Dose-Dependent Effects on Replication and Persistence of Salmonella Enterica Serovar Typhimurium in House Flies (Diptera: Muscidae)

Adult house flies (Musca domestica L.) ingest variable numbers of bacteria when they encounter microbe-rich substrates. Bacterial abundance may affect replication within the fly gut, which subsequently impacts vector potential. This study investigated the dose-dependent replication of GFP-expressing Salmonella enterica serovar Typhimurium (ex Kauffmann and Edwards1952) Le Minor and Popoff 1987, (Enterobacteriales: Enterobacteriaceae) (GFP S. Typhimurium) within the fly alimentary canal. Adult house flies were fed two doses (colony forming units, CFU) of GFP S. Typhimurium (high, ~105 CFU and low, ~104 CFU). Bacteria were examined at 2-, 4-, 6-, 12-, and 24-h postingestion (PI) in situ in the gut via epifluorescence microscopy and enumerated by culture on selective media. In both treatment groups, GFP S. Typhimurium proliferated and persisted in flies for 24 h. In the high-dose group, proliferation peaked at 6 h PI (>500% increase). In the low-dose group, proliferation peaked at both 4 and 6 h PI (>900% increase). Dose significantly affected bacterial replication within the house fly alimentary canal, particularly at 4-, 6-, and 12-h PI. The ability of S. Typhimurium to proliferate and persist in the alimentary canal demonstrates that house flies may serve as significant reservoirs and probable disseminators of this pathogen. Our results show that bacterial abundance should be considered when assessing the potential of house flies to harbor and transmit pathogens.