Correlation of Oxacillin MIC with mecA Gene Carriage in Coagulase-Negative Staphylococci

Abstract

The National Committee for Clinical Laboratory Standards has recently changed the oxacillin breakpoint from ≥4 mg/liter to ≥0.5 mg/liter to detect methicillin-resistant coagulase-negative staphylococci (CoNS) because the previous breakpoint lacked sensitivity. To determine the correlation between the new oxacillin breakpoint and the presence of themecA gene, 493 CoNS of 11 species were tested. The presence of the mecA gene was determined by PCR, and oxacillin susceptibility was determined by the agar dilution method with Mueller-Hinton agar containing 2% NaCl and oxacillin (0.125 to 4.0 mg/liter). The new breakpoint correctly classified all CoNS strains with mecA as methicillin resistant and strains ofStaphylococcus epidermidis, S. haemolyticus, and S. hominiswithout mecA as methicillin susceptible. The breakpoint of ≥0.5 mg/liter was not specific for S. cohnii, S. lugdunensis, S. saprophyticus, S. warneri, and S. xylosus, in that it categorized 70 of 74 strains of these species withoutmecA (94.6%) as methicillin resistant. The results of this study indicate that the new oxacillin breakpoint accurately identifies strains of CoNS with mecAbut is not specific for strains of certain species of CoNS withoutmecA.

Evaluation of methods for detecting oxacillin resistance in coagulase-negative staphylococci including cefoxitin disc diffusion

“…The combination between these tests improved the sensitivity for detecting oxacillin resistance in CoNS as well as a negative predictive value. As expected, the sensitivity and specificity of the tests used for S. aureus are lower for CoNS strains because standard susceptibility testing is less accurate in determining oxacillin resistance in mecA-positive CoNS than in MRSA (Hussain et al, 2000;Caieirão et al, 2004;Ghoshal et al, 2004).…”

Staphylococcus lugdunensis: clinical spectrum, antibiotic susceptibility, and phenotypic and genotypic patterns of 39 isolates

“…The oxacillin MIC breakpoints for CoNS recommended by NCCLS are ‡ 0.5 mg ⁄ L for resistance and £ 0.25 mg ⁄ L for susceptibility when using the agar dilution method [30]. Hussain et al [30] found that many isolates of S. lugdunensis have oxacillin MICs of 0.5-2 mg ⁄ L (i.e., resistant according to NCCLS guidelines); however, these isolates were mecA-negative. Only one mecApositive isolate of S. lugdunensis has been reported, with an oxacillin MIC of > 256 mg ⁄ L [22].…”

Rapid Detection of mecA-Positive andmecA-Negative Coagulase-Negative Staphylococci by an Anti-Penicillin Binding Protein 2a Slide Latex Agglutination Test

“…The detection of oxacillin resistance in staphylococci is complex and time-consuming because of the heterogeneous nature of mecA gene expression. Furthermore, in some species of CoNS the differentiation between mecA-negative and mecA-positive strains may not be possible by susceptibility methods (11). Alternative methods for rapid and accurate detection of oxacillin-resistant strains are desirable.…”

“…Overnight growth from plates containing Columbia agar supplemented with 5% sheep blood was used to perform a PCR for the nuc and mecA genes. DNA extraction, PCR master mixture concentrations, and amplification conditions were described previously (11). The primers used to detect the nuc and mecA genes were published previously (4,18).…”