Comparative Evaluation of a Prototype Chromogenic Medium (ChromID CARBA) for Detecting Carbapenemase-Producing Enterobacteriaceae in Surveillance Rectal Swabs

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-g ertapenem disk followed by plating onto MacConkey agar (CDC-TS), (ii) short selective enrichment in 9 ml brain heart infusion broth with a 10-g 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 g/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%) (P0.631). 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%) (P0.014). In conclusion, chromID CARBA was found to be a rapid and accurate culture screening method for active CPE surveillance.

Carbapenems are used as a last-resort antibiotic class for the treatment of infections due to multidrug-resistant Enterobacteriaceae. However, during the last decade carbapenem resistance has been increasingly reported and carbapenemase-producing Enterobacteriaceae (CPE) are emerging as a growing challenge in health care facilities (22). These isolates produce different types of -lactamases capable of hydrolyzing carbapenems. Among these carbapenemases, metallo–lactamases (MBLs; Ambler class B) and Klebsiella pneumoniae carbapenemase (KPC; Ambler class A) are prevailing in Enterobacteriaceae from countries or large geographic regions, including the United States, Israel, Italy, Greece, the Far East, and South America (9, 12, 14, 15, 17, 18, 22, 26, 27). Moreover, oxacillinase-48 (OXA-48; Ambler class D) has recently been isolated in Enterobacteriaceae from Turkey (6), and it has since been reported from other countries in the Mediterranean Basin and Western Europe (13, 22). Carbapenemase-producing pathogens have been associated with high rates of morbidity and mortality, particularly among critically ill patients with prolonged hospitalization (4, 21, 32, 37). Furthermore, CPE are usually multi-drug resistant pathogens, making them even more worrisome, since the treatment options are very restricted (4). It is also of note that the carbapenemase genes harbored by CPE are mostly transposon- and/or integron-encoded determinants that can easily disseminate to other enterobacterial strains and species (11, 30, 35). These facts suggest the need to implement adequate preventive measures, including active surveillance, in order to contain the spread of these pathogens.

Since gastrointestinal carriers of CPE are thought to be the reservoir of cross-transmission in health care settings, surveillance has been deemed necessary (2, 3, 7, 18). Therefore, collection of rectal swab specimens seems to be the most appropriate sampling method for microbiologic surveillance, which can be accomplished using either culture or molecular techniques. Although direct detection by molecular assays exhibits high sensitivity and has the advantage of rapid identification of CPE (16, 31), these methods are not available for daily use in many laboratories. It should also be noted that their use is limited to the detection of isolates that harbor the target -lactamase gene (31), and the residue of swabs with low inocula may not be sufficient for successful DNA extraction (29). Even more, molecular methods do not give the possibility for further strain typing and susceptibility testing.

Thus, several culture techniques for screening carbapenem-resistant Enterobacteriaceae have been tested, including methods that use in-house-prepared selective media, such as MacConkey agar or tryptic soy broth containing a 10-g carbapenem disk (3, 8, 19, 20), or commercial chromogenic agar media, like CHROMagar KPC (Hy- Labs, Rehovot, Israel) (1, 23, 29) and chromID ESBL medium (bio-Mérieux, Marcy l’Etoile, France) (5). However, these screening methods are designed to detect carbapenem-resistant Enterobacteriaceae and not specifically CPE. chromID CARBA (bioMérieux) is a chromogenic solid medium particularly designed for CPE detection and supplemented with specific agents that inhibit the growth of Grampositive and noncarbapenemase producers (24). The aim of the present study was to evaluate the performance of chromID CARBA and compare it to that of four other culture-based screening methods for CPE detection directly from rectal swabs.