Use of Mastalex to detect methicillin resistance in coagulase-negative staphylococci

Extract

Sir,

The British Society for Antimicrobial Chemotherapy (BSAC) has provisionally recommended a method to detect methicillin resistance in coagulase-negative staphylococci (CNS) that requires incubation of isolates for 48 h.¹ A more rapid susceptibility test may reduce dependence on glycopeptides for CNS infection allowing the use of β-lactams with their better pharmacokinetic properties and lower associated costs and toxicity.

Methicillin resistance in staphylococci is due to the presence of the penicillin binding protein 2′ (PBP2′) which is encoded by the mecA gene. Amplification of mecA by PCR enables rapid detection of the resistant genotype. This technique, although sensitive, is not readily accessible to most microbiology laboratories and it would be advantageous to use a commercially available slide agglutination kit. Detection of PBP2′ production by Staphylococcus aureus using a latex agglutination kit allows determination of methicillin resistance, but this test is not validated for CNS. Andrews et al.² have recently compared MIC data from 200 strains of CNS with Mastalex (Mast Diagnostics, Bootle, UK) a rapid latex test for PBP2′ or by PCR when discordant results were obtained. They found only two isolates in which there was disagreement between the latex and PCR results and in both cases the latex test was positive and PCR was negative. We decided to evaluate this latex agglutination test on a group of CNS with known sensitivities and mecA PCR results in order to determine the usefulness of this test in clinical practice.