The tad locus: postcards from the widespread colonization island

The tad (tight adherence) genes of Aggregatibacter actinomycetemcomitans specify the production of adhesive pili that function in biofilm formation, colonization and pathogenesis of this periodontal pathogen. The article reviews the evidence that the Tad secretion system represents a widespread new type of prokaryotic secretion system that has important functions in different bacterial pathogens.