LAUSR

OBJECTIVES Colibacillosis caused by avian pathogenic E. coli (APEC) is considered as a major hindrance in poultry farming worldwide. This study aimed to characterize the genetic content and the relatedness between multidrug-resistant E. coli isolates from broiler chickens died due to colibacillosis in three farms from Tunisia. METHODS One hundred samples were collected from chickens' fresh carcasses from 3 poultry farms in Tunisia. E. coli isolation and identification were performed. Then, antimicrobial susceptibility regarding antibiotics, the ability to produce β-lactamases and Minimal Inhibitory Concentration for colistin were determined according to CLSI guidelines. β-lactams and non-β-lactams antimicrobial resistance genes, integrons, virulence genes, and phylogenetic groups were investigated by PCRs. The genetic relatedness of the E. coli isolates was analyzed by PFGE and MLST. RESULTS A high infection rate of E. coli (50%) in infected organs of chickens was observed. The majority of E. coli isolates were multidrug-resistant (96%) among them 24% were colistin-resistant and 30% were ESBL- producing isolates. Seven out of 12 colistin-resistant isolates harbored the mcr-1 gene among them, 10 were ESBL- producing and carried blaCTX-M-1, blaTEM and blaSHV β-lactamases encoding genes. E coli isolates were assigned to different phylogroups but most of them (74%) belonged to the pathogenic phylogroup B2. Molecular typing by PFGE showed that some E. coli isolates harboring ESBL-mcr-1 genes were clonally related. MLST revealed the presence of 4 different ST lineages among ESBL- and mcr-1 carrying E. coli: ST4187, ST3882; ST5693 and ST8932 with clonal dissemination of E. coli ST4187 between two farms. CONCLUSION This is the first report of ESBL-mcr-1 carrying E. coli isolates of a clinically relevant phylogenetic group (B2) from chickens having died due to colibacillosis in Tunisian poultry farms.