Bacteria in dental plaque experience a wide range of stresses. The intermittent ingestion of food by the host results in dramatic changes in nutrient availability and pH, but significant variability in oxygen tension and osmolality are also observed (Lemos et al., 2005). Although it has been noted that oral biofilms experience a “feast or famine” lifestyle, organisms residing in the oral cavity are really not exposed to severely-oligotrophic environments. For this reason, studies of the stress responses of S. mutans have served as an excellent model to reveal critical differences in the ways that obligately host-associated bacteria cope with environmental stresses when compared with bacteria that have both free-living and host-associated lifestyles. These studies have revealed important contrasts in the way this organism copes with stresses and uses stress regulons/enzymes to coordinate virulence and survival compared to more widely-studied bacterial paradigms (Fig. 1).

The membrane-bound F-ATPase (H⁺-translocating ATPase) is considered the primary determinant of acid tolerance of S. mutans because it allows the organism to maintain a cytoplasmic pH that is more alkaline than the extracellular environment (Lemos et al., 2005). Work from the laboratory of Marquis correlates acid tolerance in oral streptococci with the pH optima and absolute level of activity of the F₁F_O-ATPase (Bender et al., 1986). Moreover, it was recently demonstrated that the F-ATPase of S. mutans, and other oral bacteria, can function as an ATP synthase in starved cells grown at low pH (Sheng & Marquis, 2006). The authors demonstrated that in starved cells, a sudden drop in pH result in a rapid increase in ATP, followed by a rapid loss, that enhance protection against acid killing. By using specific inhibitors of F-ATPase, the authors were able to demonstrate that this increase in ATP comes from the enzyme acting as an ATP synthase. Thus, the F-ATPase may play a dual role in acid tolerance - extruding protons out of the cells and, under certain conditions, generating ATP for growth and persistence (Fig. 2).

The nutritional alarmone (p)ppGpp also appears to play an important role in orchestrating an appropriate response to multiple environmental and physiologic inputs that S. mutans encounters in the oral cavity (Fig. 3). When limited for essential amino acids, bacteria accumulate (p)ppGpp by enzymatic phosphorylation of GDP and GTP, resulting in down-regulation of genes for macromolecular biosynthesis and up-regulation of genes for amino acid biosynthesis and stress tolerance. In Gram-positive bacteria (GPB), RelA is a bi-functional enzyme with potent (p)ppGpp-synthetic and -degradative activities. In S. mutans, RelA was shown to play major roles in the regulation of phenotypic traits that are required for establishment, persistence and survival (Lemos et al., 2004; Nascimento et al., 2008), further supporting an overlap between circuits that govern nutrient starvation, general stress tolerance and biofilm fomation. Until recently, RelA was considered the sole enzyme responsible for synthesis and degradation of (p)ppGpp in Gram-positive bacteria. However, our group recently identified two novel enzymes, designated RelP and RelQ, with (p)ppGpp-synthase activities in S. mutans that could be found in a number of related GPB (Lemos et al., 2007a). A relAPQ triple mutant was auxotrophic for the branched-chain amino acids (BCAA) leucine and valine, but not isoleucine, a phenotype that was directly related to CodY-dependent repression of genes involved in the synthesis of BCAA (Lemos et al., 2008). As mentioned above, RelP is co-transcribed with, and apparently regulated by, the RelRS TCS (Lemos et al., 2007a) suggesting that S. mutans may use environmental signals to optimize cell growth and survival in a manner that allows the organism to balance growth during dietary intake by the host with the capacity to rapidly mount an adaptive response during fasting periods. Consistent with the role of (p)ppGpp in bacteria, homologues of RelRS in S. pyogenes, designated SptRS, were shown to be critical for this bacterium to survive in saliva (Shelburne et al., 2005).