Staphylococci isolated from animals and food with phenotypically reduced susceptibility to β-lactamase-resistant β-lactam antibiotics

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Abstract
The antibiotic resistance pattern of 1921Staphylococcusstrains isolated from animals and food within the last two years were examined using diffusion tests. Among them there were only 35 strains ofS. aureushaving an inhibition zone diameter of 15 mm or less, and 4 strains of coagulase-negative staphylococci (CNS) having a zone diameter of 18 mm or less to 1-µg oxacillin disk. These 39 strains were examined also by E-test to oxacillin and for the detection of themecAgene by PCR in order to determine whether they might be real methicillin-resistant staphylococci. Among the 39 strains there were only two that were susceptible to penicillin by disk diffusion method; however, further examination by the penicillinase test showed that they produced ß-lactamase. While 19 (15S. aureus,4 CNS) strains were resistant and 7 strains were intermediate to oxacillin in disk diffusion test, the E-test gave 8 resistant and 5 intermediate results. Six out of the 8 oxacillin-resistant strains examined by disk diffusion and E-test harboured themecAgene. Thus only 6 out of the examined 1921 strains proved to bemecApositive. These methicillin-resistant,mecA-positive strains (5 of theS. aureusstrains and 1 of theS. epidermidis) originated from two dairy herds. The results prove that methicillin-resistantS. aureus(MRSA) strains in animals are really rare in Hungary. Eighteen strains were chosen and screened for minimal inhibitory concentration (MIC) of oxacillin with or without clavulanic acid or sulbactam, and three of them produced methicillinase enzyme.

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