Environmental and sex effects on bacterial carriage by adult house flies (Musca domestica)

Authors: Nayduch, Dana; NEUPANE, SARASWOTI - Kansas State University; THOMSON, JESSICA - Kansas State University

Submitted to: Entomological Society of America Regional Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 5/21/2019
Publication Date: 11/12/2019
Citation: Nayduch, D., Neupane, S., Thomson, J. 2019. Environmental and sex effects on bacterial carriage by adult house flies (Musca domestica) . Entomological Society of America Regional Meeting Proceedings. 1:1.

Interpretive Summary: Adult house flies feed and breed in sites that contain a wide variety and abundance of microbes, such as dumpsters and animal manure. When flies visit these sites, they become contaminated with and ingest bacteria which can be harbored, disseminated and transmitted to other locations, such as human habitats. Over the course of 3 months, we collected male and female house flies from two environments in Kansas: urban (restaurant dumpsters) and agricultural (dairy farm). We homogenized flies individually and cultured that homogenate on two types of media: TSA, to assess total bacterial counts and VRBA, to assess and enumerate coliform bacteria, indicators of contact with feces. A subset of coliforms that grew on VRBA was identified to species and then each isolate was tested for resistance to antimicrobial drugs. Overall, female flies harbored more bacteria than males (P<0.0001) and there was a sex by site interaction (P=0.02) where the sex of the fly had a significant effect on bacterial counts at the urban location (P<0.0001). Surprisingly, although we expected flies from the dairy farm, where manure is abundant, to have more coliforms than the urban site, coliform abundance did not differ by site (P=0.97). Coliform abundance also did not differ in male and female flies overall (P=0.66) or within each site (urban, P=0.18; agricultural, P=0.52). House flies carried bacterial strains that were resistant to antimicrobial drugs; 37 of 39 bacterial isolates were resistant to at least 1 antimicrobial and 65% of resistant strains were resistant to 4 or more antimicrobials. Our results show the importance of house flies not only in harboring bacteria, but also in posing a risk to human and animal health by carrying and potentially disseminating bacteria that are antimicrobial-resistant.

Technical Abstract: Adult house flies frequent microbe-rich sites such as dumpsters and animal manure for feeding and reproductive purposes. Flies become contaminated with and ingest a wide variety of bacteria which can be disseminated to other locations, including human habitation. We investigated total culturable bacteria and coliform abundance in male and female house flies collected from two environments: urban (restaurant dumpsters) and agricultural (dairy farm). House flies (107 total) were collected across 3 dates in 2017 and whole-fly homogenate was cultured on nonselective (TSA, total bacteria) and selective (VRBA, coliforms) media for bacterial enumeration. A subset of VRBA isolates was identified to species using API20E strips and tested for antimicrobial resistance (AMR) via microplate dilution (MIC method). We hypothesized that female flies would harbor more bacteria due to their increased association with substrates for oviposition, and that coliform abundance would be greater in agricultural flies where abundant manure is accessible. Overall, female flies harbored more bacteria than males (P<0.0001) and there was a sex by site interaction (P=0.02) with sex effects present at the urban location (P<0.0001). Coliform abundance did not differ by sex (P=0.66), site (P=0.97) or by sex within site (urban, P=0.18; agricultural, P=0.52). House flies carried AMR strains of bacteria: 37/39 isolates were resistant to 1 or more antimicrobials and 65% of AMR strains were resistant to 4 or more antimicrobials. Our study further illuminates the role house flies play in harboring bacteria, some of which pose a risk to human and animal health.