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Sensitivity and specificity of various beta-lactam antibiotics and phenotypical methods for detection of TEM, SHV and CTX-M extended-spectrum beta-lactamases

The aim of this study was to compare sensitivity and specificity of six different β lactam antibiotics in five phenotypic tests for detection of ESBLs based on synergism of β -lactam antibiotics and clavulanate. Experiments were performed on the set of 80 Klebsiella pneumoniae strains and 105 Escherichia coli strains with previously characterized ESBLs (SHV, TEM and CTX-M). ESBLs were detected by five different phenotypic methods: MIC (minimum inhibitory concentration) determination of β -lactam antibiotics with and without clavulanate, double-disk synergy test (DDST), inhibitor-potentiated disk-diffusion test (IPDDT), CLSI-Clinical and Laboratory Standard Institution (former NCCLS) combined-disk-test, and modified MAST-disk-diffusion test (MAST-DD-test). Seven antibiotics were tested as indicators of ESBL production: ceftazidime, cefotaxime, ceftriaxone, aztreonam, ceftibuten, cefpodoxime and cefepime. Ceftazidime and aztreonam were the best indicators for SHV-5, SHV-12 and TEM β -lactamases whereas cefotaxime and ceftriaxone were the most sensitive in detection of SHV-2 and CTX-M β -lactamases in DDST, IPDDT and CLSI test. MIC determination of β -lactam antibiotics with and without clavulanate was the most sensitive method. DDST was the least sensitive test. Double-disk synergy test, which is the most frequently used test for detection of ESBLs in routine laboratories, was the least sensitive independently of the indicator antibiotic. Since MIC determination is very laborious and time consuming method, we would recommend NCCLS combined disk test or IPDD test for detection of ESBLs in routine laboratories with 5 mm zone augmentation breakpoint.

extended-spectrum beta-lactamases; sensitivity; specificity; detection

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