Horizontal Transfer of the Tetracycline Resistance Gene tetM Mediated by pCF10 Among Enterococcus faecalis in the House Fly (Musca domestica L.) Alimentary Canal

The house fly (Musca domestica L.) alimentary canal was evaluated for the potential of horizontal transfer of tetM on plasmid pCF10 among Enterococcus faecalis. Two sets of experiments were conducted: (1) house flies without surface sterilization and (2) surface-sterilized flies. Both sets of flies were exposed to E. faecalis OG1RF:pCF10 as donor for 12 h and then E. faecalis OG1SSp as recipient for 1 h. Another group of flies received the recipient first for 12 h followed by exposure to the donor strain for 1 h. House flies were screened daily to determine the donor, recipient, and transconjugant bacterial load for up to 5 days. In addition, the sponge-like mouth parts used for food uptake (labellum) of surface-sterilized house flies were removed and analyzed for donors, recipients, and transconjugants, separately. In both groups of flies (n = 90 flies/group), transfer occurred within 24 h after exposure with a transconjugant/donor rate from 8.6 x 10(-5) to 4.5 x 10(1). Transconjugants were also isolated from the house fly labellum. Our data suggest that the house fly digestive tract provides a suitable environment for horizontal transfer of conjugative plasmids and antibiotic resistance genes among enterococci. Our results emphasize the importance of this insect as a potential vector of antibiotic-resistant bacterial strains.