Journal of Antimicrobial Chemotherapy, Vol 42, 49-54, Copyright © 1998 by The British Society for Antimicrobial Chemotherapy
PL Ho, KH Chow, KY Yuen, WS Ng and PY Chau
A novel, inhibitor-potentiated disc-diffusion test for detecting
extended-spectrum beta-lactamases (ESBLs) in bacteria was evaluated. This
test uses the principle of augmentation (by > or = 10 mm) of inhibition
zones produced by ceftazidime, cefotaxime, ceftriaxone or aztreonam discs
on Mueller-Hinton agar supplemented with clavulanate (4 mg/L). The test was
initially compared with the double-disc synergy test, Kirby-Bauer
disc-diffusion test and Etest ESBL screen with a panel of 45 reference
strains with known resistance profiles. This panel consisted of 27
ESBL-positive Escherichia coli strains expressing 14 Bush group 2be enzymes
and 18 other E. coli and Klebsiella pneumoniae strains (14 non-ESBL
beta-lactamase producers and four non- beta-lactamase producers). The
Kirby-Bauer disc-diffusion test was the least sensitive method: 11-44% of
the ESBL-positive control strains were misclassified as susceptible to
ceftazidime, cefotaxime, ceftriaxone or aztreonam when interpreted by
National Committee for Clinical Laboratory Standards (NCCLS) criteria. The
sensitivities of the inhibitor-potentiated disc-diffusion test, the
double-disc synergy test (when discs were 25 or 30 mm apart) and the Etest
ESBL screen (with a breakpoint of > 4-fold reduction in ceftazidime MIC
in the presence of clavulanate) were 100%, 96% and 89-96%, respectively.
The inhibitor-potentiated disc-diffusion test was further evaluated with 81
E. coli and K. pneumoniae clinical isolates, which were identified as
putative ESBL-producers by the double-disc synergy test. For these
isolates, the sensitivity of both the inhibitor-potentiated disc- diffusion
test and the Etest ESBL screen was 100%. In conclusion, the
inhibitor-potentiated disc-diffusion test is a sensitive, convenient and
inexpensive method of screening for ESBLs in E. coli and K. pneumoniae
isolates, with potential for incorporation into routine clinical laboratory
service.
ORIGINAL ARTICLES
Comparison of a novel, inhibitor-potentiated disc-diffusion test with other methods for the detection of extended-spectrum beta-lactamases in Escherichia coli and Klebsiella pneumoniae
Department of Microbiology, University of Hong Kong. plho@hkucc.hku.hk
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