Effect of acid shock on protein expression by biofilm cells of Streptococcus mutans

Small RNA SmsR1 modulates acidogenicity and cariogenic virulence by affecting protein acetylation in Streptococcus mutans

Post-transcriptional regulation by small RNAs and post-translational modifications (PTM) such as lysine acetylation play fundamental roles in physiological circuits, offering rapid responses to environmental signals with low energy consumption. Yet, the interplay between these regulatory systems remains underexplored. Here, we unveil the cross-talk between sRNAs and lysine acetylation in Streptococcus mutans, a primary cariogenic pathogen known for its potent acidogenic virulence. Through systematic overexpression of sRNAs in S. mutans, we identified sRNA SmsR1 as a critical player in modulating acidogenicity, a key cariogenic virulence feature in S. mutans. Furthermore, combined with the analysis of predicted target mRNA and transcriptome results, potential target genes were identified and experimentally verified. A direct interaction between SmsR1 and 5'-UTR region of pdhC gene was determined by in vitro binding assays. Importantly, we found that overexpression of SmsR1 reduced the expression of pdhC mRNA and increased the intracellular concentration of acetyl-CoA, resulting in global changes in protein acetylation levels. This was verified by acetyl-proteomics in S. mutans, along with an increase in acetylation level and decreased activity of LDH. Our study unravels a novel regulatory paradigm where sRNA bridges post-transcriptional regulation with post-translational modification, underscoring bacterial adeptness in fine-tuning responses to environmental stress.

Limosilactobacillus reuteri inhibits the acid tolerance response in oral bacteria

Probiotic bacteria show promising results in prevention of the biofilm-mediated disease caries, but the mechanisms are not fully understood. The acid tolerance response (ATR) allows biofilm bacteria to survive and metabolize at low pH resulting from microbial carbohydrate fermentation. We have studied the effect of probiotic strains: Limosilactobacillus reuteri and Lacticaseibacillus rhamnosus on ATR induction in common oral bacteria. Communities of L. reuteri ATCC PTA5289 and Streptoccus gordonii, Streptococcus oralis, Streptococcus mutans or Actinomyces naeslundii in the initial stages of biofilm formation were exposed to pH 5.5 to allow ATR induction, followed by a low pH challenge. Acid tolerance was evaluated as viable cells after staining with LIVE/DEAD®BacLight™. The presence of L. reuteri ATCC PTA5289 caused a significant reduction in acid tolerance in all strains except S. oralis. When S. mutans was used as a model organism to study the effects of additional probiotic strains (L. reuteri SD2112, L. reuteri DSM17938 or L. rhamnosus GG) as well as L. reuteri ATCC PTA5289 supernatant on ATR development, neither the other probiotic strains nor supernatants showed any effect. The presence of L. reuteri ATCC PTA5289 during ATR induction led to down-regulation of three key genes involved in tolerance of acid stress (luxS, brpA and ldh) in Streptococci. These data suggest that live cells of probiotic L. reuteri ATCC PTA5289 can interfere with ATR development in common oral bacteria and specific strains of L. reuteri may thus have a role in caries prevention by inhibiting development of an acid-tolerant biofilm microbiota.

TNF-α, IL-1B and IL-6 affect the differentiation ability of dental pulp stem cells

BACKGROUND

This in vitro study examined the effect of the inflammatory cytokines (tumour necrosis factor-α (TNF-α), interleukin (IL)-1β, and IL-6) on osteogenic, chondrogenic, and adipogenic differentiation of dental pulp stem cells (DPSCs) which have significant relevance in future regenerative therapies.

METHODS

DPSCs were isolated from the impacted third molar dental pulp and determined with flow cytometry analysis. DPSCs were divided into into 5 main groups with 3 subdivisions for each group making a total of 15 groups. Experimental groups were stimulated with TNF-α, IL-1β, IL-6, and a combination of all three to undergo osteogenic, chondrogenic, and adipogenic differentiation protocols. Next, the differentiation of each group was examined with different staining procedures under a light microscope. Histological analysis of osteogenic, chondrogenic, and adipogenic differentiated pellets was assessed using a modified Bern score. Statistical significance determined using one-way analysis of variance, and correlations were assessed using Pearson's test (two-tailed).

RESULTS

Stimulation with inflammatory cytokines significantly inhibited the osteogenic, chondrogenic and adipogenic differentiation of DPSCs in terms of matrix and cell formation resulting in weak staining than the unstimulated groups with inflammatory cytokines. On contrary, the unstimulated groups of MSCs have shown to be highly proliferative ability in terms of osteogenic, chondrogenic, and adipogenic differentiation.

CONCLUSIONS

DPSCs have high osteogenic, chondrogenic, and adipogenic differentiation capabilities. Pretreatment with inflammatory cytokines decreases the differentiation ability in vitro, thus inhibiting tissue formation.

Exopolysaccharides metabolism and cariogenesis of Streptococcus mutans biofilm regulated by antisense vicK RNA

Background

Streptococcus mutans (S. mutans) is a pivotal cariogenic pathogen contributing to its multiple virulence factors, one of which is synthesizing exopolysaccharides (EPS). VicK, a sensor histidine kinase, plays a major role in regulating genes associated with EPS synthesis and adhesion. Here we first identified an antisense vicK RNA (ASvicK) bound with vicK into double-stranded RNA (dsRNA).

Objective

This study aims to investigate the effect and mechanism of ASvicK in the EPS metabolism and cariogenesis of S. mutans.

