Deletion of competence genes represses expression of genes associated with anaerobic respiration/metabolism in Aggregatibacter actinomycetemcomitans

Biofilm formation contributes to the virulence of various pathogens, as the extracellular polymer matrix provides protection against the host immune defense and antimicrobial drugs. Biofilm- associated diseases often become chronic and recurring. The periodontal pathogen Aggregatibacter actinomycetemcomitans , which resides in a multispecies biofilm in the subgingival pocket, produces multiple virulence factors that can contribute to disease progression. Certain strains of the species are naturally competent, which allows uptake of extracellular DNA that can be incorporated into the bacterial genome or used as a nutrient. Earlier studies indicated that bacterial interleukin receptor I (BilRI) and the type IV pilus subunit PilA protein are needed for efficient transformation in A. actinomycetemcomitans . In this study, we show that the outer membrane secretin HofQ is required for natural competence, as deletion of the hofQ gene results in a nontransformable strain. Furthermore, we studied the gene expression profiles of three single-gene mutants of the naturally competent A. actinomycetemcomitans strain D7S, all of which have decreased transformation efficiency compared to the wild-type strain. Additionally, as A. actinomycetemcomitans can bind to and internalize interleukin (IL)-1β, the effect of IL-1β on bacterial gene expression was also studied. However, in our experimental setup, the addition of IL-1β did not change gene expression in the A. actinomycetemcomitans strains used. The mutant strain lacking the bilRI gene exhibited a gene expression pattern similar to that of the wild-type strain. However, deletion of hofQ or pilA resulted in altered gene expression. Interestingly, genes associated with anaerobic growth, biofilm formation, and virulence were downregulated in the Δ hofQ and Δ pilA deletion mutants, which could indicate a decreased colonization ability and reduced virulence.