Adult house fly (Musca domestica L.) carriage of bacteria and antimicrobial resistance at Kansas cattle operations

Adult house flies acquire and transmit bacteria of medical and veterinary importance. Considering house flies rely on microbe-rich substrates like manure for development, adult flies may be integral in the persistence and movement of bacteria and antimicrobial resistance (AMR) at cattle operations. House flies were collected from Kansas dairy and beef facilities for five alternating weeks in summer 2019. Bacteria were enumerated from flies on non-selective (TSA, overall bacteria) and selective media (VRBA, coliforms). Distinct coliform morphotypes were screened for tetracycline resistance on tetracycline-infused agar. Resistant coliforms were then confirmed by Kirby Bauer disc susceptibility and tested for multidrug resistance (MDR) to florfenicol, enrofloxacin, ceftiofur, and ampicillin. Bacterial and coliform abundance were significantly affected by fly sex, with female flies carrying more bacteria than males. Approximately 46% of coliform morphotypes from flies were AMR, and 13% were MDR. 73% of flies carried at least one AMR coliform, and 24% of flies carried MDR coliforms. In addition to tetracycline resistance, most MDR isolates were resistant to ampicillin and/or florfenicol. Only female flies carried enrofloxacin- or ceftiofur-resistant coliforms, and only female flies from beef operations carried enrofloxacin resistance. One coliform isolated from a beef operation female fly was resistant to all five antibiotics. AMR and MDR coliforms will be identified by 16S rDNA and characterized with whole genome sequencing. Our results show that house flies play a role in harboring and transmitting AMR and MDR bacteria in confined cattle operations and fly management should be considered in AMR mitigation strategies.