Trends of resistance to extended-spectrum β-lactams in commensal and clinical Escherichia coli from the broiler production pyramid.

The presence of extended-spectrum and AmpC β-lactamase-producing Escherichia coli (ESBL/AmpC-EC) in food-producing animals can negatively affect human and animal health. This study aimed to determine the occurrence of ESBL/AmpC-EC in the whole broiler production pyramid in Italy by sampling three production chains. For each chain, we sampled Parent Stock (PS) chickens at the age of one-day and during the laying period. The offspring of the sampled PS was tracked and sampled in four commercial broiler farms during the first week of life and during the last week before slaughter. Carcasses of these broilers at the slaughterhouse and tissue swabs from colibacillosis cases were also collected. All samples (cloacal swabs from chickens and rinsates from carcasses) were processed on both Eosin Methylene Blue agar (EMB) and EMB supplemented with 1mg/L cefotaxime (CTX-EMB) except for tissue samples, which were analysed only on EMB. Three confirmed E. coli colonies (2 from EMB and 1 from CTX-EMB) were subjected to disk-diffusion test for ESBL/AmpC-production according to CLSI guidelines. Detection rates using CTX-EMB demonstrated the presence of ESBL/AmpC-EC at all levels of the broiler production pyramid. Extremely-high percentages (92.5% on average) were encountered in one-day-old PS chickens, which dropped significantly to moderate ratios (20% on average) during the laying period. Persistent occurrence of ESBL/AmpC-EC was found in broiler farms at both sampling times with an average detection rate of 64.2% (range 25-100%) for day-old broilers to 42.9% before slaughter (range 15-95%). Also at the bottom of the production pyramid, carcasses were highly contaminated with ESBL/AmpC-EC (51.3% on average, range 15-75%). Conversely, detection rates of ESBL/AmpC-EC with EMB were substantially lower. The proportion of positive samples was seven times lower on average compared to CTX-EMB and for 14 out of 42 sampled flocks, no samples were found positive on EMB. Additionally, only 6.6% (10/152) of clinical E. coli showed phenotypic ESBL/AmpC-production. This finding suggests that, although present, ESBL/AmpC-EC colonise the broiler gut at low levels.