Occurrence of β-lactam resistant Escherichia coli along the broiler production chain.

In Italy, few studies have investigated the occurrence of β-lactamase producing E. coli in broilers. This paper aimed to determine the level of phenotypic β-lactam resistance, in particular to third-generation cephalosporins (3GCs), in commensal and clinical E. coli isolated from the whole broiler production pyramid. To this end, several samples were collected from three production chains (A, B, C), including breeders, growing broilers and carcasses; and they were analysed on both Eosin Methylene Blue agar (EMB) and EMB agar supplemented with 1mg/L cefotaxime (CTX-EMB). A low occurrence of ESBL/AmpC-producing E. coli was found when the non-selective medium (EMB) was considered. In contrast, for 55.3% of samples, at least one ESBL/AmpC-producing commensal E. coli was isolated when the selective medium (CTX-EMB) was used. Resistant strains were present at all steps of the broiler production pyramid with a relative sample frequency of ESBL/AmpC-producing E. coli ranging from moderate (15%) to extremely-high ratios (85%). In contrast to commensal E. coli, only 6.6% of clinical strains were phenotypically resistant. The high occurrence of ESBL/AmpC-producers is surprising considering that 3GCs have never been licenced for use in poultry in Italy. Nevertheless, the circulation of 3GC-resistant E. coli in all steps of the broiler production pyramid is worrying. Studying the genetic background of β-lactam resistance is crucial to elucidate the source and the transmission routes of ESBL/AmpC-producing E. coli along the broiler production chain.