The Adc regulon mediates zinc homeostasis in Streptococcus mutans

Inhibition of Streptococcus mutans growth and biofilm formation through protein acetylation

Numerous cellular processes are regulated in response to the metabolic state of the cell, and one such regulatory mechanism involves lysine acetylation. Lysine acetylation has been proven to play an important role in the virulence of Streptococcus mutans, a major cariogenic bacterial species. S. mutans' glucosyltransferases (Gtfs) are responsible for synthesizing extracellular polysaccharides (EPS) and contributing to biofilm formation. One of the most common nonsteroidal anti-inflammatory drugs is acetylsalicylic acid (ASA), which can acetylate proteins through a nonenzymatic transacetylation reaction. Herein, we investigated the inhibitory effects of ASA on S. mutans. ASA treatment was observed to impede the growth of S. mutans, leading to a reduction in the production of water-insoluble EPS and the formation of biofilm. Moreover, ASA decreased the enzyme activity of Gtfs while increasing the protein acetylation level. The in vivo anticaries efficacy of ASA has further been proved using the rat caries model. In conclusion, ASA as an acetylation agent attenuated the cariogenic virulence of S. mutans, suggesting the potential value of protein acetylation on antimicrobial and anti-biofilm applications to S. mutans.

Competitive dynamics and balance between Streptococcus mutans and commensal streptococci in oral microecology

Dental caries, as a biofilm-related disease, is closely linked to dysbiosis in microbial ecology within dental biofilms. Beyond its impact on oral health, bacteria within the oral cavity pose systemic health risks by potentially entering the bloodstream, thereby increasing susceptibility to bacterial endocarditis, among other related diseases. Streptococcus mutans, a principal cariogenic bacterium, possesses virulence factors crucial to the pathogenesis of dental caries. Its ability to adhere to tooth surfaces, produce glucans for biofilm formation, and metabolize sugars into lactic acid contributes to enamel demineralization and the initiation of carious lesions. Its aciduricity and ability to produce bacteriocins enable a competitive advantage, allowing it to thrive in acidic environments and dominate in changing oral microenvironments. In contrast, commensal streptococci, such as Streptococcus sanguinis, Streptococcus gordonii, and Streptococcus salivarius, act as primary colonizers and compete with S. mutans for adherence sites and nutrients during biofilm formation. This competition involves the production of alkali, peroxides, and antibacterial substances, thereby inhibiting S. mutans growth and maintaining microbial balance. This dynamic interaction influences the balance of oral microbiota, with disruptions leading to shifts in microbial composition that are marked by rapid increases in S. mutans abundance, contributing to the onset of dental caries. Thus, understanding the dynamic interactions between commensal and pathogenic bacteria in oral microecology is important for developing effective strategies to promote oral health and prevent dental caries. This review highlights the roles and competitive interactions of commensal bacteria and S. mutans in oral microecology, emphasizing the importance of maintaining oral microbial balance for health, and discusses the pathological implications of perturbations in this balance.

Exopolysaccharides from agriculturally important microorganisms: Conferring soil nutrient status and plant health

A wide variety of microorganisms secretes extracellular polymeric substances or commonly known as exopolysaccharides (EPS), which have been studied to influence plant growth via various mechanisms. EPS-producing microorganisms have been found to have positive effects on plant health such as by facilitating nutrient entrapment in the soil, or by improving soil quality, especially by helping in mitigating various abiotic stress conditions. The various types of microbial polysaccharides allow for the compartmentalization of the microbial community enabling them to endure undressing stress conditions. With the growing population, there is a constant need for developing sustainable agriculture where we could use various PGPR to help the plant cope with various stress conditions and simultaneously enhance the crop yield. These polysaccharides have also found application in various sectors, especially in the biomedical fields, manifesting their potential to act as antitumor drugs, play a significant role in immune evasion, and reveal various therapeutic potentials. These constitute high levels of bioactive polysaccharides which possess a wide range of implementation starting from industrial applications to novel food applications. In this current review, we aim at presenting a comprehensive study of how these microbial extracellular polymeric substances influence agricultural productivity along with their other commercial applications.

