Carbapenem Disks on MacConkey Agar in Screening Methods for Detection of Carbapenem-Resistant Gram-Negative Rods in Stools

Abstract

Direct plating of simulated stool specimens on MacConkey agar (MCA) with 10-g ertapenem, meropenem, and imipenem disks allowed the establishment of optimal zone diameters for the screening of carbapenem-resistant Gram-negative rods (CRGNR) of <24 mm (ertapenem), <34 mm (meropenem), and <32 mm (imipenem). Screening of stool specimens is recommended by the Centers for Disease Control and Prevention as well as the Institut national de santé publique du Québec to identify carriers of carbapenem-resistant Gram-negative rods (CRGNR) and initiate appropriate infection control measures (1, 2). Lolans and colleagues reported that an ertapenem zone diameter of Յ27 mm on MacConkey agar (MCA) was highly sensitive for the detection of Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae in rectal swab specimens (3). However, the zone diameter interpretive criteria for imipenem and meropenem placed directly on MCA have not yet been established. This study compares the performances of the screening method using MCA and 10-g carbapenem disks (ertapenem, meropenem, and imipenem) and defines the optimal inhibition zone diameters for detecting CRGNR using simulated stool specimens.Thirty-nine clinical isolates have been well characterized, phenotypically and genotypically, as described in Table 1. Twenty carbapenemase-producing isolates (17 Enterobacteriaceae and 3 nonfermenters) were selected based on the presence of genes coding for different carbapenemases. Nineteen non-carbapenemase-producing Enterobacteriaceae (18 extended-spectrum ␤-lactamase [ESBL]-or plasmid-mediated AmpC [pAmpC] lactamase-producing isolates) and a susceptible wild-type Escherichia coli strain were also selected as negative controls. The MICs of ceftazidime, cefotaxime, ertapenem, meropenem, and imipenem were determined by the microdilution method according to the Clinical and Laboratory Standards Institute (4).A stool specimen obtained from a normal volunteer was used to prepare all the simulated clinical specimens. To ensure that the specimen did not harbor any ␤-lactam-resistant bacteria, screening tests were performed using ChromID ESBL, CHROMagar KPC, and MCA with ertapenem, meropenem, and imipenem disks. Each plate was inoculated with 100 l of liquefied stool and incubated 24 h aerobically at 35°C. There was no growth on the two selective chromogenic agar plates. For the MCA, inhibition diameters around the antibiotic disks were 29 mm, 39 mm, and 35 mm for ertapenem, meropenem, and imipenem, respectively.Dilutions of the 39 isolates were mixed with aliquots of stool. The simulated fecal material was inoculated onto the screening MCA medium to obtain final challenge concentrations of 10 4 to 10 1 CFU/ml for each strain. The fecal inoculum was spread on MCA by rotation using a rake spreader, and disks of the carbapenems were individually placed onto MCA. After incubation, the diameters of the inhibition zones around each carbapenem disk were measured.The results of all inhibition diameters obtained with the 4 dilutions tested for each...

Carbapenem-Resistant Non-Glucose-Fermenting Gram-Negative Bacilli: the Missing Piece to the Puzzle

“…Defined zone diameters for screening CROs were established using spiked stool specimens; however, clinical studies demonstrating the utility of this method for the detection of CR-NF from clinical specimens are still required. The method does have the advantage of a 24-to 48-h turnaround time (TAT) compared to the CDC broth enrichment method (47).…”

The Likelihood of Developing a Carbapenem-Resistant Enterobacteriaceae Infection during a Hospital Stay

“…Ertapenem and meropenem disks were added manually to plates and incubated overnight at 37°C (11). Colonies growing within Յ27 mm of ertapenem and Յ32 mm of meropenem were further identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry (Bruker Daltonics, Inc., Billerica, MA), as previously described (11)(12)(13). For Enterobacteriaceae isolates growing within the aforementioned zone diameters (including both discrete colonies and confluent growth), carbapenem (ertapenem, meropenem, and imipenem) antimicrobial susceptibility testing was performed to identify CRE by the disk diffusion method (14).…”

Evaluation of Multiple Methods for Detection of Gastrointestinal Colonization of Carbapenem-Resistant Organisms from Rectal Swabs

“…Optimal screening methods for rapid detection of carbapenem-resistant organisms (CROs) have yet to be established (2). Currently described methods include broth enrichment, direct selective culture, chromogenic media, and detection of carbapenemase genes directly from rectal swabs (3)(4)(5)(6)(7)(8)(9). The objectives of this study were to (i) evaluate multiple methods for screening CROs from rectal swabs and (ii) to determine the prevalence of gastrointestinal colonization with CROs among high-risk inpatient populations.…”

“…From each ESwab broth, five different methods for the detection of CRO were set up in parallel. The five methods included (i) the CDC broth enrichment method with ertapenem for selection (3), (ii) a modified CDC broth enrichment method using ertapenem and vancomycin for selection (3), (iii) a direct MacConkey plate with ertapenem disks (4,6), (iv) a chromogenic chromID CARBA agar plate method (with the new reformulated medium; package insert version 20157 A-en-2013/02, reference no. 414012 [bioMérieux, Marcy l'Étoile, France]), and (v) the Check-Direct CPE screen assay for the BD MAX instrument (Check-Points, Wageningen, The Netherlands; Becton Dickinson, Sparks, MD).…”