The influence of salt concentration on the detection of methicillin resistance in coagulase-negative staphylococci

The detection of methicillin resistance by the breakpoint method was examined using three different media containing varying quantities of added salt and 4 mg/L methicillin or 1 mg/L oxacillin. Three hundred clinical isolates of eight species of coagulase-negative staphylococci were tested. In 68 strains methicillin resistance was expressed only at certain salt concentrations and four distinct susceptibility phenotypes were observed. A correlation between the susceptibility phenotype and the species of the isolate was found. Testing on Columbia agar (CA) containing 4 mg/L methicillin with 0% and 4% added salt was required to detect resistance in all 68 strains. Resistance was detected less frequently using Balanced Sensitivity Test (BST) agar or Diagnostic Sensitivity Test (DST) agar containing methicillin or CA, BST or DST agar containing oxacillin. Increased production of beta-lactamase was shown to be an unlikely cause of MR in these strains. Disc sensitivity tests were performed on the 68 strains using five different media. Columbia agar gave optimum results as the other media gave enhanced zones of inhibition for some isolates. Further tests were performed on CA containing varying salt concentrations using both oxacillin and methicillin discs. A close relationship between the staphylococcal species, and the influence of increasing salt concentration on zone size was found. Discrepancies were noted between results obtained by breakpoint and the results obtained with methicillin discs particularly with Staphylococcus simulans and some Staphylococcus epidermidis strains. Results obtained with oxacillin discs more closely correlated with those obtained by breakpoint, but only when disc tests were performed on media with low and high salt content. To identify methicillin resistance in strains of CNS by disc tests, the use of Columbia agar with 0% and 5% added salt and oxacillin discs is recommended.