Performance of chromID ESBL, a chromogenic medium for detection of Enterobacteriaceae producing extended-spectrum {beta}-lactamases

1 Service de Bactériologie, Hôpital Cochin, Assistance Publique Hôpitaux de Paris, Faculté de Médecine Paris Descartes, 27 Rue du Faubourg Saint Jacques, 75014 Paris, France 2 Service de Bactériologie-Virologie, INSERM U914: Emerging Resistance to Antibiotics, Hôpital de Bicêtre, Assistance Publique/Hôpitaux de Paris, Faculté de Médecine Paris-Sud, Université Paris XI, 78 Rue du Général Leclerc, 94275 Le Kremlin-Bicêtre, France Correspondence Thierry Naas thierry.naas{at}bct.aphp.fr Received 11 September 2007 Accepted 28 November 2007 The chromogenic agar medium chromID ESBL (bioMérieux) was compared with BLSE agar medium (AES) for selective isolation and presumptive identification of extended-spectrum β -lactamase (ESBL)-producing Enterobacteriaceae from clinical samples. A total of 765 samples (468 rectal swabs, 255 urine samples and 42 pulmonary aspirations) obtained from 547 patients was processed. All bacterial strains isolated on either medium were further characterized using biochemical tests, and ESBL producers were confirmed by synergy testing. Genetic characterization of ESBL genes was determined by PCR. A total of 33 ESBL-producing Enterobacteriaceae strains [ Escherichia coli ( n =16), Klebsiella pneumoniae ( n =8), Enterobacter spp. ( n =3), Citrobacter spp. ( n =5) and Proteus mirabilis ( n =1)] was recovered. The sensitivity after 24 h incubation was 88 % for chromID ESBL and 85 % for BLSE agar. At 48 h, the sensitivity of chromID ESBL increased to 94 % and was higher than that obtained with BLSE agar. The positive predictive value at 24 h for chromID ESBL was 38.7 % [95 % confidence interval (95 % CI) 28.3 –50.2 %)], which was significantly higher than that for BLSE agar [15.4 %, 95 % CI 10.1 –21.5 %]. On both media, false-positive results were mostly due to Pseudomonas aeruginosa and to Enterobacteriaceae overproducing chromosomal cephalosporinase ( Enterobacter spp.) or a chromosomal penicillinase ( Klebsiella oxytoca ). This study showed that chromID ESBL, a ready-to-use chromogenic selective medium, is sensitive and specific for rapid, presumptive identification of ESBL-producing Enterobacteriaceae . Its chromogenic properties and its selectivity are particularly useful in specimens containing resident associated flora. Abbreviations: 95 % CI, 95 % confidence interval; ESBL, extended-spectrum β - lactamase; PPV, positive predictive value.