Methods

The phenotypes of biofilm were detected by scanning electron microscopy (SEM), gas chromatography-mass spectrometery (GC-MS) , gel permeation chromatography (GPC) , transcriptome analysis and Western blot. Co-immunoprecipitation (Co-ip) assay and enzyme activity experiment were adopted to investigate the mechanism of ASvicK regulation. Caries animal models were developed to study the relationship between ASvicK and cariogenicity of S. mutans.

Results

Overexpression of ASvicK can inhibit the growth of biofilm, reduce the production of EPS and alter genes and protein related to EPS metabolism. ASvicK can adsorb RNase III to regulate vicK and affect the cariogenicity of S. mutans.

Conclusions

ASvicK regulates vicK at the transcriptional and post-transcriptional levels, effectively inhibits EPS synthesis and biofilm formation and reduces its cariogenicity in vivo.

Antiplaque Efficacy of a Novel Moringa oleifera Dentifrice: A Randomized Clinical Crossover Study

OBJECTIVES

 The use of herbal dentifrices has grown exponentially over the years. They are categorically referred to as ethnomedicines. Various agents have been tried with contradicting findings based on phytopharmacological analysis. Miswak is one agent which has been used over the years. A novel Moringa oleifera-based dentifrice has shown promising results in terms of its cytotoxicity, biocompatibility, and as a potent anti-inflammatory agent. Therefore, the present study aims to compare the efficacy of two commercially available miswak- and moringa-based herbal dentifrices on the reduction of plaque and gingivitis scores.

MATERIALS AND METHODS

 This randomized clinical crossover study included 20 subjects with mild to moderate gingivitis. The study was conducted over a total examination period of 20 days with a wash-out period of 2 weeks between the use of both the toothpastes. The plaque index and gingival index of the study subjects were recorded at the designated time intervals throughout the study period.

STATISTICAL ANALYSIS

 The data collected were entered on Microsoft Excel, and statistical analysis using SPSS software (SPSS version 28, IBM Corp, Armonk, New York, United States) was done. The statistical test used was the Wilcoxon signed-rank test. Moreover, p ≤0.05 was considered significant.

RESULTS

 The results showed that the reduction in mean gingival index scores from baseline to day 3 was more statistically significant in the moringa-based dentifrice. Similarly, the plaque index scores showed statistically significant reduction following the use of the moringa-based dentifrice when compared with the miswak dentifrice. This study reveals that the moringa dentifrice is a safe and effective agent in reducing plaque accumulation and treating gingival inflammation.

CONCLUSION

 The current study aims to provide an insight into the possible role of moringa dentifrice as a possible adjunctive oral hygiene aid.

pdh modulate virulence through reducing stress tolerance and biofilm formation of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (S. suis 2) is a zoonotic pathogen. It causes meningitis, arthritis, pneumonia and sepsis in pigs, leading to extremely high mortality, which seriously affects public health and the development of the pig industry. Pyruvate dehydrogenase (PDH) is an important sugar metabolism enzyme that is widely present in microorganisms, mammals and higher plants. It catalyzes the irreversible oxidative decarboxylation of pyruvate to acetyl-CoA and reduces NAD+ to NADH. In this study, we found that the virulence of the S. suis ZY05719 sequence type 7 pdh deletion strain (Δpdh) was significantly lower than the wild-type strain (WT) in the mouse infection model. The distribution of viable bacteria in the blood and organs of mice infected with the Δpdh was significantly lower than those infected with WT. Bacterial survival rates were reduced in response to temperature stress, salt stress and oxidative stress. Additionally, compared to WT, the ability to adhere to and invade PK15 cells, biofilm formation and stress resistance of Δpdh were significantly reduced. Moreover, real-time PCR results showed that pdh deletion reduced the expression of multiple adhesion-related genes. However, there was no significant difference in the correlation biological analysis between the complemented strain (CΔpdh) and WT. Moreover, the survival rate of Δpdh in RAW264.7 macrophages was significantly lower than that of the WT strain. This study shows that PDH is involved in the pathogenesis of S. suis 2 and reduction in virulence of Δpdh may be related to the decreased ability to resist stress of the strain.

Relationship between Pyruvate Kinase Activity and Cariogenic Biofilm Formation in Streptococcus mutans Biotypes in Caries Patients

Streptococcus mutans (MS) and its biotype I are the strains most frequently found in dental plaque of young children. Our results indicate that in children pyruvate kinase (PK) activity increases significantly in dental plaque, and this corresponds with caries progression. The MS strains isolated in this study or their main glycolytic metabolism connected with PK enzymes might be useful risk factors for studying the pathogenesis and target points of novel therapies for dental caries. The relationship between PK activity, cariogenic biofilm formation and selected biotypes occurrence was studied. S. mutans dental plaque samples were collected from supragingival plaque of individual deciduous molars in 143 subjects. PK activity was measured at different time points during biofilm formation. Patients were divided into two groups: initial stage decay, and extensive decay. Non-parametric analysis of variance and analysis of covariance were used to determine the connections between S. mutans levels, PK activity and dental caries biotypes. A total of 143 strains were derived from subjects with caries. Biotyping data showed that 62, 23, 50, and 8 strains were classified as biotypes I, II, III, IV, respectively. PK activity in biotypes I, II, and IV was significantly higher in comparison to that in biotype III. The correlation between the level of S. mutans in dental plaque and PK activity was both statistically significant (p < 0.05) and positive. The greater the level of S. mutans in the biofilm (colony count and total biomass), the higher the PK activity; similarly, a low bacterial count correlated with low PK activity.