SMU_1361c regulates the oxidative stress response of Streptococcus mutans

Dental caries is the most common chronic infectious disease around the world and disproportionately affects the marginalized socioeconomic group. Streptococcus mutans, considered a primary etiological agent of caries, depends on the coordinated physiological response to tolerate the oxidative stress generated by commensal species within dental plaque, which is a critical aspect of its pathogenicity. Here, we identified and characterized a novel tetracycline repressor family regulator, SMU_1361c, which appears to be acquired by the bacteria via horizontal gene transfer. Surprisingly, smu_1361c functions as a negative transcriptional regulator to regulate gene expression outside its operon and is involved in the oxidative stress response of S. mutans. The smu_1361c overexpression strain UA159/pDL278-1361c was more susceptible to oxidative stress and less competitive against hydrogen peroxide generated by commensal species Streptococcus gordonii and Streptococcus sanguinis. Transcriptomics analysis revealed that smu_1361c overexpression resulted in the significant downregulation of 22 genes, mainly belonging to three gene clusters responsible for the oxidative stress response. The conversed DNA binding motif of SMU_1361c was determined by electrophoretic mobility shift and DNase I footprinting assay with purified SMU_1361c protein; therefore, smu_1361c is directly involved in gene transcription related to the oxidative stress response. Crucially, our finding provides a new understanding of how S. mutans deals with the oxidative stress that is required for pathogenesis and will facilitate the development of new and improved therapeutic approaches for dental caries.IMPORTANCEStreptococcus mutans is the major organism associated with the development of dental caries, which globally is the most common chronic disease. To persist and survive in biofilms, S. mutans must compete with commensal species that occupy the same ecological niche. Here, we uncover a novel molecular mechanism of how tetracycline repressor family regulator smu_1361c is involved in the oxidative stress response through transcriptomics analysis, electrophoretic mobility shift assay, and DNase I footprinting assay. Furthermore, we demonstrated that smu_1361c mediates S. mutans sensitivity to oxidative stress and competitiveness with commensal streptococci. Therefore, this study has revealed a previously unknown regulation between smu_1361c and genes outside its operon and demonstrated the importance of smu_1361c in the oxidative stress response and the fitness of S. mutans within the plaque biofilms, which can be exploited as a new therapy to modulate ecological homeostasis and prevent dental caries.

ZrgA contributes to zinc acquisition in Vibrio parahaemolyticus

Metals are nutrients essential for almost all lifeforms. Bacteria have evolved several mechanisms to overcome the metal restrictions imposed by the host. Vibrio parahaemolyticus causes severe threats to public health and significant economic losses in shrimp aquaculture. Herein, we report that ZrgA contributes to zinc acquisition in this pathogen. The operon VP_RS01455 to VP_RS01475 of V. parahaemolyticus encodes the putative Zn transporter ZrgABCDE, whose homologs are widely distributed in Vibrionaceae. RNA sequencing analysis revealed that V. parahaemolyticus modulates the transcriptome in response to Zn limitation. Genes in the Zinc uptake regulator (Zur) regulon are upregulated during Zn limitation, including three genes annotated to encode Zn-binding proteins. Significant upregulation of these three genes during Zn limitation was also confirmed by quantitative real-time PCR (qRT-PCR) analysis. However, only the mutants containing a VP_RS01470 (zrgA) deletion exhibited impaired growth under Zn-deficient conditions, indicating that VP_RS01470 plays the predominant role in V. parahaemolyticus Zn acquisition. The VP_RS01470 deletion mutant displayed a false appearance of decreased swimming motility under Zn-deficient conditions, as revealed by the fact that the polar flagellar-related genes were not downregulated in the mutant. Moreover, VP_RS01470 deletion produced no noticeable impact on the swarming motility and virulence in mice. qRT-PCR analysis and β-galactosidase activity assays indicated that Zur negatively regulates VP_RS01470 expression in V. parahaemolyticus. Collectively, our findings suggest that ZrgA is required for Zn acquisition in V. parahaemolyticus and highlight the importance of detecting the expression of flagellar genes during analysis of motility of a mutant deficient in growth.

