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Escherichia coli  is a normal inhabitant of the intestinal microbiota of chickens, a small proportion of which may be avian pathogenic  E .  coli  (APEC) or potential extraintestinal pathogenic  E .  coli  (ExPEC), capable of causing disease in humans. These  E .  coli  may also be resistant to antimicrobials of critical importance in human or veterinary health. This study aims to 1) determine the prevalence of antimicrobial resistance (AMR) and resistance genes, multidrug resistance (MDR), chromosomal mechanisms of quinolone-resistance and virulence profiles of  E .  coli  isolated from healthy chicken farms in the region of Dakar, Senegal, 2) investigate the spread of third-generation cephalosporins (3GC) resistance in  E .  coli  isolated from healthy chicken farms with respect to virulence and resistance genes, serogroups, Pulsed-Field Gel Electrophoresis (PFGE), phylogenetic groups, plasmid types and transferability and 3) determine whether nonsusceptibility against 3GC on farms could be linked to risk factors. More than 68% of isolates from environmental faecal and drinking water samples, carcasses and carcass washes collected on 32 healthy chicken farms were multidrug resistant (MDR), resistance to antimicrobials critical in human health (3GC or ciprofloxacin) being found in all types of samples. Ciprofloxacin resistance was due to mutations in the  gyrA  and  parC  genes, 95% of tested farms harboring isolates carrying three mutations, in  gyrA  (Ser83Ile and Asp87Asn) and  parC  (Ser80Ile). Nine of the 32 farms (28.1%) demonstrated the presence of one or more 3GC-nonsusceptible indicator isolates but none of the potential risk factors were significantly associated with this presence on farms. Following ceftriaxone enrichment, presumptive extended-spectrum beta-lactamase/AmpC-beta-lactamase (ESBL/AmpC)-producer isolates were found in 17 of the 32 farms. 3GC resistance was mediated by  bla   CMY-2   or  bla   CTX-M   genes,  bla   CTX-M   being of genotypes  bla   CTX-M-1  ,  bla   CTX-M-8   and for the first time in chickens in Senegal, the genotype  bla   CTX-M-15 .  Clonally related ESBL/AmpC-producer isolates were found on different farms. In addition,  bla   CTX-M   genes were identified on replicon plasmids I1 and K/B and  bla   CMY-2   on K/B, I1 and B/O. These plasmids were found in isolates of different clusters. In addition, 18 isolates, some of which were ESBL/AmpC-producers, were defined as potential human ExPEC. In conclusion,  E .  coli  isolates potentially pathogenic for humans and demonstrating MDR, with resistance expressed against antimicrobials of critical importance in human health were found in healthy chickens in Senegal. Our results suggest that both clonal spreading and horizontal gene transfer play a role in the spread of 3GC-resistance and that chickens in Senegal could be a reservoir for AMR and ExPEC for humans. These results highlight the importance of raising awareness about compliance with biosecurity measures and prudent use of antimicrobials.

plos one

Prevalence of antimicrobial resistance and potential pathogenicity, and possible spread of third generation cephalosporin resistance, in Escherichia coli isolated from healthy chicken farms in the region of Dakar, Senegal