Effect of the Biofilm Age and Starvation on Acid Tolerance of Biofilm Formed by Streptococcus mutans Isolated from Caries-Active and Caries-Free Adults

Streptococcus mutans (S. mutans) is considered a leading cause of dental caries. The capability of S. mutans to tolerate low pH is essential for its cariogenicity. Aciduricity of S. mutans is linked to its adaptation to environmental stress in oral cavity. This study aimed to investigate the effect of biofilm age and starvation condition on acid tolerance of biofilm formed by S. mutans clinical isolates. S. mutans clinical strains isolated from caries-active (SM593) and caries-free (SM18) adults and a reference strain (ATCC25175) were used for biofilm formation. (1) Both young and mature biofilms were formed and then exposed to pH 3.0 for 30 min with (acid-adapted group) or without (non-adapted group) pre-exposure to pH 5.5 for three hours. (2) The mature biofilms were cultured with phosphate-buffered saline (PBS) (starved group) or TPY (polypeptone-yeast extract) medium (non-starved group) at pH 7.0 for 24 h and then immersed in medium of pH 3.0 for 30 min. Biofilms were analyzed through viability staining and confocal laser scanning microscopy. In all three strains, mature, acid-adapted and starved biofilms showed significantly less destructive structure and more viable bacteria after acid shock than young, non-adapted and non-starved biofilms, respectively (all p < 0.05). Furthermore, in each condition, SM593 biofilm was denser, with a significantly larger number of viable bacteria than that of SM18 and ATCC25175 (all p < 0.05). Findings demonstrated that mature, acid-adapted and starvation might protect biofilms of all three S. mutans strains against acid shock. Additionally, SM593 exhibited greater aciduricity compared to SM18 and ATCC25175, which indicated that the colonization of high cariogenicity of clinical strains may lead to high caries risk in individuals.

The Challenging World of Biofilm Physiology

Worldwide, infectious diseases are one of the leading causes of death among children. At least 65% of all infections are caused by the biofilm mode of bacterial growth. Bacteria colonise surfaces and grow as multicellular biofilm communities surrounded by a polymeric matrix as a common survival strategy. These sessile communities endow bacteria with high tolerance to antimicrobial agents and hence cause persistent and chronic bacterial infections, such as dental caries, periodontitis, otitis media, cystic fibrosis and pneumonia. The highly complex nature and the rapid adaptability of the biofilm population impede our understanding of the process of biofilm formation, but an important role for oxygen-binding proteins herein is clear. Much research on this bacterial lifestyle is already performed, from genome/proteome analysis to in vivo antibiotic susceptibility testing, but without significant progress in biofilm treatment or eradication. This review will present the multiple challenges of biofilm research and discuss possibilities to cross these barriers in future experimental studies.

Historical and contemporary hypotheses on the development of oral diseases: are we there yet?

Dental plaque is an oral biofilm that much like the rest of our microbiome has a role in health and disease. Specifically, it is the cause of very common oral diseases such as caries, gingivitis, and periodontitis. The ideas about oral disease development have evolved over time. In the nineteenth century, scientists could not identify bacteria related to disease due to the lack of technology. This led to the "Non-Specific Plaque Hypothesis" or the idea that the accumulation of dental plaque was responsible for oral disease without discriminating between the levels of virulence of bacteria. In the twentieth century this idea evolved with the techniques to analyze the changes from health to disease. The first common hypothesis was the "Specific Plaque Hypothesis" (1976) proposing that only a few species of the total microflora are actively involved in disease. Secondly, the "Non-Specific Plaque Hypothesis" was updated (1986) and the idea that the overall activity of the total microflora could lead to disease, was enriched by taking into account difference in virulence among bacteria. Then, a hypothesis was considered that combines key concepts of the earlier two hypotheses: the "Ecological Plaque Hypothesis" (1994), which proposes that disease is the result of an imbalance in the microflora by ecological stress resulting in an enrichment of certain disease-related micro-organisms. Finally, the recent "Keystone-Pathogen Hypothesis" (2012) proposes that certain low-abundance microbial pathogens can cause inflammatory disease by interfering with the host immune system and remodeling the microbiota. In this comprehensive review, we describe how these different hypotheses, and the ideas around them, arose and test their current applicability to the understanding of the development of oral disease. Finally, we conclude that an all-encompassing ecological hypothesis explaining the shifts from health to disease is still lacking.

Insights into the virulence of oral biofilms: discoveries from proteomics

This review covers developments in the study of polymicrobial communities, biofilms and selected areas of host response relevant to dental plaque and related areas of oral biology. The emphasis is on recent studies in which proteomic methods, particularly those using mass spectrometry as a readout, have played a major role in the investigation. The last 5-10 years have seen a transition of such methods from the periphery of oral biology to the mainstream, as in other areas of biomedical science. For reasons of focus and space, the authors do not discuss biomarker studies relevant to improved diagnostics for oral health, as this literature is rather substantial in its own right and deserves a separate treatment. Here, global gene regulation studies of plaque-component organisms, biofilm formation, multispecies interactions and host-microbe interactions are discussed. Several aspects of proteomics methodology that are relevant to the studies of multispecies systems are commented upon.

