The pga gene cluster in Aggregatibacter actinomycetemcomitans is necessary for the development of natural competence in Ca2+-promoted biofilms

Natural competence is the ability of bacteria to incorporate extracellular DNA into their genomes. This competence is affected by a number of factors, including Ca2+ utilization and biofilm formation. As bacteria can form thick biofilms in the presence of extracellular Ca2+, the additive effects of Ca2+-promoted biofilm formation on natural competence should be examined. We evaluated natural competence in Aggregatibacter actinomycetemcomitans, an important periodontal pathogen, in the context of Ca2+-promoted biofilms, and examined whether the pga gene cluster, required for bacterial cell aggregation, is necessary for competence development. The A.actinomycetemcomitans cells grown in the presence of 1mm CaCl2 exhibited enhanced cell aggregation and increased levels of cell-associated Ca2+. Biofilm-derived cells grown in the presence of Ca2+ exhibited the highest levels of natural transformation frequency and enhanced expression of the competence regulator gene, tfoX. Natural competence was enhanced by the additive effects of Ca2+-promoted biofilms, in which high levels of pga gene expression were also detected. Mutation of the pga gene cluster disrupted biofilm formation and competence development, suggesting that these genes play a critical role in the ability of A.actinomycetemcomitans to adapt to its natural environment. The Ca2+-promoted biofilms may enhance the ability of bacteria to acquire extracellular DNA.