Zn2+ regulates human oxalate metabolism by manipulating oxalate decarboxylase to treat calcium oxalate stones

A high concentration of oxalate is associated with an increased risk of kidney calcium oxalate (CaOx) stones, and the degradation of exogenous oxalate mostly depends on oxalate-degrading enzymes from the intestinal microbiome. We found that zinc gluconate supplement to patients with CaOx kidney stones could significantly improve the abundance of oxalate metabolizing bacteria in humans through clinical experiments on patients also subjected to antibiotic treatment. The analysis of clinical samples revealed that an imbalance of Lactobacillus and oxalate decarboxylase (OxDC) was involved in the formation of CaOx kidney stones. Then, we identified that Zn²⁺ could be used as an external factor to improve the activity of OxDC and promote Lactobacillus in the intestinal flora, and this treatment achieved a therapeutic effect on rats with stones aggravated by antibiotics. Finally, by analyzing the three-dimensional structure of OxDC and completing in vitro experiments, we propose a model of the Zn²⁺-induced reduction of CaOx kidney stone symptoms in rats by increasing the metabolism of oxalate through the positive effects of Zn²⁺ on Lactobacillus and OxDC.

ZccE, a P-type ATPase contributing to biofilm formation and competitiveness in Streptococcus mutans

Most living organisms require zinc for survival; however, excessive amounts of this trace element can be toxic. Therefore, the frequent fluctuations of salivary zinc, caused by the low physiological level and the frequent introduction of exogenous zinc ions, present a serious challenge for bacteria colonizing the oral cavity. Streptococcus mutans is considered one of the main bacterial pathobiont in dental caries. Here, we verified the role of a P-type ATPase ZccE as the main zinc-exporting transporter in S. mutans and delineated the effects of zinc toxification caused by zccE deletion in the physiology of this bacterium. The deletion of the gene zccE severely impaired the ability of S. mutans to grow under high zinc stress conditions. Intracellular metal quantification using inductively coupled plasma optical emission spectrometer revealed that the zccE mutant exhibited approximately two times higher zinc accumulation than the wild type when grown in the presence of a subinhibitory zinc concentration. Biofilm formation analysis revealed less single-strain biofilm formation and competitive weakness in the dual-species biofilm formed with Streptococcus sanguinis for zccE mutant under high zinc stress. The quantitive reverse transcription polymerase chain reaction test revealed decreased expressions of gtfB, gtfC, and nlmC in the mutant strain under excessive zinc treatment. Collectively, these findings suggest that ZccE plays an important role in the zinc detoxification of S. mutans and that zinc is a growth-limiting factor for S. mutans within the dental biofilm.

Acetylation of Lactate Dehydrogenase Negatively Regulates the Acidogenicity of Streptococcus mutans

Lysine acetylation, a ubiquitous and dynamic regulatory posttranslational modification (PTM), affects hundreds of proteins across all domains of life. In bacteria, lysine acetylation can be found in many essential pathways, and it is also crucial for bacterial virulence. However, the biological significance of lysine acetylation events to bacterial virulence factors remains poorly characterized. In Streptococcus mutans, the acetylome profiles help identify several lysine acetylation sites of lactate dehydrogenase (LDH), which catalyzes the conversion of pyruvate to lactic acid, causing the deterioration of teeth. We investigated the regulatory mechanism of LDH acetylation and characterized the effect of LDH acetylation on its function. We overexpressed the 15 Gcn5 N-acetyltransferases (GNAT) family members in S. mutans and showed that the acetyltransferase ActA impaired its acidogenicity by acetylating LDH. Additionally, enzymatic acetyltransferase reactions demonstrated that purified ActA could acetylate LDH in vitro, and 10 potential lysine acetylation sites of LDH were identified by mass spectrometry, 70% of which were also detected in vivo. We further demonstrated that the lysine acetylation of LDH inhibited its enzymatic activity, and a subsequent rat caries model showed that ActA impaired the cariogenicity of S. mutans. Collectively, we demonstrated that ActA, the first identified and characterized acetyltransferase in S. mutans, acetylated the LDH enzymatically and inhibited its enzymatic activity, thereby providing a starting point for the further analysis of the biological significance of lysine acetylation in the virulence of S. mutans.

