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Extended spectrum beta-lactamases (ESBL) belong to the most widely disseminated resistance mechanisms in Enterobacteriacae. According to a common isolation protocol, also recommended by the European Food Safety Authority (EFSA), the isolation starts with an enrichment culture followed by cultivation on MacConkey agar plates containing 1 mg/l cefotaxime. ESBL-production is then confirmed by antimicrobial susceptibility testing with specific panels and PCR of the ESBL-genes.
We were interested in the positive predictive value (PPV) of the cultural isolation on MacConkey agar plates containing 1 mg/l cefotaxime for ESBL-production. Therefore ESBL-producing E. coli samples from broiler, fattening pig and cattle farms were first isolated culturally. Subsequently ESBL suspicious isolates were confirmed by antimicrobial susceptibility testing and PCR of ESBL-genes.
From samples collected on fattening pig farms 121 isolates growing on MacConkey agar plates containing 1 mg/l cefotaxime were obtained. Of these isolates 107 could be confirmed by PCR. It follows a PPV of 87%. In the presentation, the results of PPV for the different sample sources will be reported. The calculation of the sensitivity and specificity from these data and the possible limitations of this method will be explained.