Species-specific fate of bacteria in house flies and impact on vector potential for pathogens

House flies ingest bacteria during filth-feeding and consequently can transport microbes from septic environments to human habitats and food. Vector potential is influenced both by flies encountering pathogens and by the fate of bacteria in the fly alimentary canal. In order for pathogens to be transmitted, they must exit the fly in feces or vomit, and maintain viability. We investigated the fate of non-pathogenic E. coli and two human pathogens, Salmonella typhimurium and Streptococcus pyogenes, in house flies. Using GFP-transformed strains of bacteria, we assessed the temporal and spatial fate and viability of bacteria in the alimentary canal. Interestingly, we observed a dose-dependent effect on bacterial fate, as determined by visual examination (epiflourescent microscopy) and culture recovery. In addition, all three species, irrespective of dose, showed different temporal fates in the house fly gut. Non-motile S. pyogenes had the quickest transit time in the gut, and rapidly experienced great declines, possibly due to antimicrobial effectors secreted in the gut lumen. Even though S. typhimurium and E. coli are both motile bacteria, S. typhimurium persisted longer in flies, exhibited more sustained motility, and multiplied. In contrast, E. coli exhibited some early motility, but became enveloped in food boluses and declined in number over a 24 h period. All three species were recovered from fly excreta, with S. pyogenes showing the lowest recoverable amounts. Characteristics of these three microbes that enhance or inhibit survival in the house fly will be discussed in terms of vector potential.