Enterococcus faecalis OG1RF:pMV158 survives and proliferates in the house fly (Musca domestica L.) digestive tract

Enterococcus faecalis is an important nosocomial pathogen and house flies have been implicated in the dissemination and transfer of this bacterium in agricultural and urban environments. In this study, a GFP-expressing strain of Enterococcus faecalis OG1RF:pMV158 was used to track the fate of the bacterium in the digestive tract of the common house fly to assess the vector potential of this insect for E. faecalis under laboratory conditions. Colony forming unit (CFU) counts were obtained from viable fluorescing E. faecalis recovered from mouthparts and digestive tract regions (labellum, foregut, midgut, hindgut) at 1, 4, 8, 24, 48, 72 and 96 hours after the initial bacterial exposure. Counts were highest in the midgut at 1 hour and declined during the first 24 hours. In the labellum, foregut and hindgut, E. faecalis concentrations were more variable but overall higher after 24 hours. Observations of the digestive tract under a dissecting microscope with UV light revealed that E. faecalis peaked in the crop after 48 hours. Our data suggest that E. faecalis was digested in the midgut; however, microscopy and CFU counts indicated the proliferation in the crop. Both drinking water and feed (flaked corn), sampled at the end of the assay (96h) were contaminated by fluorescing E. faecalis, demonstrating that the flies disseminated E. faecalis throughout the experiment. The role of house flies in the ecology of E. faecalis will be discussed.