No effect of natural transformation on the evolution of resistance to bacteriophages in the Acinetobacter baylyi model system

The adaptive benefits of natural transformation, the active uptake of free DNA molecules from the environment followed by incorporation of this DNA into the genome, may be the improved response to selection resulting from increased genetic variation. Drawing analogies with sexual reproduction, transformation may be particularly beneficial when selection rapidly fluctuates during coevolution with virulent parasites ('the Red Queen Hypothesis'). Here we test this hypothesis by experimentally evolving the naturally transformable and recombinogenic species Acinetobacter baylyi with a cocktail of lytic phages. No increased levels of resistance to phage were found in the wild type compared to a recombination deficient ΔdprA strain after five days of evolution. When exposed to A. baylyi DNA and phage, naturally transformable cells show greater levels of phage resistance. However, increased resistance arose regardless of whether they were exposed to DNA from phage-sensitive or -resistant A. baylyi, suggesting resistance was not the result of transformation, but was related to other benefits of competence. Subsequent evolution in the absence of phages did not show that recombination could alleviate the cost of resistance. Within this study system we found no support for transformation-mediated recombination being an advantage to bacteria exposed to parasitic phages.