Emergence of a competence-reducing filamentous phage from the genome of acinetobacter baylyi ADP1

Bacterial genomes commonly contain prophage sequences as a result of past infections with lysogenic phages. Many of these integrated viral sequences are believed to be cryptic, but prophage genes are sometimes coopted by the host, and some prophages may be reactivated to form infectious particles when cells are stressed or mutate. We found that a previously uncharacterized filamentous phage emerged from the genome of Acinetobacter baylyi ADP1 during a laboratory evolution experiment. This phage has a genetic organization similar to that of the Vibrio cholerae CTXφ phage. The emergence of the ADP1 phage was associated with the evolution of reduced transformability in ourexperimental populations, so we named it the competence-reducing acinetobacter phage (CRAφ). Knocking out ADP1 genes required for competence leads to resistance to CRAφ infection. Although filamentous bacteriophages are known to target type IV pili, this is the first report of a phage that apparently uses a competence pilus as a receptor. A. baylyi may be especially susceptible to this route of infection because every cell is competent during normal growth, whereas competence is induced only under certain environmental conditions or in a subpopulation of cells in other bacterial species. It is possible that CRAφ-like phages restrict horizontal gene transfer in nature by inhibiting the growth of naturally transformable strains. We also found that prophages with homology to CRAφ exist in several strains of Acinetobacter baumannii. These CRAφ-like A. baumannii prophages encode toxins similar to CTXφ that might contribute to the virulence of this opportunistic multidrug-resistant pathogen.