Streptococcus mutans protein synthesis during mixed-species biofilm development by high-throughput quantitative proteomics

Biofilms formed on tooth surfaces are comprised of mixed microbiota enmeshed in an extracellular matrix. Oral biofilms are constantly exposed to environmental changes, which influence the microbial composition, matrix formation and expression of virulence. Streptococcus mutans and sucrose are key modulators associated with the evolution of virulent-cariogenic biofilms. In this study, we used a high-throughput quantitative proteomics approach to examine how S. mutans produces relevant proteins that facilitate its establishment and optimal survival during mixed-species biofilms development induced by sucrose. Biofilms of S. mutans, alone or mixed with Actinomyces naeslundii and Streptococcus oralis, were initially formed onto saliva-coated hydroxyapatite surface under carbohydrate-limiting condition. Sucrose (1%, w/v) was then introduced to cause environmental changes, and to induce biofilm accumulation. Multidimensional protein identification technology (MudPIT) approach detected up to 60% of proteins encoded by S. mutans within biofilms. Specific proteins associated with exopolysaccharide matrix assembly, metabolic and stress adaptation processes were highly abundant as the biofilm transit from earlier to later developmental stages following sucrose introduction. Our results indicate that S. mutans within a mixed-species biofilm community increases the expression of specific genes associated with glucan synthesis and remodeling (gtfBC, dexA) and glucan-binding (gbpB) during this transition (P<0.05). Furthermore, S. mutans up-regulates specific adaptation mechanisms to cope with acidic environments (F1F0-ATPase system, fatty acid biosynthesis, branched chain amino acids metabolism), and molecular chaperones (GroEL). Interestingly, the protein levels and gene expression are in general augmented when S. mutans form mixed-species biofilms (vs. single-species biofilms) demonstrating fundamental differences in the matrix assembly, survival and biofilm maintenance in the presence of other organisms. Our data provide insights about how S. mutans optimizes its metabolism and adapts/survives within the mixed-species community in response to a dynamically changing environment. This reflects the intricate physiological processes linked to expression of virulence by this bacterium within complex biofilms.

Photodynamic therapy as an alternative treatment for disinfection of bacteria in oral biofilms

BACKGROUND AND OBJECTIVES

Biofilm-related diseases such as caries and periodontal disease are prevalent chronic oral infections which pose significant oral and general health risks. Biofilms are sessile communities attached to surfaces. Photodynamic therapy (PDT) has been demonstrated to have a significant anti-microbial effect and presents as an alternative to treating biofilm-related disease. The aim of this study was to determine the ability of porfimer sodium induced PDT to treat localized infections of Streptococcus mutans in biofilm communities.

MATERIALS AND METHODS

Reproducible biofilms were formed by S. mutans strain ATCC 27351 growing in log phase at 37°C in Brain Heart Infusion medium, circulating through flow cells at 3 ml/minute for 36-48 hours. The photosensitizer used was porfimer sodium (Photofrin®) at 125 µg/ml with biofilm immersion times of 5 minutes and increasing energy density of post-immersion laser illumination at 630 nm (100 mW/cm(2) ). Resulting effects on bacterial viability in the biofilms were tracked by monitoring alamarBlue® conversion. Supplementary data characterizing the biofilms before and after exposure to PDT were acquired by Multiple Attenuated Internal Reflection Infrared Spectroscopy (MAIR-IR).

RESULTS

The results of this study show that PDT using porfimer sodium and 630 nm laser light was effective in significantly reducing the viability of S. mutans biofilms. Maximum effectiveness was seen when biofilms were exposed to both photosensitizer and light versus controls. Porfimer sodium incubation times as short as 5 minutes in solutions as dilute as 25 µg/ml and illuminated with as little as 30 J/cm(2) resulted in significant decreases in viability of bacteria in biofilms. Optimum parameters appear to be 125 µg/ml porfimer sodium concentration and incubated for 5 minutes and 60 J/cm(2) of light energy density.

CONCLUSIONS

This study has demonstrated that significant killing of the cariogenic organism S. mutans by the combination of a photosensitizer and the appropriate wavelength of laser light was possible even when the bacteria are embedded in an extracellular matrix.

Comparative proteomic analysis of Streptococcus suis biofilms and planktonic cells that identified biofilm infection-related immunogenic proteins

Streptococcus suis (SS) is a zoonotic pathogen that causes severe disease symptoms in pigs and humans. Biofilms of SS bind to extracellular matrix proteins in both endothelial and epithelial cells and cause persistent infections. In this study, the differences in the protein expression profiles of SS grown either as planktonic cells or biofilms were identified using comparative proteomic analysis. The results revealed the existence of 13 proteins of varying amounts, among which six were upregulated and seven were downregulated in the Streptococcus biofilm compared with the planktonic controls. The convalescent serum from mini-pig, challenged with SS, was applied in a Western blot assay to visualize all proteins from the biofilm that were grown in vitro and separated by two-dimensional gel electrophoresis. A total of 10 immunoreactive protein spots corresponding to nine unique proteins were identified by MALDI-TOF/TOF-MS. Of these nine proteins, five (Manganese-dependent superoxide dismutase, UDP-N-acetylglucosamine 1-carboxyvinyltransferase, ornithine carbamoyltransferase, phosphoglycerate kinase, Hypothetical protein SSU05_0403) had no previously reported immunogenic properties in SS to our knowledge. The remaining four immunogenic proteins (glyceraldehyde-3-phosphate dehydrogenase, hemolysin, pyruvate dehydrogenase and DnaK) were identified under both planktonic and biofilm growth conditions. In conclusion, the protein expression pattern of SS, grown as biofilm, was different from the SS grown as planktonic cells. These five immunogenic proteins that were specific to SS biofilm cells may potentially be targeted as vaccine candidates to protect against SS biofilm infections. The four proteins common to both biofilm and planktonic cells can be targeted as vaccine candidates to protect against both biofilm and acute infections.

