Mobile Microenvironments

Horizontal Transfer of the Tetracycline Resistance Gene tetM Mediated by pCF10 Among Enteroccus faecalis in the House Fly Alimentary Canal
Mastura Akhtar, Helmut Hirt, Ludek Zurek
Environmental Microbiology

Vectors have largely aided the spread of microorganisms. Vectors move bacteria from one place to another as they themselves go about their life cycle. The movements of a house fly would be a prime example of this type of vector. However, a role of the vector not considered as often is its role as a habitat for a bacterium itself. During transport, or as a permanent environment, bacteria encounter a unique combination of other bacteria and nutrients that only the vector could assemble. This provides for a microenvironment that can play a key role in the evolution of and dissemination of traits beneficial to bacterial species. While inside the fly these traits can be transferred horizontally through conjugation, transduction and transformation. This study considers how plasmid mediated horizontal gene transfer in the gut of a house fly can mediate tetracycline resistance to transfer between bacterial species.
Two strains of enterocci, bacteria normally residing in the gastrointestinal tract, were selected for donor and recipient. The donor contained the tetM gene to identify transformants using selective media. Flies were separated into two groups, one that received the donor first, via infected food supply, and the other received the recipient strain first. After twelve hours flies were given food source containing the opposite strain for one hour. Each group was then subdivided so that while flies were checked for the presence of donor, recipient and transformants over the next five days and half would have their eating appendage sterilized and half would not.
Results showed that regardless of whether the donor or recipient was introduced first, both groups established concentrations of donor and recipient cells that were similar. The was also no statistical difference in rate of gene transfer between the two groups. Concentration of donor and recipient cells in the digestive tract was similar to that of the surface sterilized, suggesting that observed donor, recipient and transformants were localized to the gut.
Transformants began to be detected 24 hours after both stains were combined. Their presence was screened for using selective media. Groups of flies were sterilized at the surface to eliminate the possibility of surface contamination and transformation outside the vector. Portions of the food supplied to the flies were periodically screened for transformants with very little occurrence, suggesting this did not play a key role. However, it is possible that transfer is taking place on the eating appendage. The conditions in which intestinal gene transfer are best suited are not well understood. The final possible explanation could be that transformants could be the product of high plasmid transfer rate and subsequent rapid clonal expansion of transformants.
Horizontal transfer in a vector could lead to the spread of various genes and provide a unique microenvironment for evolution. The house fly’s unique combination of contact with decaying organic matter and food provides ample opportunity for transfer of traits between bacteria evolved to live in harsh environments to those that are common in food. This potential introduces a need to better understand horizontal gene transfer and evolution in microenvironments that can directly affect humans.

Brian Lohman
University of Idaho