Epidemiology of antimicrobial resistance in commensal Escherichia coli from healthy dairy cattle on a Mediterranean pasture-based system of Australia: A cross-sectional study

This study aimed to determine the prevalence of antimicrobial resistance (AMR) in commensal Escherichia coli from healthy lactating cows and calves in the Mediterranean pasture-based feeding dairy system of Western Australia (WA). Fecal samples were collected from healthy adult lactating cows and healthy calves from dairy farms in WA. Presumptive commensal E. coli was isolated from these samples and confirmed using matrix- assisted laser-desorption/ionization time-of-flight mass spectrometry. Broth microdilution was used to assess the prevalence and the phenotypic AMR profiles of the E. coli isolates to 8 antimicrobial agents of dairy industry and human importance. The minimum inhibitory concentration for each isolate was interpreted using the epidemiologic cutoff (ECOFF) and Clinical and Laboratory Standards Institute breakpoints. Genomic characterization provided multilocus sequence types and AMR genes for a selection of isolates categorized as nonwild type (NWT) by ECOFF values for the combination of ampicillin, trimethoprim-sulfamethoxazole, and tetracycline. From a total of 1,117 fecal samples (633 adult, 484 calf) collected across 26 randomly selected farms, 891 commensal E. coli isolates were recovered (541 adult, 350 calf). Commensal E. coli classified as NWT was highest for ampicillin for both adult (68.8%; 95% CI [64.7, 72.7]) and calf feces (67.1%; 95% CI [62.0, 72.0]). A large proportion of tetracycline NWT and trimethoprimsulfamethoxazole NWT organisms were also identified from calf feces, being 44.0% (95% CI [38.7, 49.4]) and 24.6% (95% CI [20.2, 29.4]), respectively. Clinical resistance prevalence was low, being higher for calves than for adult feces for ampicillin (adult: 7.8%, 95% CI [5.7, 10.3]; calf: 30.0%, 95% CI [25.2, 35.1]), tetracycline (adult: 6.3%, 95% CI [4.4, 8.7]; calf: 40.3%, 95% CI [35.1, 45.6]), and trimethoprim-sulfamethoxazole (adult: 2.6%, 95% CI [1.4, 4.3]; calf: 22.0%, 95% CI [17.7, 26.7]). Commensal E. coli originating from calf feces was significantly higher in NWT prevalence compared with adult feces for ciprofloxacin, gentamicin, tetracycline, and trimethoprim-sulfamethoxazole. The overall number of antimicrobials an isolate was classified as NWT toward varied among farms and was significantly higher for isolates originating from calf rather than adult feces. The strain type and sampling source of the commensal E. coli investigated were both associated with the commonality of the resultant resistance genome. Clinical resistance and NWT classification were highest for ampicillin, tetracycline, and trimethoprim-sulfamethoxazole, all antimicrobials commonly used in the treatment of dairy cattle in Australia. Although highly variable across farms, commensal E. coli isolated from healthy dairy calf feces had significantly higher NWT and multidrug resistance (MDR) prevalence compared with feces from healthy adult lactating dairy cows. The resistant genome identified in MDR isolates, although not always consistent with the phenotype, included QnrS1 and genes encoding AmpC beta-lactamase and aminoglycoside phosphotransferase.