The activity of a small lytic peptide PTP-7 on Staphylococcus aureus biofilms

One of the most important features of bacterial biofilms is their resistance to antibiotics and to the host immune system. In this study, we have found that a small lytic peptide, PTP-7, is very potent to Gram-positive bacteria and is able to kill antibiotic sensitive and resistant Staphylococcus aureus indiscriminately. Further studies have revealed that despite being a cationic peptide, the antibacterial activity of PTP-7 was not affected by the negatively charged extracellular polymeric substance (EPS) of biofilms. PTP-7 could diffuse into the deep layer of S. aureus biofilms to kill bacteria inside biofilms efficiently and effectively. Neither the high concentrations of metal ions nor the acidic pH in biofilms affected the activity of peptide PTP-7. It seems that the unique sequence/structure together with the resistant bacteria killing ability of peptide PTP-7 confers its anti-biofilm activity. This study sheds new light on the treatment of bacterial biofilms, especially various biofilm related infections.

[Streptococcus mutans and oral streptococci in dental plaque]

The human oral microbial biota represents a highly diverse biofilm. Twenty-five species of oral streptococci inhabit the human oral cavity and represent about 20 % of the total oral bacteria. Taxonomy of these bacteria is complex and remains provisional. Oral streptococci encompass friends and foes bacteria. Each species has developed specific properties for colonizing the different oral sites subjected to constantly changing conditions, for competing against competitors, and for resisting external agressions (host immune system, physico-chemical shocks, and mechanical frictions). Imbalance in the indigenous microbial biota generates oral diseases, and under proper conditions, commensal streptococci can switch to opportunistic pathogens that initiate disease in and damage to the host. The group of "mutans streptococci" was described as the most important bacteria related to the formation of dental caries. Streptococcus mutans, although naturally present among the human oral microbiota, is the microbial species most strongly associated with carious lesions. This minireview describes the oral streptococci ecology and their biofilm life style by focusing on the mutans group, mainly S. mutans. Virulence traits, interactions in the biofilm, and influence of S. mutans in dental caries etiology are discussed.

"Biofilmology": a multidisciplinary review of the study of microbial biofilms

The observation of biofilm formation is not a new phenomenon. The prevalence and significance of biofilm and aggregate formation in various processes have encouraged extensive research in this field for more than 40 years. In this review, we highlight techniques from different disciplines that have been used to successfully describe the extracellular, surface and intracellular elements that are predominant in understanding biofilm formation. To reduce the complexities involved in studying biofilms, researchers in the past have generally taken a parts-based, disciplinary specific approach to understand the different components of biofilms in isolation from one another. Recently, a few studies have looked into combining the different techniques to achieve a more holistic understanding of biofilms, yet this approach is still in its infancy. In order to attain a global understanding of the processes involved in the formation of biofilms and to formulate effective biofilm control strategies, researchers in the next decade should recognise that the study of biofilms, i.e. biofilmology, has evolved into a discipline in its own right and that mutual cooperation between the various disciplines towards a multidisciplinary research vision is vital in this field.

Characterization of the Streptococcus sobrinus acid-stress response by interspecies microarrays and proteomics

Streptococcus mutans and Streptococcus sobrinus are considered the primary organisms responsible for human dental caries. The ability to generate acids and to adapt to low pH conditions is directly associated with the cariogenic potential of these bacteria. To survive acidic conditions, both species have been shown to mount an acid-tolerance response (ATR). However, previous characterization of the S. sobrinus ATR identified critical differences in the mechanisms of acid adaptation between S. mutans and S. sobrinus. Here, interspecies microarray and proteomic approaches were used to identify novel, previously unrecognized genes and pathways that participate in the S. sobrinus acid-stress response. The results revealed that, among other things, metabolic alterations that enhance energy generation and upregulation of the malolactic fermentation enzyme activity constitute important acid-resistance properties in S. sobrinus. Some of these acid adaptive traits are shared by S. mutans and might be considered optimal targets for therapeutic treatments designed to control dental caries.

Real-time solvent tolerance analysis of pseudomonas sp. strain VLB120{Delta}C catalytic biofilms

Biofilms are ubiquitous surface-associated microbial communities embedded in an extracellular polymeric (EPS) matrix, which gives the biofilm structural integrity and strength. It is often reported that biofilm-grown cells exhibit enhanced tolerance toward adverse environmental stress conditions, and thus there has been a growing interest in recent years to use biofilms for biotechnological applications. We present a time- and locus-resolved, noninvasive, quantitative approach to study biofilm development and its response to the toxic solvent styrene. Pseudomonas sp. strain VLB120ΔC-BT-gfp1 was grown in modified flow-cell reactors and exposed to the solvent styrene. Biofilm-grown cells displayed stable catalytic activity, producing (S)-styrene oxide continuously during the experimental period. The pillar-like structure and growth rate of the biofilm was not influenced by the presence of the solvent. However, the cells experience severe membrane damage during styrene treatment, although they obviously are able to adapt to the solvent, as the amount of permeabilized cells decreased from 75 to 80% down to 40% in 48 h. Concomitantly, the fraction of concanavalin A (ConA)-stainable EPS increased, substantiating the assumption that those polysaccharides play a major role in structural integrity and enhanced biofilm tolerance toward toxic environments. Compared to control experiments with planktonic grown cells, the Pseudomonas biofilm adapted much better to toxic concentrations of styrene, as nearly 65% of biofilm cells were not permeabilized (viable), compared to only 7% in analogous planktonic cultures. These findings underline the robustness of biofilms under stress conditions and its potential for fine chemical syntheses.

