Evaluation of a new selective chromogenic agar medium for detection of extended-spectrum β-lactamase-producing Enterobacteriaceae

A novel chromogenic agar medium (ESBL-Bx; bioMerieux, Marcy l'Etoile, France) was compared to Mac-Conkey agar supplemented with 2 mg ceftazidime/liter (MCKC) for the selective isolation and presumptive identification of extended-spectrum beta-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], and 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 isolated from 32 specimens on MCKC. The specificities by specimens were 89 and 91% for ESBL-Bx and MCKC, respectively. On either 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.