Development of an algorithm for phenotypic screening of carbapenemase-producing Enterobacteriaceae in the routine laboratory.

BACKGROUND: Carbapenemase-producing Enterobacteriaceae (CPE) are difficult to identify among carbapenem non-susceptible Enterobacteriaceae (NSE). We designed phenotypic strategies giving priority to high sensitivity for screening putative CPE before further testing. METHODS: Presence of carbapenemase-encoding genes in ertapenem NSE (MIC > 0.5 mg/l) consecutively isolated in 80 French laboratories between November 2011 and April 2012 was determined by the Check-MDR-CT103 array method. Using the Mueller-Hinton (MH) disk diffusion method, clinical diameter breakpoints of carbapenems other than ertapenem, piperazicillin+tazobactam, ticarcillin+clavulanate and cefepime as well as diameter cut-offs for these antibiotics and temocillin were evaluated alone or combined to determine their performances (sensitivity, specificity, positive and negative likelihood ratios) for identifying putative CPE among these ertapenem-NSE isolates. To increase the screening specificity, these antibiotics were also tested on cloxacillin-containing MH when carbapenem NSE isolates belonged to species producing chromosomal cephalosporinase (AmpC) but Escherichia coli. RESULTS: Out of the 349 ertapenem NSE, 52 (14.9%) were CPE, including 39 producing OXA-48 group carbapenemase, eight KPC and five MBL. A screening strategy based on the following diameter cut offs, ticarcillin+clavulanate <15 mm, temocillin <15 mm, meropenem or imipenem <22 mm, and cefepime <26 mm, showed 100% sensitivity and 68.1% specificity with the better likelihood ratios combination. The specificity increased when a diameter cut-off <32 mm for imipenem (76.1%) or meropenem (78.8%) further tested on cloxacillin-containing MH was added to the previous strategy for AmpC-producing isolates. CONCLUSION: The proposed strategies that allowed for increasing the likelihood of CPE among ertapenem-NSE isolates should be considered as a surrogate for carbapenemase production before further CPE confirmatory testing.

10-Fold increase (2006-11) in the rate of healthy subjects with extended-spectrum ß-lactamase-producing Escherichia coli faecal carriage in a Parisian check-up centre.

OBJECTIVES: In 2006, 0.6% of healthy subjects living in the Paris area had extended-spectrum ß-lactamase (ESBL)-producing Escherichia coli in their gut. To assess the evolution of this rate, a study identical to that of 2006 was conducted in 2011. PARTICIPANTS AND METHODS: Healthy adults who visited the IPC check-up centre in February-March 2011 and agreed to participate, provided stools and answered a questionnaire on the visit day. Stools were analysed to detect ESBL producers and to isolate the dominant E. coli population. ESBLs were molecularly characterized. For the subjects harbouring ESBL-producing E. coli, the phylogenetic group and sequence type (ST) were determined for both ESBL-producing and dominant E. coli isolates. PFGE profiles were also determined when two types of isolates had the same ST. RESULTS: Among the 345 subjects included, 21 (6%) had ESBL-producing E. coli faecal carriage. None of the previously published risk factors was identified. CTX-M accounted for 86% and SHV-12 for 14%. Dominant and ESBL-producing E. coli were similarly distributed into phylogenetic groups (A, 52%-48%; B1, 5%; B2, 24%-14%; and D, 19%-33%). Dominant and ESBL-producing E. coli displayed a polyclonal structure (18 STs each). However, ST10 and ST131 were identified in dominant and ESBL-producing E. coli isolates from different subjects. Most (20/21) ESBL producers were subdominant and belonged (16/21) to STs different from that of the corresponding dominant E. coli. CONCLUSIONS: The 10-fold increase in the rate of healthy subjects with ESBL-producing E. coli faecal carriage over a 5 year period suggests wide dissemination of these isolates in the Parisian community.