Trichloroethene and cis-1,2-dichloroethene concentration-dependent toxicity model simulates anaerobic dechlorination at high concentrations. II: continuous flow and attached growth reactors

A model that was used to describe toxicity from high concentrations of chlorinated aliphatic hydrocarbons (CAHs) on reductively dechlorinating cultures in batch reactors (Sabalowsky and Semprini (in press)) was extended here to simulate observations in continuous flow suspended and attached growth reactors. The reductively dechlorinating anaerobic Evanite subculture (EV-cDCE) was fed trichloroethene (TCE) and excess electron donor to accumulate cis-1,2-dichloroethene (cDCE) in a continuous flow stirred tank reactor (CFSTR); and an attached growth recirculating packed column (RPC). A concentration-dependent toxicity model used to simulate the results of batch reactors in part I (Sabalowsky and Semprini (in press) Biotechnol Bioeng) also simulated well the observations for the CFSTR and RPC growth modes. The toxicity model incorporates cDCE and TCE toxicity coefficients that directly increase the cell decay coefficient in proportion with cDCE and TCE concentrations. Simulated estimates of the cDCE and TCE toxicity coefficients indicate reductively dechlorinating cells are most sensitive to high concentrations of cDCE and TCE in batch-fed growth, followed by CFSTR, with attached growth being least sensitive. The greater toxicity of TCE than cDCE, and ratio of the modeled toxicity coefficients, agrees with previously proposed models relating toxicity to partitioning in the cell wall (K(M/B)), proportional to octanol-water partitioning (K(OW)) coefficients.

Cytoplasmic pH measurement and homeostasis in bacteria and archaea

Of all the molecular determinants for growth, the hydronium and hydroxide ions are found naturally in the widest concentration range, from acid mine drainage below pH 0 to soda lakes above pH 13. Most bacteria and archaea have mechanisms that maintain their internal, cytoplasmic pH within a narrower range than the pH outside the cell, termed "pH homeostasis." Some mechanisms of pH homeostasis are specific to particular species or groups of microorganisms while some common principles apply across the pH spectrum. The measurement of internal pH of microbes presents challenges, which are addressed by a range of techniques under varying growth conditions. This review compares and contrasts cytoplasmic pH homeostasis in acidophilic, neutralophilic, and alkaliphilic bacteria and archaea under conditions of growth, non-growth survival, and biofilms. We present diverse mechanisms of pH homeostasis including cell buffering, adaptations of membrane structure, active ion transport, and metabolic consumption of acids and bases.

Inactivation of VicK affects acid production and acid survival of Streptococcus mutans

The regulation of acid production in and the tolerance to low pH of the cariogenic bacterium Streptococcus mutans have garnered considerable attention since both of these properties contribute substantially to the virulence of this organism. Frequent or prolonged exposure to acid end products, mainly lactic acid, that are present following the consumption of dietary sugars erodes the dental enamel, thereby initiating dental caries. Here we report the involvement of the S. mutans VicK sensor kinase in both the acidogenicity and the aciduricity of this bacterium. When cultures were supplemented with glucose, the glycolytic rate of a VicK null mutant was significantly decreased compared to the glycolytic rate of the wild type (P < 0.05), suggesting that there was impaired acid production. Not surprisingly, the VicK deletion mutant produced less lactic acid, while an acid tolerance response assay revealed that loss of VicK significantly enhanced the survival of S. mutans (P < 0.05). Compared to the survival rates of the wild type, the survival rates of the VicK-deficient mutant were drastically increased when cultures were grown at pH 3.5 with or without preexposure to a signal pH (pH 5.5). Global transcriptional analysis using DNA microarrays and S. mutans wild-type UA159 and VicK deletion mutant strains grown at neutral and low pH values revealed that loss of VicK significantly affected expression of 89 transcripts more than twofold at pH 5.5 (P < 0.001). The affected transcripts included genes with putative functions in transport and maintenance of cell membrane integrity. While our results provide insight into the acid-inducible regulon of S. mutans, here we imply a novel role for VicK in regulating intracellular pH homeostasis in S. mutans.

Final pH affects the interference capacity of naturally occurring oral Lactobacillus strains against mutans streptococci

OBJECTIVES

To establish the effects of three factors: previous caries experience; colonization of Streptococcus mutans; and final pH on autologous lactobacilli-mediated inhibition against a panel of mutans streptococci in young subjects with different caries experiences.

DESIGN

The inhibition capacity was determined by the use of the agar overlay method and the final pH in culture medium was measured after 20 h. Using a logistic regression model, the risk of having an incomplete lactobacilli-mediated inhibition was calculated.

RESULTS

All three factors significantly influenced the interference outcome in the order; final pH of the Lactobacillus strains, oral colonization of autologous S. mutans and caries experience. A high risk occurred at a lower pH and at a wider pH range for individuals with previous caries experience and autologous colonization of S. mutans compared with caries-free subjects who were not colonized. At a final pH of 4.0, this risk was approximately eight times higher than that of the latter group. Two mutans Streptococcus strains in the test panel demonstrated high individual predictive values of inhibition mediated by oral lactobacilli.

CONCLUSIONS

Generation of a low pH either directly via organic acid production and/or production of bacteriocins or metabolites at a low pH may promote mutans Streptococcus growth inhibition, in vitro. Furthermore, a shift of pH range for the risk of incomplete inhibition of mutans streptococci suggests a less effective inhibition at a wider pH range for naturally occurring lactobacilli from individuals with earlier caries experience containing own S. mutans.

