Plasmid transfer in the animal intestine and other dynamic bacterial populations: the role of community structure and environment

The transfer of the R1drd19 plasmid between isogenic strains of Escherichia coil BJ4 in batch cultures of laboratory media and intestinal extracts was compared. Using an estimate of plasmid transfer rate that is independent o f cell density, of donor:recipient ratios and of mating time, it was found that transfer occurs at a much lower rate in intestinal extracts than in la boratory media. Furthermore, the results suggest that the majority of intes tinal plasmid transfer takes place in the viscous mucus layer covering the epithelial cells. Investigation of plasmid transfer in different flow syste ms harbouring a dynamic, continuously growing population of constant size s howed that transfer kinetics were strongly influenced by bacterial biofilm formation. When donor and recipient populations were subjected to continuou s mixing, as in a chemostat, transfer continued to occur at a constant rate . When donor and recipient populations retained fixed spatial locations, as in a biofilm, transfer occurred very rapidly in the initial phase, after w hich no further transfer was detected. From in vivo studies of plasmid tran sfer in the intestine of streptomycin-treated mice, results were obtained w hich were similar to those obtained in the biofilm, but differed markedly f rom those obtained in the chemostat. In spite of peristaltic movements in t he gut, and of apparently even distribution of E. coil as single cells in t he intestinal mucus, the intestinal environment displays transfer kinetics different from those expected of a mixed, liquid culture, but quite similar to those of a biofilm.