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Extended-spectrum b-lactamases screening agar with AmpC inhibition

Abstract number: P871

al Naiemi N., Murk J.L., Savelkoul P.H.M., Vandenbroucke-Grauls C.M., Debets-Ossenkopp Y.J.

Objectives: The serious increase in the prevalence of Extended-Spectrum Beta-Lactamases (ESBLs) worldwide creates a need for effective screening methods to detect these resistance genes. The currently used screening agars have low specificity, mainly due to growth of isolates of species with inducible AmpC b-lactamases. In this study we evaluated an ESBL Screening Agar (ESA) which inhibits growth of AmpC-producing isolates and Enterococci, and compared it with the commercially available BLSE agar (AES Laboratory, France) for selective isolation and presumptive identification of ESBL-producing Enterobacteriaceae.

Methods: The ESA consists of two MacConkey Agars containing either ceftazidime 1.0 mg/l or cefotaxime 1.0 mg/l + cloxacilline 400 mg/l + vancomycine 64 mg/l. The BLSE agar is a commercial double-plate agar (MacConkey + ceftazidime 2 mg/l and Drigalski + cefotaxime 1.5 mg/l). The agars were evaluated with 208 Enterobacteriaceae isolates, 70 of them had been previously genotypically characterised as ESBL-producers. The other 138 isolates were ESBL negative. The ESA was further evaluated with 100 clinical specimens. All clinical specimens were futher characterised for ESBL production with ESBL combined disc test and ESBL Etest.

Results: The sensitivity and specificity of the ESA and the BLSE agar tested with the 208 Enterobacteriaceae isolates were 100% (70/70) and 84.7% (117/138), and 100% (70/70) and 57.2% (79/138) respectively. Isolates of species with inducible AmpC were most commonly the false positives on BLSE agar. The ESA detected all 5 ESBL-positive clinical specimens correctly, 4 specimens were false positive. The sensitivity and specificity of the ESA in the clinical specimens in this study were 100% and 95.7% respectively.

Conclusion: The specificity of ESA for screening of ESBL-producing strains was significantly better than the specificity of BLSE agar; the better performance of the ESA was mainly due to less false positive results due to Amp-C-producing strains. The ESA performed well also when inoculated directly with clinical specimens. We conclude that ESA is a sensitive and convenient method to directly screen for ESBL-producing organisms in clinical specimens.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Subject:
Location: Barcelona, Spain
Presentation type:
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