ABSTRACTThe gram-negative coccobacillus,Actinobacillus actinomycetemcomitans, is the putative agent for localized juvenile periodontitis, a particularly destructive form of periodontal disease in adolescents. This bacterium has also been isolated from a variety of other infections, notably endocarditis. Fresh clinical isolates ofA. actinomycetemcomitansform tenacious biofilms, a property likely to be critical for colonization of teeth and other surfaces. Here we report the identification of a locus of seven genes required for nonspecific adherence ofA. actinomycetemcomitansto surfaces. The recently developed transposon IS903φkanwas used to isolate mutants of the rough clinical isolate CU1000 that are defective in tight adherence to surfaces (Tad−). Unlike wild-type cells, Tad−mutant cells adhere poorly to surfaces, fail to form large autoaggregates, and lack long, bundled fibrils. Nucleotide sequencing and genetic complementation analysis revealed a 6.7-kb region of the genome with seven adjacent genes (tadABCDEFG) required for tight adherence. The predicted TadA polypeptide is similar to VirB11, an ATPase involved in macromolecular transport. The predicted amino acid sequences of the other Tad polypeptides indicate membrane localization but no obvious functions. We suggest that thetadgenes are involved in secretion of factors required for tight adherence ofA. actinomycetemcomitans. Remarkably, complete and highly conservedtadgene clusters are present in the genomes of the bubonic plague bacillusYersinia pestisand the human and animal pathogenPasteurella multocida. Partialtadloci also occur in strikingly diverseBacteriaandArchaea. Our results show that thetadgenes are required for tight adherence ofA. actinomycetemcomitansto surfaces and are therefore likely to be essential for colonization and pathogenesis. The occurrence of similar genes in a wide array of microorganisms indicates that they have important functions. We propose thattad-like genes have a significant role in microbial colonization.