Evaluation of CHROMagar CTX, a novel medium for isolating CTX-M-ESBL-positive Enterobacteriaceae while inhibiting AmpC-producing strains.

OBJECTIVES: The aim of this study was to evaluate CHROMagar CTX (CHROMagar France), a novel agar for the selective isolation of Enterobacteriaceae expressing the bla(CTX-M) gene in the presence of enteric bacteria expressing AmpC enzymes.
METHODS: A panel of 150 Gram-negative bacteria (mainly Escherichia coli, Enterobacter, Klebsiella, Pseudomonas and Salmonella) isolated from humans and animals were assembled for the purpose of evaluating CHROMagar CTX and comparing it with CHROMagar ECC with the addition of 1, 2, 4 and 8 mg/L cefotaxime or ceftazidime and with bioMérieux extended-spectrum beta-lactamase (ESBL)-Bx agar. CHROMagar CTX was also assessed for its ability to isolate bla(CTX-M) strains from farm animal faeces (n = 342).
RESULTS: The panel contained CTX-M-positive (n = 70) strains (CTX-M types 1, 9, 14 and 15), ESBLs (n = 31) belonging to other families (OXA, PER, SHV, TEM, VEB), strains positive for ampC genes (n = 31), strains that overexpressed ampC (n = 6), non-ESBL/AmpC strains (n = 11) and Klebsiella oxytoca (n = 1). CHROMagar CTX was superior to other agars tested for selective isolation of Enterobacteriaceae expressing the bla(CTX-M) gene with 100% sensitivity and 64.2% specificity for CTX-M strains in the panel and 90.1% of the colonies from animal faeces plated on CHROMagar CTX were CTX-M strains.
CONCLUSIONS: CHROMagar CTX is a valuable agar in situations where it is important to isolate bla(CTX-M) strains in the presence of AmpC strains. The agar may be particularly useful in veterinary studies, where AmpC-producing commensal E. coli can be encountered reasonably frequently in the enteric flora of some animal species and may also be useful, following further evaluation, for samples from humans.