Metal Homeostasis in Pathogenic Streptococci

Streptococcus spp. are an important genus of Gram-positive bacteria, many of which are opportunistic pathogens that are capable of causing invasive disease in a wide range of populations. Metals, especially transition metal ions, are an essential nutrient for all organisms. Therefore, to survive across dynamic host environments, Streptococci have evolved complex systems to withstand metal stress and maintain metal homeostasis, especially during colonization and infection. There are many different types of transport systems that are used by bacteria to import or export metals that can be highly specific or promiscuous. Focusing on the most well studied transition metals of zinc, manganese, iron, nickel, and copper, this review aims to summarize the current knowledge of metal homeostasis in pathogenic Streptococci, and their role in virulence.

Expert consensus on early childhood caries management

Early childhood caries (ECC) is a significant chronic disease of childhood and a rising public health burden worldwide. ECC may cause a higher risk of new caries lesions in both primary and permanent dentition, affecting lifelong oral health. The occurrence of ECC has been closely related to the core microbiome change in the oral cavity, which may be influenced by diet habits, oral health management, fluoride use, and dental manipulations. So, it is essential to improve parental oral health and awareness of health care, to establish a dental home at the early stage of childhood, and make an individualized caries management plan. Dental interventions according to the minimally invasive concept should be carried out to treat dental caries. This expert consensus mainly discusses the etiology of ECC, caries-risk assessment of children, prevention and treatment plan of ECC, aiming to achieve lifelong oral health.

The AdcR-regulated AdcA and AdcAII contribute additively to zinc acquisition and virulence in Streptococcus suis

Metals are necessary elements for bacteria. Typically, vertebrate hosts restrict invading bacterial pathogens from accessing metals. Therefore, bacteria have evolved high-affinity metal importers to acquire metals. Streptococcus suis is a major swine pathogen and an emerging zoonotic agent that endangers the swine industry and human health worldwide. Herein, we aimed to identify the zinc acquisition systems in S. suis and evaluate their roles in bacterial virulence. Bioinformatic analyses revealed that S. suis encodes homologues of AdcA and AdcAII, two well-characterised Zn-binding lipoproteins in certain streptococci. Quantitative reverse transcription PCR (qRT-PCR) analysis revealed that the expressions of adcA and adcAII were significantly upregulated in response to Zn limitation, with a higher expression level of adcAII than adcA. Gene deletion mutants and complementation strains were constructed; their growth characteristics under Zn-deficient and Zn-replete conditions indicated that AdcA and AdcAII have overlapping functionality in Zn acquisition. A mouse infection model was used to evaluate the roles of AdcA and AdcAII in S. suis virulence. Mice infected with the double mutant ΔadcAΔadcAII exhibited a significantly higher survival rate, decreased bacterial burden, and lower production of inflammatory cytokines compared to those infected with the wild type (WT) strain. Furthermore, ΔadcAΔadcAII showed reduced competitiveness in infection establishment compared with the WT strain. RNA sequencing, qRT-PCR, and electrophoretic mobility shift assays revealed that AdcR negatively regulates the expressions of adcA and adcAII. Collectively, our results demonstrated that AdcA and AdcAII, which are negatively regulated by AdcR, contribute additively to zinc acquisition and virulence in S. suis.