A model of efficiency: stress tolerance by Streptococcus mutans

The complete genome sequence of Streptococcus mutans, a bacterial pathogen commonly associated with human dental caries, was published in 2002. The streamlined genome (2.03 Mb) revealed an organism that is well adapted to its obligately host-associated existence in multispecies biofilms on tooth surfaces: a dynamic environment that undergoes rapid and substantial fluctuations. However, S. mutans lacks many of the sensing systems and alternative sigma factors that bacteria often use to coordinate gene expression in response to stress and changes in their environment. Over the past 7 years, functional genomics and proteomics have enhanced our understanding of how S. mutans has integrated the stress regulon and global transcriptional regulators to coordinate responses to environmental fluctuations with modulation of virulence in a way that ensures persistence in the oral cavity and capitalizes on conditions that are favourable for the development of dental caries. Here, we highlight advances in dissection of the stress regulon of S. mutans and its intimate interrelationship with pathogenesis.

Effects of sucrose on the extracellular matrix of plaque-like biofilm formed in vivo, studied by proteomic analysis

Previous studies have shown that sucrose promotes changes in the composition of the extracellular matrix (ECM) of plaque-like biofilm (PLB), but its effect on protein expression has not been studied in vivo. Therefore, the protein compositions of ECM of PLB formed with and without sucrose exposure were analyzed by two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). For this purpose, a crossover study was conducted during two phases of 14 days each, during which a volunteer wore a palatal appliance containing eight enamel blocks for PLB accumulation. In each phase, a 20% sucrose solution or distilled and deionized water (control) were extraorally dripped onto the blocks 8x/day. On the 14th day, the PLB were collected, the ECM proteins were extracted, separated by two-dimensional gel electrophoresis, digested by in-gel trypsin and MALDI-TOF MS analyzed. In the ECM of PLB formed under sucrose exposure, the following changes compared with the control PLB were observed: (1) the presence of upregulated proteins that may be involved in bacterial response to environmental changes induced by sucrose and (2) the absence of calcium-binding proteins that may partly explain the low inorganic concentration found in ECM of PLB formed under sucrose exposure. The findings showing that sucrose affected the ECM protein composition of PLB in vivo provide further insight into the unique cariogenic properties of this dietary carbohydrate.

Acid tolerance of biofilm cells of Streptococcus mutans

Streptococcus mutans, a member of the dental plaque community, has been shown to be involved in the carious process. Cells of S. mutans induce an acid tolerance response (ATR) when exposed to sublethal pH values that enhances their survival at a lower pH. Mature biofilm cells are more resistant to acid stress than planktonic cells. We were interested in studying the acid tolerance and ATR-inducing ability of newly adhered biofilm cells of S. mutans. All experiments were carried out using flow-cell systems, with acid tolerance tested by exposing 3-h biofilm cells to pH 3.0 for 2 h and counting the number of survivors by plating on blood agar. Acid adaptability experiments were conducted by exposing biofilm cells to pH 5.5 for 3 h and then lowering the pH to 3.5 for 30 min. The viability of the cells was assessed by staining the cells with LIVE/DEAD BacLight viability stain. Three-hour biofilm cells of three different strains of S. mutans were between 820- and 70,000-fold more acid tolerant than corresponding planktonic cells. These strains also induced an ATR that enhanced the viability at pH 3.5. The presence of fluoride (0.5 M) inhibited the induction of an ATR, with 77% fewer viable cells at pH 3.5 as a consequence. Our data suggest that adhesion to a surface is an important step in the development of acid tolerance in biofilm cells and that different strains of S. mutans possess different degrees of acid tolerance and ability to induce an ATR.

Involvement of the detoxifying enzyme lactoylglutathione lyase in Streptococcus mutans aciduricity

Streptococcus mutans, a normal inhabitant of dental plaque, is considered a primary etiological agent of dental caries. Its main virulence factors are acidogenicity and aciduricity, the abilities to produce acid and to survive and grow at low pH, respectively. Metabolic processes are finely regulated following acid exposure in S. mutans. Proteome analysis of S. mutans demonstrated that lactoylglutathione lyase (LGL) was up-regulated during acid challenge. The LGL enzyme catalyzes the conversion of toxic methylglyoxal, derived from glycolysis, to S-D-lactoylglutathione. Methylglyoxal inhibits the growth of cells in all types of organisms. The current study aimed to investigate the relationship between LGL and aciduricity and acidogenicity in S. mutans. An S. mutans isogenic mutant defective in lgl (LGLKO) was created, and its growth kinetics were characterized. Insertional inactivation of lgl resulted in an acid-sensitive phenotype. However, the glycolytic rate at pH 5.0 was greater for LGLKO than for S. mutans UA159 wild-type cells. LGL was involved in the detoxification of methylglyoxal, illustrated by the absence of enzyme activity in LGLKO and the hypersensitivity of LGLKO to methylglyoxal, compared with UA159 (MIC of 3.9 and 15.6 mM, respectively). Transcriptional analysis of lgl conducted by quantitative real-time PCR revealed that lgl was up-regulated (approximately sevenfold) during the exponential growth phase compared with that in the stationary growth phase. Gene expression studies conducted at low pH demonstrated that lgl was induced during acidic growth (approximately 3.5-fold) and following acid adaptation (approximately 2-fold). This study demonstrates that in S. mutans, LGL functions in the detoxification of methylglyoxal, resulting in increased aciduricity.

Response to alkaline stress by root canal bacteria in biofilms

AIM

To determine whether bacteria isolated from infected root canals survive alkaline shifts better in biofilms than in planktonic cultures.

