Comparative survivability of GFP-expressing Streptococcus pyogenes and Salmonella typhimurium in the alimentary canal of house flies (Musca domestica L.)

When house flies feed and oviposit on septic substrates and their bodies become internally and externally contaminated with microorganisms. As a result, house flies have been implicated in the dissemination of bacterial diseases affecting humans and animals. In assessing vector potential, it is critical to determine the survivability of bacteria within house flies as well as the ability for microbes to be reintroduced to the environment. Further, the persistence and proliferation of bacteria within the house fly alimentary canal increases the potential of bacteria being disseminated through excretion. This study investigated the survivability of green fluorescent protein (GFP)-expressing Streptococcus pyogenes and Salmonella typhimurium within the alimentary canal of house flies both spatially (via uv microscopy) and quantitatively (via culture-recovery). Adult house flies were fed known amounts of bacteria, and were dissected to remove the entire alimentary canal or homogenized and cultured at intervals within 24 h post-ingestion (hPI). The two bacterial species demonstrated different survivability patterns within flies. Within 1-4 hPI S. pyogenes were visualized inside the midgut in chains, while between 6-24 hPI the bacteria were visualized only as single cells in the gut. Both bacterial species were recovered via gut culture throughout the 24 h period. Culture recovery showed a reduction in number of viable cells, however, S. typhimurium survivability exceeded that of S. pyogenes by as much as a factor of 10. Since house flies are cosmopolitan and synanthropic, this discussion will focus on their potential as reservoirs and possibly vectors of S. pyogenes and S. typhimurium.