K+ modulates genetic competence and the stress regulon of Streptococcus mutans

Bacterial second messenger cyclic di-AMP in streptococci

Cyclic dimeric adenosine monophosphate (c-di-AMP) has been well studied in bacteria, including those of the genus Streptococcus, since the first recognition of this dinucleotide in 2008. Streptococci possess a sole diadenylate cyclase, CdaA, and distinct c-di-AMP phosphodiesterases. Interestingly, cdaA is required for viability of some streptococcal species but not all when streptococci are grown in standard laboratory media. Bacteria of this genus also have distinct c-di-AMP effector proteins, diverse c-di-AMP-signaling pathways, and subsequent biological outcomes. In streptococci, c-di-AMP may influence bacterial growth, morphology, biofilm formation, competence program, drug resistance, and bacterial pathogenesis. c-di-AMP secreted by streptococci has also been shown to interact with the mammalian host and induces immune responses including type I interferon production. In this review, we summarize the reported c-di-AMP networks in seven species of the genus Streptococcus, which cause diverse clinical manifestations, and propose future perspectives to investigate the signaling molecule in these streptococcal pathogens.

Complete Genome Sequence of Streptococcus mutans 27-3, an Active Extracellular Membrane Vesicle Producer

Here, we report the complete genome sequence of Streptococcus mutans 27-3. Isolated from a caries-active patient, 27-3 produces significantly more extracellular membrane vesicles than the commonly used laboratory strain UA159. This study provides useful information for comparative genomic analysis and better understanding of regulation of vesiculogenesis in this bacterium.

The Many Roles of the Bacterial Second Messenger Cyclic di-AMP in Adapting to Stress Cues

Bacteria respond to changes in environmental conditions through adaptation to external cues. Frequently, bacteria employ nucleotide signaling molecules to mediate a specific, rapid response. Cyclic di-AMP (c-di-AMP) was recently discovered to be a bacterial second messenger that is essential for viability in many species. In this review, we highlight recent work that has described the roles of c-di-AMP in bacterial responses to various stress conditions. These studies show that depending on the lifestyle and environmental niche of the bacterial species, the c-di-AMP signaling network results in diverse outcomes, such as regulating osmolyte transport, controlling plant attachment, or providing a checkpoint for spore formation. c-di-AMP achieves this signaling specificity through expression of different classes of synthesis and catabolic enzymes as well as receptor proteins and RNAs, which will be summarized.

Expression of an Extracellular Protein (SMU.63) Is Regulated by SprV in Streptococcus mutans

In Streptococcus mutans, SprV (SMU.2137) is a pleiotropic regulator that differentially regulates genes related to competence, mutacin production, biofilm formation, and the stress tolerance response, along with some other pathways. In this study, we established a link between SprV and an ∼67-kDa protein in the culture supernatant of strain UA159 that was later confirmed as SMU.63 by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis. We discovered that SprV downregulates the transcription and translation of SMU.63. We found that the seven amino acids from the C-terminal region of SprV were also crucial for the expression of SMU.63. Deletion of smu.63 led to increased sucrose-independent biofilm formation and competence. The sprV deletion also increased biofilm formation although this could be partially attributed to the downregulation of smu.63 In an smu.63 sprV double mutant, a synergistic effect was observed in biofilm formation in contrast to effects on competence development. We found that low or excess magnesium ion repressed sprV transcription that, in turn, affected the expression of smu.63 As expected, a magnesium ion-dependent effect of competence and biofilm formation was observed in the UA159 strain. We also replicated the results of SMU.63 expression and competence in S. mutans GS5 that encodes both SprV and SMU.63 homologs and found that the GS5 strain behaves similarly to the UA159 strain, indicating that SprV's effect is strain independent.IMPORTANCE We previously identified a pleiotropic regulator, SprV, in Streptococcus mutans This regulator appears to be highly conserved among streptococci. Here, we showed that SprV regulates the expression of a secreted protein encoded by SMU.63 in S. mutans SMU.63 has been known to impact biofilm formation and genetic competence, two important characteristics that help in colonization of the organism. SMU.63 is also unique since it is known to form amyloid fiber. We found that SprV regulates the expression of SMU.63 at both the transcriptional and translational levels. We also found that the expression of SprV is regulated by magnesium ion concentration. Interestingly, both low and high magnesium ion concentrations affected biofilm formation and genetic competence. Since SMU.63 is also highly conserved among streptococci, we hypothesized that SprV will have a similar effect on its expression.

