Dose-dependent fate of GFP-expressing Escherichia coli in the alimentary canal of adult house flies

The adult house fly Musca domestica (L.) (Diptera: Muscidae) can disseminate bacteria from microbe-rich substrates to areas in which humans and domesticated animals reside. Because bacterial abundance fluctuates widely across substrates, flies encounter and ingest varying amounts of bacteria. This study investigated the dose-dependent survival of bacteria in house flies. Flies were fed four different doses' of green fluorescent protein (GFP)-expressing Escherichia coli (GFPE.coli) (very low, low, medium, high) and survival was determined at 1, 4, 10 and 22h post-ingestion by culture and epifluorescent microscopy. Over 22h, the decline in GFPE.coli was significant in all treatments (P<0.04) except the very low dose treatment (P=0.235). Change in survival (S) did not differ between flies fed low and very low doses of bacteria across all time-points, although S in both treatments differed from that in flies fed high and medium doses of bacteria at several time-points. At 4, 10 and 22h, GFPE.coli S significantly differed between medium and high dose-fed flies. A threshold dose, above which bacteria are detected and destroyed by house flies, may exist and is likely to be immune-mediated. Understanding dose-dependent bacterial survival in flies can help in predicting bacteria transmission potential.