Horizontal gene transfer among bacteria in terrestrial and aquatic habitats as assessed by microcosm and field studies

Genetic interactions among bacteria are mediated by one of the three distin ct gene-exchange mechanisms: conjugation, transformation or transduction. C onjugative gene exchange relies on mobile elements, such as plasmids, which transfer between donor and recipient cells. In natural transformation, com petent cells take up DNA and incorporate it into their genome. Gene transfe r via transduction is mediated by bacteriophages which accidentally package donor DNA in their phage head and transfer it to recipient cells. Driven m ainly by biosafety research and research into the rapid dissemination of an tibiotic resistance, the evaluation of gene flux among bacteria in their na tural habitats has become a focus of scientific interest in recent years. A ccordingly, gene transfer has been assessed in laboratory-based studies emp loying model ecosystems, as well as in field experiments. Conjugative gene exchange has been shown to occur under a wide range of environmental condit ions. Factors identified as conducive for conjugation include the presence of nutrients provided by the rhizosphere of plants. Studies addressing gene transfer via transformation have demonstrated that naturally transformable bacteria develop competence and take up DNA under in situ conditions. More over, DNA has been shown to persist to some extent in the environment, and thus be available for uptake by naturally competent cells. Gene exchange vi a transduction has been demonstrated under conditions of nutrient depletion and low densities of host cells. Whereas gene transfer is readily observed in the laboratory, more importantly, field studies have provided direct ev idence that all three gene transfer mechanisms also occur in nature. DNA tr ansfer frequencies observed in the environment in some cases differed consi derably from those obtained under laboratory conditions. Transfers of low f requency observed in laboratory-based experiments have been readily detecte d in the environment in the presence of selective forces.