Bacterial Second Messenger Cyclic di-AMP Modulates the Competence State in Streptococcus pneumoniae

Streptococcus pneumoniae (the pneumococcus) is a naturally competent organism that causes diseases such as pneumonia, otitis media, and bacteremia. The essential bacterial second messenger cyclic di-AMP (c-di-AMP) is an emerging player in the stress responses of many pathogens. In S. pneumoniae, c-di-AMP is produced by a diadenylate cyclase, CdaA, and cleaved by phosphodiesterases Pde1 and Pde2. c-di-AMP binds a transporter of K+ (Trk) family protein, CabP, which subsequently halts K+ uptake via the transporter TrkH. Recently, it was reported that Pde1 and Pde2 are essential for pneumococcal virulence in mouse models of disease. To elucidate c-di-AMP-mediated transcription that may lead to changes in pathogenesis, we compared the transcriptomes of wild-type (WT) and Δpde1 Δpde2 strains by transcriptome sequencing (RNA-Seq) analysis. Notably, we found that many competence-associated genes are significantly upregulated in the Δpde1 Δpde2 strain compared to the WT. These genes play a role in DNA uptake, recombination, and autolysis. Competence is induced by a quorum-sensing mechanism initiated by the secreted factor competence-stimulating peptide (CSP). Surprisingly, the Δpde1 Δpde2 strain exhibited reduced transformation efficiency compared to WT bacteria, which was c-di-AMP dependent. Transformation efficiency was also directly related to the [K+] in the medium, suggesting a link between c-di-AMP function and the pneumococcal competence state. We found that a strain that possesses a V76G variation in CdaA produced less c-di-AMP and was highly susceptible to CSP. Deletion of cabP and trkH restored the growth of these bacteria in medium with CSP. Overall, our study demonstrates a novel role for c-di-AMP in the competence program of S. pneumoniaeIMPORTANCE Genetic competence in bacteria leads to horizontal gene transfer, which can ultimately affect antibiotic resistance, adaptation to stress conditions, and virulence. While the mechanisms of pneumococcal competence signaling cascades have been well characterized, the molecular mechanism behind competence regulation is not fully understood. The bacterial second messenger c-di-AMP has previously been shown to play a role in bacterial physiology and pathogenesis. In this study, we provide compelling evidence for the interplay between c-di-AMP and the pneumococcal competence state. These findings not only attribute a new biological function to this dinucleotide as a regulator of competence, transformation, and survival under stress conditions in pneumococci but also provide new insights into how pneumococcal competence is modulated.

The Biology of Streptococcus mutans

As a major etiological agent of human dental caries, Streptococcus mutans resides primarily in biofilms that form on the tooth surfaces, also known as dental plaque. In addition to caries, S. mutans is responsible for cases of infective endocarditis with a subset of strains being indirectly implicated with the onset of additional extraoral pathologies. During the past 4 decades, functional studies of S. mutans have focused on understanding the molecular mechanisms the organism employs to form robust biofilms on tooth surfaces, to rapidly metabolize a wide variety of carbohydrates obtained from the host diet, and to survive numerous (and frequent) environmental challenges encountered in oral biofilms. In these areas of research, S. mutans has served as a model organism for ground-breaking new discoveries that have, at times, challenged long-standing dogmas based on bacterial paradigms such as Escherichia coli and Bacillus subtilis. In addition to sections dedicated to carbohydrate metabolism, biofilm formation, and stress responses, this article discusses newer developments in S. mutans biology research, namely, how S. mutans interspecies and cross-kingdom interactions dictate the development and pathogenic potential of oral biofilms and how next-generation sequencing technologies have led to a much better understanding of the physiology and diversity of S. mutans as a species.