Introduction. Dairy calves, particularly pre-weaned calves have been identified as a common source of multidrug resistant (MDR) Escherichia coli . Gap statement. E. coli strains isolated from dairy calves and the location of their resistance genes (plasmid or chromosomal) have not been well characterised. Aim. To characterise the phenotypic and genotypic features as well as the population structure of antimicrobial-resistant E. coli isolated from calves located on dairy farms that feed waste-milk to their replacement calves. Methodology. Recto-anal swab enrichments from 40 dairy calves (≤ 14 days old) located on four dairy farms were examined for tetracycline, streptomycin, ciprofloxacin, and third-generation cephalosporin resistant E. coli . Whole genome sequencing was performed using both short- and long-read technologies on selected antimicrobial resistant E. coli . Results. Fifty-eight percent (23/40) of calves harboured antimicrobial resistant E. coli : 43 % (17/40) harboured tetracycline resistant, and 23 % (9/40) harboured chromosomal mediated AmpC producing E. coli . Whole genome sequencing of 27 isolates revealed five sequence types, with ST88 being the dominant ST (17/27, 63 % of the sequenced isolates) followed by ST1308 (3/27, 11 %), along with the extraintestinal pathogenic E. coli lineages ST69 (3/27, 11 %), ST10 (2/27, 7 %), and ST58 (2/27, 7 %). Additionally, 16 isolates were MDR, harbouring additional resistance genes that were not tested phenotypically. Oxford Nanopore long-read sequencing technologies enabled the location of multiple resistant gene cassettes in IncF plasmids to be determined. Conclusion. Our study identified a high incidence of tetracycline and streptomycin-resistant E. coli in dairy calves, and highlighted the presence of multidrug-resistant strains, emphasising the need for further investigation into potential associations with farm management practices.

Dairy calves, particularly pre-weaned calves have been identified as a common source of multidrug (MDR) resistant E. coli. However, the strains and whether their resistance genes are plasmid or chromosomally located have not been well characterised. Our study examined the phenotype and genotype of antimicrobial resistant E. coli isolated from young calves (≤ 14 days old). Recto-anal swab enrichments from 40 dairy calves located on four dairy farms were examined for tetracycline, streptomycin, ciprofloxacin, and third-generation cephalosporin resistant E. coli. Fifty-eight percent (23/40) of calves harboured antimicrobial resistant E. coli: 18/40 (45%) harboured tetracycline resistant and 25% (10/40) harboured chromosomal mediated AmpC producing E. coli. Whole genome sequencing of 27 isolates revealed five sequence types, with ST88 being the dominant ST (17/27, 63% of the sequenced isolates) followed by ST1308 (3/27, 11%), along with the extraintestinal pathogenic E. coli lineages ST69 (3/27), ST10 (2/27, 7%), and ST58 (1/27, 4%). Additionally, 16 isolates were MDR, harbouring additional resistance genes that were not tested phenotypically. Oxford Nanopore long-read sequencing technologies enabled the location of multiple resistant gene cassettes in IncF plasmids to be determined. A phylogenetic comparison of the ST10 and ST69 isolates demonstrated that the calf derived isolates were distinct from other New Zealand animal, human, and environmental isolates. and highlights the importance of understanding the sources of antimicrobial resistance.