Periodontal pathogens interfere with quorum-sensing-dependent virulence properties in Streptococcus mutans

Periodontal diseases are among the most common chronic infectious diseases occurring in humans. According to the National Institute of Dental and Craniofacial Research, periodontal diseases affect the majority of the American population (1). Oral chronic infectious diseases not only lead to tooth loss but may also act as a risk factor for systemic diseases such as cardiovascular diseases, respiratory diseases, and pre-term birth (2-4). Quorum sensing involves the control of gene expression in response to cell density when a minimum population unit, or quorum, is reached (5). In streptococci such as Streptococcus mutans, one of the quorum sensing signaling systems depends on a competence-stimulating peptide (CSP, encoded by the comC gene) and a two-component signal transduction system which is encoded by the comD and comE genes, corresponding to a histidine protein kinase (receptor for CSP) and a response regulator, respectively (6-8). Quorum sensing systems modulate a variety of virulence activities in S. mutans. Disruption of the Com quorum sensing system in S. mutans results in attenuated virulence activities such as biofilm formation (9, 10), acid tolerance (11), bacteriocin production (12) and genetic competence (13, 14). In addition, Matsumoto-Nakano and Kuramitsu (15) have recently demonstrated that the comC gene of strain GS5 also modulates the sensitivity of S. mutans to a variety of antimicrobial agents such as triclosan, fluoride, antibiotics and histatin-5. Therefore, CSP represents a potential target for control of S. mutans infection. Some bacteria such as S. mutans and Streptococcus gordonii are naturally transformable, being able to take up naked DNA from the extracellular environment (16). Homologous recombination of foreign DNA into the host chromosome following transformation is believed to play a major role in the evolution of bacteria. This was demonstrated by both the rapid emergence of penicillin resistance following the acquisition of low-affinity penicillin binding proteins (17) and evidence for the occurrence of frequent recombination events in the evolution of virulence factors in Streptococcus pneumoniae (18, 19). Experimental evidence also has shown that oral bacteria such as S. gordonii can take up free extracellular DNA from saliva in vitro (20). Furthermore, transformation of bacteria has been demonstrated after infection in animal hosts (21). Such events suggest that the expanded capacities conferred by acquired genes could result in more virulent bacterial species. Since bacteria can acquire resistance to compromising environments by taking up extracellular DNA, the ability to transfer genetic material among bacteria should be considered as a possible virulence attribute of bacteria involved in caries and periodontal disease. The production of bacteriocins by microorganisms is also one of the important mechanisms used by bacteria for antagonistic interference (22). Although these peptide molecules are not required for growth, they may help the microorganisms that produce them to compete for the limited nutrients in their environment (23). Some late colonizers of dental plaque such as Porphyromonas gingivalis and Treponema denticola have been implicated as pathogens causing chronic periodontitis (24, 25). Colonization by these pathogens can result in a proportional decrease in the population of early colonizers such as streptococci leading to the domination of sites of periodontal diseases by the former (24). How these periodontal pathogens dominate in dental plaque is not yet understood. One possibility is that these late colonizers antagonize the quorum sensing of the early colonizers and therefore render the early colonizers less virulent and less resistant to endogenous antimicrobial agents such as histatins, peroxide, and lysozyme. The objective of the present study was to determine if these late colonizers interfere with quorum sensing in S. mutans. The Com quorum-sensing system has been identified in several early colonizers of dental plaque (8, 26, 27) and may be generalized targets for bacterial antagonism by late colonizers. In this study, we used S. mutans, a well-documented oral streptococcus with many CSP-dependent quorum-sensing properties, as an example of an earlier colonizer which is antagonized by periodontal pathogens such as P. gingivalis and T. denticola.