Journal of Bacteriology, November 2000, p. 6169-6176, Vol. 182, No. 21
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Department of Microbiology, College of Physicians and Surgeons, Columbia University, New York, New York 100321; Department of Biology, Bard College, Annandale-on-Hudson, New York 125042; Molecular Laboratories, American Museum of Natural History, New York, New York 100243; and Department of Oral Pathology and Biology, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 071034
Received 12 April 2000/Accepted 5 July 2000
The 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 of A. actinomycetemcomitans form 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 of A. actinomycetemcomitans to surfaces. The recently developed
transposon IS903
kan was 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 the
tad genes are involved in secretion of factors required for
tight adherence of A. actinomycetemcomitans. Remarkably,
complete and highly conserved tad gene clusters are present
in the genomes of the bubonic plague bacillus Yersinia pestis and the human and animal pathogen Pasteurella
multocida. Partial tad loci also occur in strikingly
diverse Bacteria and Archaea. Our results show
that the tad genes are required for tight adherence of
A. actinomycetemcomitans to 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 that tad-like genes have a significant role in microbial colonization.
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