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Infection and Immunity, March 2004, p. 1431-1440, Vol. 72, No. 3
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.3.1431-1440.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Effects of RelA on Key Virulence Properties of Planktonic and Biofilm Populations of Streptococcus mutans

José A. C. Lemos, Thomas A. Brown Jr., and Robert A. Burne*

Department of Oral Biology, University of Florida College of Dentistry, Gainesville, Florida 32610

Received 18 June 2003/ Returned for modification 23 September 2003/ Accepted 10 December 2003

Streptococcus mutans is a biofilm-forming bacterium that is adapted to tolerate rapid and dramatic fluctuations in nutrient availability, carbohydrate source, and pH in its natural environment, the human oral cavity. Dissecting the pathways used to form stable biofilms and to tolerate environmental stress is central to understanding the virulence of this organism. Here, we investigated the role of the S. mutans relA gene, which codes for a guanosine tetraphosphate and guanosine pentaphosphate [(p)ppGpp] synthetase/hydrolase, in biofilm formation and acid tolerance. Two mutants in which relA was insertionally inactivated or replaced by an antibiotic resistance determinant were constructed. Under normal growth and stress conditions, the mutants grew slower than the wild-type strain, although the final yields were similar. The mutants, which were still able to accumulate (p)ppGpp after the induction of a stringent response, showed significant reductions in biofilm formation on microtiter plates or hydroxylapatite disks. There was no difference in the sensitivities to acid killing of the parent and relA strains grown in planktonic cultures. However, when cells were grown in biofilms, the mutants became more acid resistant and could lower the pH through glycolysis faster and to a greater extent than the wild-type strain. Differences in acid resistance were not correlated with increases in F-ATPase activity, although bacterial sugar:phosphotransferase activity was elevated in the mutants. Expression of the luxS gene was increased as much as fivefold in the relA mutants, suggesting a link between AI-2 quorum sensing and the stringent response.

* Corresponding author. Mailing address: Department of Oral Biology, University of Florida, College of Dentistry, P.O. Box 100424, Gainesville, FL 32610. Phone: (352) 392-4370. Fax: (352) 392-7357. E-mail: rburne{at}dental.ufl.edu.

Editor: V. J. DiRita

Infection and Immunity, March 2004, p. 1431-1440, Vol. 72, No. 3
0019-9567/04/$08.00+0     DOI: 10.1128/IAI.72.3.1431-1440.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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