Background: Extended-spectrum β-lactamase (ESBL) producing Enterobacteriaceae are a serious problem worldwide because of their resistance to all β-lactam antibiotics, except carbapenem or cephamycin. To prevent erroneous selection of antibiotics for ESBL producers, the role of clinical microbiology laboratory in accurate detection of this organism is important. Several ESBL detection methods has been proposed, and recently ESBL detection could become possible without additional test using automated microbiology system was developed for used in which detect ESBL by comparing obtained AST results with the pooled data about species-resistance mechanisms. This study was designed to evaluate the abilities of VITEK 2 system (bioMerieux, Marcy l’Etoile, France) and it’s computer program, Advanced Expert System (AES) to detect ESBL-producing Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae).

Methods: A total 54 isolates of ESBL-positive (12 strains of E. coli, 42 strains of K. pneumoniae) and 33 ESBL-negative gram-negative isolates (21 of E. coli, 12 of K. pneumoniae) from hopitalized patients of Hanyang university Kuri hospital were evaluated. ESBL detection was done first by screening and confirmatory disk diffusion method recommended by NCCLS. Next, double disk synergy (DDS) test was performed. And also antimicrobial susceptibility of each isolate was determined with the VITEK 2 system and the results were analyzed with the expert system, AES. 

Results: All the 54 ESBL-positive isolates by the NCCLS confirmatory test, demonstrated ESBL positive by both DDS test and VITEK 2 AES. Also all the 33 ESBL-negatives by the NCCLS confirmatory test, demonstrated ESBL negative by ESBL-negative isolates were confirmed as ESBL negative both DDS test and VITEK 2 AES. The sensitivities of ceftazidime and cefotaxime disks used in disk diffusion confirmatory test recommended by NCCLS were 92.6%, 81.5% respectively. The specificities were 100% in both disks. VITEK 2 AES predicted ESBLs as “ESBL”phenotype of 85.2% and as “ESBL+impermeability”phenotype of 14.8% according to resistance mechanisms. The 6 strains of K. pneumoniae revealing “ESBL+impermeability”phenotype were all resistant to cefoxitin, but 2 strains of E. coli demonstrating “ESBL+impermeability”phenotype was susceptible to cefoxitin.

Conclusion: These results suggest that the VITEK 2 AES can identify ESBL accurately at least for K. pneumoniae and E. coli without additional confirmatory test. Also, AES can predict resistance mechanisms of ESBL, which is difficult to detect by routine AST results. Additional study is needed to reveal the molecular mechanism of the “ESBL+impermeability”phenotype. (Korean J Clin Microbiol 2002;5(1):15-20)