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

Abstract

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 <or=20 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.

Correlation of phenotypic tests with the presence of the blaZ gene for detection of beta-lactamase

“…The breakpoint recommended by the CLSI to classify penicillin-resistant isolates by the disk diffusion method showed a sensitivity of 72.2%, a value higher than the 57.1% reported by Kaase et al 15 In this study, the sensitivity of the nitrocefin test (28.9%) was lower than that obtained by these authors (39.3%) and by others: 82.0%, 33 86.2%, 34 95.6%, 23 70.8–97.9% depending on the induction method and agar used, 24 and 62.1–100% depending on the manufacturer 26 . The large difference in the sensitivity of the nitrocefin test between the present study and the studies cited may be related to the fact that we mainly evaluated S. saprophyticus .…”

Comparison of phenotypic methods for penicillinase detection in Staphylococcus aureus

“…In contrast to previous reports, an unacceptably low sensitivity of 39.3% was found for nitrocefin tests, even after induction by penicillin and incubation for 60 min. Sensitivities reported previously for nitrocefin tests were 82% [6], 86.2% [7], 95.6% [8], 70.8–97.9%, depending on the induction method and agar used [4], and 62.1–100%, depending on the manufacturer of the nitrocefin assay [9]. In addition, a sensitivity of 100% has been reported previously for the starch–iodine plate method following penicillinase induction [4].…”

Molecular Detection of Enterotoxin Genes of Multiresistant Staphylococcus aureus Isolates from Different Sources of Food

“…Food is the main source for the transfer of antibiotic resistance, possibly through the ingestion of resistant food-borne strains [30,31]. The inhibition zone of mecillinam can be considered as an indicator for the critical qualification of producer/non-producer β lactamase S.aureus [32]. All the sensitive isolates within the mecillinam inhibition zone of less than 17mm could be considered as β lactamase producer S.aureus isolates.…”