Etest as a Method of Detecting Extended-Spectrum beta-Lactamase

Shin, Haeng Seop; Ro, Dong Woo; Cho, Dong Taek

Abstract

BACKGROUND: Detection of extended-spectrum beta-lactamase (ESBL) expression is difficult in ordinary clinical laboratories. The Etest has been introduced into clinical settings for the rapid identification of ESBL. The principle behind the Etest is to compare the minimal inhibitory concentration (MIC) of ceftazidime alone with the MIC of ceftazidime with clavulanic acid. The aim of this study was to evaluate the efficiency of the Etest for the detection of ESBL in Korea, where antimicrobial resistance rates are high.
METHODS
The double disk synergy test and the Etest were performed simultaneously. The results of the clinical isolates were compared to those of strains producing TEM-1, TEM-2, and SHV-1 as negative controls. The results of the double disk synergy test and the E-test were confirmed by isoelectric focusing of beta-lactamase extracted from suspicious ESBL-producing strains.
RESULTS
MIC determination using the standard agar dilution method according to the National Committee for Clinical Laboratory Standards revealed that a total of 48 strains were resistant or intermediate against one or more antibiotics of the third generation cephalosporins. These strains included five strains of E. coli, 14 of S. marcescens, seven of K. pneumoniae, 18 of Enterobacter spp., and four of Citrobacter spp. Sixteen (33%) of the strains, including five strains of E. coli, three of S. marcescens, five of K. pneumoniae, and three of Enterobacter spp. were ESBL- producing strains that were confirmed by double disk synergy test. Thirteen (81%) of the strains of ESBL- producing organisms were detected by Etest, but the remaining three strains (19%) were undetectable by Etest alone.
CONCLUSION
The accuracy of Etest for the detection of ESBL was not high, but the efficiency of Etest as the primary screening method of a large number of clinical isolates was appreciable regarding efficiency and rapidity.