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J. Clin. Microbiol. doi:10.1128/JCM.02221-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Evaluation of a New Selective Chromogenic Agar Medium for detection of Extended-spectrum -lactamase-producing Enterobacteriaceae

Laboratoire de Bactériologie, Cliniques Universitaires UCL de Mont-Godinne, B-5530 Yvoir, Belgique

^* To whom correspondence should be addressed. Email: youri.glupczynski{at}skynet. be.

	Abstract

A novel chromogenic agar medium (ESBL-Bx bioMérieux, Marcy L'Etoile, France) was compared to MacConkey agar supplemented with 2 mg ceftazidime/liter (MCKC) for the selective isolation and presumptive identification of extended-spectrum -lactamase (ESBL)-producing Enterobacteriaceae directly from clinical samples. Of a total of 644 clinical specimens (including 551 fecal samples), 496 yielded no growth and 148 yielded growth on one or both media. Overall, 44 ESBL-producing Enterobacteriaceae strains [Escherichia coli (n=17), Enterobacter aerogenes (n=17), Klebsiella spp. (n=5), Citrobacter freundii (n=5)] were isolated from 37 specimens by a combination of both methods after 18 to 24 h of incubation. The sensitivities were 97.7% and 84.1% for ESBL-Bx and MCKC, respectively with 43 ESBL-positive strains isolated as colored colonies from 36 specimens on ESBL-Bx versus 37 ESBL-positive organisms from 32 specimens on MCKC. The specificities by specimens were 89 and 91% for ESBL-Bx and MCKC, respectively. On any one of the two media, natural AmpC hyperproducing Enterobacter spp. (n=25) and Citrobacter spp. (n=14) were the most common false-positives as well as non-ESBL-producing Klebsiella oxytoca (n=18) on ESBL-Bx and Morganella morganii (n=10) on MCKC.

We conclude that ESBL-Bx is a sensitive and specific medium for the isolation of ESBL-producing Enterobacteriaceae from clinical samples. The main advantages of ESBL-Bx over MCKC reside in its chromogenic character and its sensitivity and selectivity which enabled the recovery and presumptive identification of most ESBL-producing Enterobacteriaceae within 24 h and reduced by 27% the need for unnecessary identification and confirmation of ESBL testing when disregarding all colorless colonies growing on this medium.

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