Opportunistic pathogens are prevalent across the culturable exogenous and endogenous microbiota of stable flies captured at a dairy facility.

Stable flies in the genus Stomoxys are highly abundant, blood-feeding pests on dairy farms; however, their role in the carriage and potential transmission of pathogens is largely understudied. Here, we report on the frequency and distribution of culturable bacteria collected from Stomoxys flies captured in free stall barns and nearby calf hutches over a three-month period on a focal research farm in Wisconsin, USA. Mastitis-associated bacterial taxa, including Staphylococcus, Escherichia, Enterobacter, and Klebsiella spp., were frequently isolated from pooled samples of the internal or external portions of the flies. Conversely, selective enrichment protocols from these samples yielded only a single isolate of Salmonella and no enterohemorrhagic Escherichia coli O157. Neither trap location nor time of capture had a significant impact on the observed frequency of most bacterial genera isolated from the flies. Our results confirm that Stomoxys flies harbor both mastitis-associated bacterial taxa and bacterial taxa associated with opportunistic infections in humans. Further research into the transmission of fly-associated microbes could be important in the control of mastitis or other bacterial diseases on dairy farms.