Mecillinam susceptibility as an indicator of β-lactamase production in Staphylococcus aureus

The use of direct susceptibility testing from specimens has led to the fortuitous observation that penicillin-susceptible strains have larger inhibition zones for mecillinam than do beta-lactamase producers. The current study was, therefore, undertaken to test 179 Staphylococcus aureus isolates for mecillinam susceptibility by Rosco Neo-Sensitabs and to compare the results with commonly used tests for beta -lactamase production, i.e. size and character of penicillin inhibition zones, chromogenic cephalosporin (nitrocefin) tests and clover leaf assays. Agreement between methods was reached for 175 of 179 strains when disregarding the results of the nitrocefin tests, 88 isolates being found susceptible and 87 being found to be beta-lactamase producers. All 88 susceptible isolates had mecillinam zones of >22 mm, with the great majority being >25 mm; double zones did, however, occur. The 87 beta-lactamase producers had zones <14 mm or no zones. Four isolates presenting problems in had mecillinam zones of less than or equal to20 mm and were without tapering growth at the penicillin inhibition zone edge. In conclusion, the size of the mecillinam inhibition zone is found to be a useful supplementary test in the clinically important distinction between beta-lactamase-producing and non-producing isolates of S. aureus.