The pga gene cluster in A ggregatibacter actinomycetemcomitans is necessary for the development of natural competence in Ca 2+ ‐promoted biofilms

Summary Natural competence is the ability of bacteria to incorporate extracellular DNA into their genomes. This competence is affected by a number of factors, including Ca 2+ utilization and biofilm formation. As bacteria can form thick biofilms in the presence of extracellular Ca 2+ , the additive effects of Ca 2+ ‐promoted biofilm formation on natural competence should be examined. We evaluated natural competence in A ggregatibacter actinomycetemcomitans , an important periodontal pathogen, in the context of Ca 2+ ‐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 1 m m CaCl 2 exhibited enhanced cell aggregation and increased levels of cell‐associated Ca 2+ . Biofilm‐derived cells grown in the presence of Ca 2+ 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 Ca 2+ ‐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 Ca 2+ ‐promoted biofilms may enhance the ability of bacteria to acquire extracellular DNA .