Comparative evaluation of a prototype chromogenic medium (ChromID CARBA) for detecting carbapenemase-producing Enterobacteriaceae in surveillance rectal swabs

Abstract

Carbapenemase-producing Enterobacteriaceae (CPE) are an increasing problem worldwide, and rectal swab surveillance is recommended as a component of infection control programs. The performance of a prototype chromogenic medium (chromID CARBA) was evaluated and compared with media tested by four other screening methods: (i) overnight selective enrichment in 5 ml tryptic soy broth with a 10-μg ertapenem disk followed by plating onto MacConkey agar (CDC-TS), (ii) short selective enrichment in 9 ml brain heart infusion broth with a 10-μg ertapenem disk followed by plating onto chromID ESBL medium (ESBL-BH), (iii) direct plating onto chromID ESBL, and (iv) direct plating onto MacConkey agar supplemented with meropenem (1 μg/ml) (MCM). The screening methods were applied to detect CPE in 200 rectal swab specimens taken from different hospitalized patients. Identification and antimicrobial susceptibility were performed by the Vitek 2 system. Carbapenem MICs were checked by Etest. Carbapenemase production was confirmed using the modified Hodge test, combined-disk tests, and PCR assays. In total, 133 presumptive CPE strains were detected. Phenotypic and genotypic assays confirmed 92 strains to be CPE (56 KPC-positive Klebsiella pneumoniae, 29 VIM-positive K. pneumoniae, and 7 KPC-positive Enterobacter aerogenes strains) recovered from 73 patients, while the remaining 41 strains were confirmed to be CPE negative (19 ESBL producers and 22 nonfermenters). chromID CARBA, ESBL-BH, and chromID ESBL exhibited the highest sensitivity (92.4%), followed by CDC-TS and MCM (89.1%) (P = 0.631). The specificity was greater for chromID CARBA (96.9%) and ESBL-BH (93.2%) than for CDC-TS (86.4%), MCM (85.2%), and chromID ESBL (84.7%) (P = 0.014). In conclusion, chromID CARBA was found to be a rapid and accurate culture screening method for active CPE surveillance.

Fig 1

Recovery of 92 CPE strains from 73 positive rectal swab specimens and analysis of discrepant results. CDC-TS, inoculation of swabs in tryptic soy broth with a 10-μg ertapenem disk followed by plating to MacConkey agar (100 μl of the selective tryptic soy broth after overnight enrichment); ESBL-BH, inoculation of swabs in brain heart infusion broth with a 10-μg ertapenem disk followed by plating to a chromID ESBL plate (100 μl of the selective brain heart infusion broth after 4 to 6 h of enrichment); MCM, supplementation of MacConkey agar plates with meropenem (1 mg/liter); KpKPC, K. pneumoniae KPC positive; KpVIM, K. pneumoniae VIM positive; EaKPC, E. aerogenes KPC positive.