High prevalence of mcr-1 encoding colistin resistance and first identification of blaCTX-M-55 in ESBL/CMY-2-producing Escherichia coli isolated from chicken faeces and retail meat in Tunisia.

Avian industries have been reported as an important contributor in the worldwide spread of antibiotic resistance owing to some particular practices especially the overuse of antibiotics. Thus in this study, we aimed to characterize extended-spectrum-beta-lactamase (ESBL) and acquired-AmpC-beta-lactamase (aAmpC)-producing Escherichia coli isolates from chicken faeces and raw meat in Tunisia. During the year 2018, 286 faecal chicken swabs and 47 raw chicken meat samples were collected and processed to recover cefotaxime-resistant E. coli. Antimicrobial susceptibility was performed by disk-diffusion and/or broth-microdilution. blaTEM, blaSHV, blaCTX-M, and blaCMY genes were investigated by PCR/sequencing. Genes encoding resistance to colistin (mcr-1 to mcr-8), tetracycline (tetA/tetB), sulfonamide (sul1/sul3), and chloramphenicol (cmlA), were analysed by PCR. Class 1 integrons were investigated by PCR/sequencing. Phylogenetic groups of all isolates were determined. PFGE and MLST were performed for representative isolates. PCR-based replicon typing was performed in mcr1-harbouring isolates. Cefotaxime-resistant E. coli was detected in 22.4% (64/286) and 63.8% (30/47) of faeces and meat samples, respectively. Ninety isolates were ESBL-producers and harboured the genes: blaCTX-M-1 +/- blaTEM-1 (n = 65), blaCTX-M-55 +/- blaTEM-1 (n = 21), blaCTX-M-14 (n = 1), and blaSHV-12 (n = 3). The blaCMY-2 gene was detected in four ESBL-negative isolates. Isolates belonged to phylogroups D (50%), A (36.2%), B1 (9.6%), and B2 (4.3%). Fifty-four were colistin-resistant and 52 carried the mcr-1 gene. The tetA, sul1/sul3 and cmlA genes were detected among resistant isolates and 76 harboured class 1 integrons. MLST analysis revealed 13 sequence types (STs). The isolates were classified into 28 PFGE types. The IncP, IncFIB, and IncI1 replicons were detected among mcr-1-positive strains. We report a high frequency of ESBL-producers and colistin-resistant E. coli in chicken and derived food and the detection for the first time of blaCTX-M-55 in poultry in Tunisia.