Evaluation of a Latex Agglutination Test (MRSA-Screen) for Detection of Oxacillin Resistance in Coagulase-Negative Staphylococci

Abstract

The MRSA-Screen (Denka-Seiken, Tokyo, Japan) latex agglutination test was evaluated for its ability to detect PBP 2a from 200 clinical isolates of coagulase-negative staphylococci (CoNS; 84 mecA-positive strains and 116 mecA-negative strains) consisting of 108 Staphylococcus epidermidis, 37 S. saprophyticus, 15 S. haemolyticus, 11 S. hominis, 10 S. capitis, 10 S. warneri, and 3 S. lugdunensis species as well as 6 other species of CoNS. The assay was compared with susceptibility testing with an agar screen plate with oxacillin at 6 g/ml (OXA6), by oxacillin disk diffusion (DD), by broth microdilution (BMDIL), by the E test, and with Vitek GPS-SV and Vitek GPS-107 susceptibility cards. PCR for the detection of the mecA gene was used as the "gold standard." The sensitivities and specificities for the methods evaluated were as follows: MRSA-Screen, 100 and 100%, respectively; OXA6, 100 and 99%, respectively; DD, 98 and 62%, respectively; BMDIL, 100 and 60%, respectively; E test, 100 and 51%, respectively; Vitek GPS-SV susceptibility card, 98 and 87%, respectively; and Vitek GPS-107 susceptibility card, 100 and 61%, respectively. The MRSA-Screen test accurately and rapidly detected oxacillin resistance in CoNS.Coagulase-negative staphylococci (CoNS) are recognized as important causes of nosocomial infection, especially in neonates, those who are immunocompromised, and patients with indwelling prosthetic devices (6). The rate of resistance to oxacillin among CoNS has been increasing (2), and the empirical treatment of choice for infections caused by these organisms is often vancomycin. The rapid and accurate identification of oxacillin resistance is essential in order to determine the most appropriate antimicrobial therapy. The major mechanism of oxacillin resistance in staphylococci is mediated by the production of a modified penicillin-binding protein, PBP 2a, specified by the mecA gene. In 1999, the National Committee for Clinical Laboratory Standards (NCCLS) lowered the breakpoint for determination of oxacillin resistance in CoNS from Ն4 to Ն0.5 g/ml (12) in order to more accurately reflect mecA-mediated resistance. In addition, the use of the oxacillin agar screen plate (Mueller-Hinton agar with 6 g of oxacillin per ml supplemented with 4% NaCl) was no longer recommended by NCCLS for the detection of oxacillin resistance in CoNS, as reports in the literature had indicated that this method may not be able to detect mecA-mediated oxacillin resistance in these strains (15,17).A rapid latex agglutination test, the MRSA-Screen (DenkaSeiken, Tokyo, Japan), has recently been developed in order to detect the presence of oxacillin resistance mediated by PBP 2a. Although several studies have evaluated the use of this test for the detection of oxacillin resistance in Staphylococcus aureus (8,18,19), its accuracy for the detection of oxacillin resistance in CoNS has not been extensively assessed (5, 9). In the study described here, we evaluated the MRSA-Screen for its ability to detect PBP 2a in clinical strains of CoNS. In ...

Resistance to methicillin of coagulase-negative staphylococci (CNS) isolated from bovine mastitis

“…Moreover, in all isolates with the mecA gene, the presence of its PBP2a product was detected. This finding correlates with the data presented by Louie et al (2001); in their study, all isolates with genetically determined methicillin resistance showed agglutination indicating the presence of PBP2a (84 mecA-positive/84 PBP2a-positive isolates). Likewise, Miller et al (2005) demonstrated 100% agreement between the presence of the mecA gene and the modified protein of low affinity to penicillin.…”

Identification and detection of methicillin resistance in Non-Epidermidis coagulase-negative staphylococci

“…We only found major errors (falseresistance) with the oxacillin disk in 2.5% of the isolates, including S. sciuri, S. saprophyticus and S. cohnii-cohnii. Similar discrepancies between phenotypic and genotypic results have been described for S. saprophyticus, S. cohnii, S. warneri, S. capitis, S. lugdunensis and S. xylosus [32].…”

“…Total concordance between the latex agglutination test for PBP2a and mecA gene detection by PCR has been observed in isolates that showed false resistance with oxacillin disk diffusion or dilution. The PBP2a test was performed with and without induction, and identical results were observed with the different methods [32,36]. When we analyzed the false positive results obtained with agar dilution (Table 2), we found that eight isolates gave MICs of 0.5 µg/mL (low level of resistance or borderline resistance).…”

“…This can be explained by the fact that cefoxitin has a strong ability to induce PBP2a and also has a high affinity for PBP4, a protein involved in resistance in Staphylococcus spp. These organisms, although less frequent in the laboratory routine, could be more affected by the lower specificity of the oxacillin disk diffusion test and/or the oxacillin agar dilution test [32,34]. According to NCCLS 2001, the disk diffusion test with oxacillin is not recommended for S. saprophyticus, since most isolates that are mecA gene negative can express resistance based on phenotypic methods [35].…”

Methods for Detection of Oxacillin Resistance among Coagulase-Negative Staphylococci Recovered from Patients with Bloodstream Infections at the University Hospital in Brazil

“…However, several studies reported that this technique is sensitive and can be used as a confirmatory test for the results of disk diffusion (Hussain et al, 1998;Cavassini et al, 1999;Louie et al, 2001). Ferreira et al (2003) found that the screening test with 4 µg/ml oxacillin showed 100% sensitivity and 100% specificity versus 94.2% sensitivity and 100% specificity when a concentration of 6 µg/ml had been used, indicating the former as a screening method for CoNS.…”