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Infect. Immun. doi:10.1128/IAI.00402-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Autoinducer-2 is Required for Biofilm Growth of Aggregatibacter (Actinobacillus) actinomycetemcomitans

Department of Periodontics, Endodontics and Dental Hygiene, University of Louisville School of Dentistry, Louisville, Kentucky; Department of Oral Biology, University of Florida School of Dentistry, Gainesville, Florida

^* To whom correspondence should be addressed. Email: drdemu01{at}louisville.edu.

	Abstract

Autoinducer-2 (AI-2) is required for the growth of A. actinomycetemcomitans in culture under iron limitation. However, in vivo this organism thrives in a complex multi-species biofilm that forms in the human oral cavity. In this report, we show that adherent growth of A. actinomycetemcomitans on a saliva coated surface, but not planktonic growth under iron replete conditions, is defective in a LuxS-deficient background. Biofilm growth of the luxS mutant exhibited lower total biomass and lower biofilm depth than the wild type strain. Normal biofilm growth of the luxS mutant was restored genetically by introduction of a functional copy of luxS and biochemically by addition of partially purified AI-2. Furthermore, introduction of S-adenosylhomocysteine hydrolase to A. actinomycetemcomitans, which restores metabolism of S-adenosylmethionine in the absence of LuxS, did not complement the luxS mutation unless AI-2 was added in trans. This suggests that AI-2 itself is required for biofilm growth by A. actinomycetemcomitans. A biofilm growth deficiency similar to that of the LuxS-deficient strain was also observed when genes encoding the AI-2 interacting proteins RbsB or LsrB were inactivated. Biofilm formation by A. actinomycetemcomitans was virtually eliminated upon inactivation of both rbsB and lsrB. In addition, biofilm growth by wild type A. actinomycetemcomitans was reduced in the presence of ribose, which competes with AI-2 for binding to RbsB. These results suggest that RbsB and LsrB function as AI-2 receptors in A. actinomycetemcomitans and that the development of A. actinomycetemcomitans biofilms requires AI-2.

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