Comparison of the chromID™ ESBL medium and MacConkey agar supplemented with ceftazidime (5mg/l) for the detection of extended-spectrum beta-lactamase producing Enterobacteriaceae from rectal swabs in hospitalized patients

Objectives

Rapid identification of patients colonized with extended-spectrum beta-lactamase (ESBL)- producing Enterobacteriaceae is useful for the early detection and control of nosocomial outbreaks. The aim of this study was to evaluate the clinical diagnostic performance of the selective chromogenic medium chromID™ ESBL (bioMérieux, Marcy-l’Etoile, France), compared with our in-house medium CTAZ (MacConkey agar + ceftazidime 5 mg/l) for the detection of ESBL-producing Enterobacteriaceae from rectal swabs in ICU patients.

Methods

From 5/11 to 17/12/2007, ICU hospitalized patients (n=299) were screened by sampling rectal swabs (n=436) using the Eswab™ (COPAN).

After homogenization by vortexing for 15 s, 100 μl of the Eswab™ were inoculated onto chromID ESBL and CTAZ. Plates were incubated at 35°C and read after 18 and 48 h.

Identification and susceptibility testing were performed by using the Vitek 2 system.

The presence of ESBL was confirmed by combined double disks according to CLSI guidelines.

Genotypic characterization was determined by PCR assays targeting blaTEM, blaSHV and blaCTX-M genes.

Isolates harboring blaTEM and blaSHV were further analyzed by sequencing to identifiy the ESBL.

A daily quality control was performed on CTAZ by using reference strains, E. coli ATCC 25922 (ESBL-negative) and K. pneumoniae ATCC 700603 (ESBL-positive), and according to CLSI guidelines for selective media.

Results

One hundred and six specimens yielded growth on at least one of the selective media (Table 1) and 330 specimens were culture negative on
both media.

Of 95 Enterobacteriaceae strains isolated from 48 patients (16.1%), 69 ESBL-positive strains were found in 32 patients (10.7%) (Table 1).

All Enterobacteriaceae strains produced the expected color colonies on chromID ESBL, except for 4 E. coli isolates (colorless after 18h, pink-burgundy after 48 h).

CTX-M derived enzymes (n=53) were the most frequently encountered ESBL, especially in E. coli and E. cloacae. They were significantly more frequently recovered on chromID ESBL than on CTAZ (p<0.0001) (Table 2).

TEM (n=10) and SHV (n=6) were found in E. coli, K. pneumoniae and Enterobacter spp. They were more frequently recovered on CTAZ than on chromID ESBL (p<0.0001).

The diagnostic performance of media was assessed by specimens, by bacterial isolates and Enterobacteriaceae isolates (Table 3). No significant difference was observed.