M. Droge et al., Horizontal gene transfer among bacteria in terrestrial and aquatic habitats as assessed by microcosm and field studies, BIOL FERT S, 29(3), 1999, pp. 221-245
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.