcus aureus Nasal Carriage by Use of Nylon Flocked Swabs

“…In contrast to these results, a recent study from De Silva et al (8) did not find any difference in the determination of S. aureus nasal carriage when comparing flocked swabs to rayon swabs; however, those authors used a different microbiological procedure than ours, consisting of an overnight preenrichment broth step, followed by plating on chromogenic agar. The latter process is known to improve the sensitivity of bacterial detection (3,10,13,15), but it is timeconsuming since the results are delayed by 1 day, which may be a determinant for the detection of nasal carriers when decontamination treatment must be applied to prevent S. aureus infection, for instance, just before a surgical procedure (4) or the arrival of the patient in an intensive care unit. In addition, De Silva et al provided no data relative to the amount of bacteria recovered by each type of swab (8).…”