engleski

Sensitivity and specificity of various beta-lactam antibiotics in detection of SHV, TEM and CTX-M beta-lactamases

Background and aim: Resistance to expanded-spectrum cephalosporins through the acquisition and expression of extended-spectrum β -lactamases (ESBLs) is increasing. The clinical implications of ESBLs are extremely serious, and sensitive diagnostic methods are urgently needed to guide therapy, monitor resistance development and implement intervention strategies. 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. Materials and methods: Experiments were performed on the set of 80 Klebsiella pneumoniae strains and 99 Escherichia coli strains with previously characterized ESBLs (SHV, TEM and CTX-M). ESBLs were detected by five different phenotypic methods: Minimum inhibitory concentrations (MIC)) determination of β -lactam antibiotics with and without clavulanate, double-disc synergy test (DDST), inhibitor-potentiated disc-diffusion test (IPDD test), NCCLS combined-disc-test, and modified MAST-disc-diffusion test (MAST-DD-test). Six antibiotics were tested as indicators of ESBL production: ceftazidime, cefotaxime, ceftriaxone, aztreonam, ceftibuten and cefepime. Results: Ceftazidime and aztreonam were the best indicators for SHV-5 and SHV-12 β -lactamases whereas cefotaxime and ceftriaxone were the most sensitive in detection of SHV-2 and CTX-M β -lactamases in DDST, IPDD test and NCCLS test. MIC determination of β -lactam antibiotics with and without clavulanate was the most sensitive method. DDST was the least sensitive test. Ceftibuten and cefepime have been shown not to be reliable indicators of ESBL production because they are more stable to hydrolysis by ESBLs than oxymino-cephalosporins and for that reason they did not perform well in all methods tested in this study except of MIC determination. Double-disc 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. Conclusions: Since MIC determination is very laborious and time consuming method, we would recommend NCCLS combined disc test or IPDD test for detection of ESBLs in routine laboratories with 5 mm zone augmentation breakpoint. The results presented here suggest that the categorization of the isolates as ESBL producers or non producers depends on the detection method, on the indicator antibiotic applied to produce synergism with clavulanate and the type of β -lactamase.

Beta-lactam antibiotics; extended-spectrum beta-lactamases

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