COMPARISON OF SCREENING METHODS FOR DETECTION OF EXTENDED-SPECTRUM BETA-LACTAMASES AND THEIR PREVALENCE AMONG BLOOD ISOLATES OF ESCHERICHIA-COLI AND KLEBSIELLA SPP IN A BELGIAN TEACHING HOSPITAL

Using a set of 33 well-defined extended-spectrum beta-lactamase (ESBL) -producing strains of Escherichia coli and Klebsiella pneumoniae, we c ompared three screening methods for ESBL detection: (i) a double-disk synergy test, (ii) a three-dimensional test (both the double-disk syne rgy test and the three-dimensional test were performed with ceftriaxon e, ceftazidime, aztreonam, and cefepime), and (iii) the Etest ESBL scr een (AB Biodisk, Solna, Sweden), based on the recognition of a reducti on in the ceftazidime MIC in the presence of clavulanic acid, In the d ouble disk test, all four indicator antibiotics scored equally and 31 of the 33 reference strains were recognized, In the three-dimensional test, ceftriaxone was the only satisfactory indicator and 30 ESBL-posi tive strains were detected by this antibiotic, Both systems produced t wo false-positive results with cefepime, With the Etest ESBL screen, 1 5 of 16 TEM-related and 11 of 16 SHV-related ESBL-producing strains sc ored positive, In 10 cases the clavulanic acid on one end of the strip interfered with the MIC determination for ceftazidime, which was read on the opposite end, This MIC had to be determined with an extra ceft azidime-only strip, No false-positive results were noted, Eighty-six b lood isolates of E, coli and Klebsiella species were screened for ESBL expression by the double-disk and three-dimensional tests, both with ceftriaxone. Six strains with suspicious antibiogram phenotypes also g ave positive results by the double-disk test, One E, coli strain remai ned undetected by the three-dimensional test, Identification of the en zymes suspected of being ESBLs by isoelectric focusing (all strains) a nd DNA sequencing (1 strain) confirmed the screening test results exce pt for one Klebsiella oxytoca strain, which proved to be a hyperproduc er of its chromosomal enzyme and which also had a negative Etest score , The five true ESBL producers were all confirmed by the Etest ESBL sc reen, Pulsed-field gel electrophoresis proved that the, coli strains w ere unrelated, but that two of the three K. pneumoniae strains were cl osely related.