The epidemiology of AmpC-producing isolated from dairy cattle faeces on pasture-fed farms.

Antibiotic use, particularly amoxicillin-clavulanic acid in dairy farming, has been associated with an increased incidence of AmpC-hyperproducing . There is limited information on the incidence of AmpC-hyperproducing from seasonal pasture-fed dairy farms. We undertook a New Zealand wide cross-sectional study to determine the prevalence of AmpC-producing carried by dairy cattle. Paddock faeces were sampled from twenty-six dairy farms and were processed for the selective growth of both extended-spectrum beta-lactamase (ESBL)- and AmpC-producing . Whole genome sequence analysis was carried out on 35 AmpC-producing . No ESBL- or plasmid mediated AmpC-producing were detected, but seven farms were positive for chromosomal mediated AmpC-hyperproducing . These seven farms were associated with a higher usage of injectable amoxicillin antibiotics. Whole genome sequence analysis of the AmpC-producing demonstrated that the same strain (<3 SNPs difference) of ST5729 was shared between cows on a single farm. Similarly, the same strain (≤15 SNPs difference) of ST8977 was shared across two farms (separated by approximately 425 km). These results infer that both cow-to-cow and farm-to-farm transmission of AmpC-producing has occurred.