Acetylation of glucosyltransferases regulates Streptococcus mutans biofilm formation and virulence

Lysine acetylation is a frequently occurring post-translational modification (PTM), emerging as an important metabolic regulatory mechanism in prokaryotes. This process is achieved enzymatically by the protein acetyltransferase (KAT) to specifically transfer the acetyl group, or non-enzymatically by direct intermediates (acetyl phosphate or acetyl-CoA). Although lysine acetylation modification of glucosyltransferases (Gtfs), the important virulence factor in Streptococcus mutans, was reported in our previous study, the KAT has not been identified. Here, we believe that the KAT ActG can acetylate Gtfs in the enzymatic mechanism. By overexpressing 15 KATs in S. mutans, the synthesized water-insoluble extracellular polysaccharides (EPS) and biofilm biomass were measured, and KAT (actG) was identified. The in-frame deletion mutant of actG was constructed to validate the function of actG. The results showed that actG could negatively regulate the water-insoluble EPS synthesis and biofilm formation. We used mass spectrometry (MS) to identify GtfB and GtfC as the possible substrates of ActG. This was also demonstrated by in vitro acetylation assays, indicating that ActG could increase the acetylation levels of GtfB and GtfC enzymatically and decrease their activities. We further found that the expression level of actG in part explained the virulence differences in clinically isolated strains. Moreover, overexpression of actG in S. mutans attenuated its cariogenicity in the rat caries model. Taken together, our study demonstrated that the KAT ActG could induce the acetylation of GtfB and GtfC enzymatically in S. mutans, providing insights into the function of lysine acetylation in bacterial virulence and pathogenicity.

Lysine acetylation is a regulatory post-translational modification (PTM) important in physiological processes across all domains of life. Although it has been well studied and characterized in eukaryotes, new insights into the lysine acetylation in bacteria have gained momentum in recent years, and hundreds to thousands of protein acetylation processes have been identified in various bacteria with novel enrichment strategies. However, the specific mechanisms of regulating lysine acetylation and function are still poorly understood. Therefore, we screened for the KAT mediating Gtfs acetylation by constructing 15 strains of S. mutans that overexpressed the GCN5-related N-acetyltransferases (GNAT) family members. Eventually, we identified and characterized ActG, a GNAT family member, that could catalyze the acetylation of GtfB and GtfC in S. mutans by the enzymatic mechanism, inversely related to their enzymatic activities, subsequently affecting the water-insoluble EPS synthesis and biofilm formation. In addition, ActG impaired the cariogenicity of S. mutans in a rat caries model. Thus, this study provides significant insights into the effect of lysine acetylation on S. mutans virulence and pathogenicity by regulating target protein functions and relative physiological processes.

Deletion of the yqeK gene leads to the accumulation of Ap4A and reduced biofilm formation in Streptococcus mutans

Diadenosine-5',5'''-P1, P4-tetraphosphate (Ap4A) is a second messenger playing a crucial role in various life activities of bacteria. The increase of Ap4A expression is pleiotropic, resulting in an impairment in the formation of biofilm and other physiological functions in some bacteria. However, Ap4A function in Streptococcus mutans, an important pathogen related to dental caries, remains unknown. In this work, the Ap4A hydrolase, YqeK, was identified and characterized in S. mutans. Then, the effects of yqeK deletion on the growth, biofilm formation, and exopolysaccharide (EPS) quantification in S. mutans were determined by the assessment of the growth curve, crystal violet, and anthrone-sulfuric acid, respectively, and visualized by microscopy. The results showed that the in-frame deletion of the yqeK gene in S. mutans UA159 led to an increase in Ap4A levels, lag phase in the early growth, as well as decrease in biofilm formation and water-insoluble exopolysaccharide production. Global gene expression profile showed that the expression of 88 genes was changed in the yqeK mutant, and among these, 42 were upregulated and 46 were downregulated when compared with the wild-type S. mutans UA159. Upregulated genes were mainly involved in post-translational modification, protein turnover, and chaperones, while downregulated genes were mainly involved in carbohydrate transport and metabolism. Important virulence genes related to biofilms, such as gtfB, gtfC, and gbpC, were also significantly downregulated. In conclusion, these results indicated that YqeK affected the formation of biofilms and the expression of biofilm-related genes in S. mutans.