METHODOLOGY

Clinical isolates of Enterococcus faecalis, Lactobacillus paracasei, Olsenella uli, Streptococcus anginosus, S. gordonii, S. oralis and Fusobacterium nucleatum in biofilm and planktonic cultures were stressed at pH 10.5 for 4 h, and cell viability determined using the fluorescent staining LIVE/DEAD BacLight bacterial viability kit. In addition, proteins released into extracellular culture fluids were identified by Western blotting.

RESULTS

Enterococcus faecalis, L. paracasei, O. uli and S. gordonii survived in high numbers in both planktonic cultures and in biofilms after alkaline challenge. S. anginosus, S. oralis and F. nucleatum showed increased viability in biofilms compared with planktonic cultures. Alkaline exposure caused all planktonic cultures to aggregate into clusters and resulted in a greater extrusion of cellular proteins compared with cells in biofilms. Increased levels of DnaK, HPr and fructose-1,6-bisphosphate aldolase were observed in culture fluids, especially amongst streptococci.

CONCLUSIONS

In general, bacteria isolated from infected roots canals resisted alkaline stress better in biofilms than in planktonic cultures, however, planktonic cells appeared to use aggregation and the extracellular transport of specific proteins as survival mechanisms.

Evidence that accumulation of mutants in a biofilm reflects natural selection rather than stress-induced adaptive mutation

The accumulation of mutant genotypes within a biofilm evokes the controversy over whether the biofilm environment induces adaptive mutation or whether the accumulation can be explained by natural selection. A comparison of the virulence of two strains of the dental pathogen Streptococcus mutans showed that rats infected with one of the strains accumulated a high proportion (average, 22%) of organisms that had undergone a deletion between two contiguous and highly homologous genes. To determine if the accumulation of deletion mutants was due to selection or to an increased mutation rate, accumulations of deletion mutants within in vitro planktonic and biofilm cultures and within rats inoculated with various proportions of deletion organisms were quantified. We report here that natural selection was the primary force behind the accumulation of the deletion mutants.

Dental plaque as a biofilm and a microbial community - implications for health and disease

Dental plaque is a structurally- and functionally-organized biofilm. Plaque forms in an ordered way and has a diverse microbial composition that, in health, remains relatively stable over time (microbial homeostasis). The predominant species from diseased sites are different from those found in healthy sites, although the putative pathogens can often be detected in low numbers at normal sites. In dental caries, there is a shift toward community dominance by acidogenic and acid-tolerating species such as mutans streptococci and lactobacilli, although other species with relevant traits may be involved. Strategies to control caries could include inhibition of biofilm development (e.g. prevention of attachment of cariogenic bacteria, manipulation of cell signaling mechanisms, delivery of effective antimicrobials, etc.), or enhancement of the host defenses. Additionally, these more conventional approaches could be augmented by interference with the factors that enable the cariogenic bacteria to escape from the normal homeostatic mechanisms that restrict their growth in plaque and out compete the organisms associated with health. Evidence suggests that regular conditions of low pH in plaque select for mutans streptococci and lactobacilli. Therefore, the suppression of sugar catabolism and acid production by the use of metabolic inhibitors and non-fermentable artificial sweeteners in snacks, or the stimulation of saliva flow, could assist in the maintenance of homeostasis in plaque. Arguments will be presented that an appreciation of ecological principles will enable a more holistic approach to be taken in caries control.

A searchable database for proteomes of oral microorganisms

An online database of proteomes for two-dimensional electrophoresis (2DE) gel data was constructed and it is now freely accessible through a web-based interface. Proteins from three oral bacteria, Streptococcus mutans UA159, Actinobacillus actinomycetemcomitans HK1651, and Porphyromonas gingivalis W83, whose genome databases are freely available, were separated by 2DE, and protein spots were analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and identified. About 1000 spots from the gels of P. gingivalis W83 were extracted and analyzed by MALDI-TOF, and 330 proteins were identified. In addition, 160 of 240 spots of A. actinomycetemcomitans and 158 of 356 spots of S. mutans were identified. Information such as spot coordinates on the gels, protein names (predicted functions), molecular weights, isoelectroric points, and links to online databases, including Oral Pathogen Sequence Databases of the Los Alamos National Laboratory Bioscience Division (ORALGEN) and National Center for Biotechnology Information (NCBI) or The Institute Genomic Research (TIGR), were stored in tables accessible through the relational database management system MySQL on an Apache web server. To test for functionality of this database system, responses of S. mutans to environmental changes were analyzed using the database and 21 spots on the gel were identified as proteins whose expression had been increased or decreased by environmental pH change without in-gel trypsin digestion, protein extraction, or MALDI-TOF/TOF-MS (mass spectrometer) analysis. The identified proteins are agreement with those reported in previous papers on acid tolerance of S. mutans, demonstrating the usefulness of the system. This database is available at http://www.myamagu.dent.kyushu-u.ac.jp/~bioinformatics/index.html or http://www.bipos.mascat.nihon-u.ac.jp/index.html.

Up-regulation of competence- but not stress-responsive proteins accompanies an altered metabolic phenotype in Streptococcus mutans biofilms

Mature biofilm and planktonic cells of Streptococcus mutans cultured in a neutral pH environment were subjected to comparative proteome analysis. Of the 242 protein spots identified, 48 were significantly altered in their level of expression (P<0.050) or were unique to planktonic or biofilm-grown cells. Among these were four hypothetical proteins as well as proteins known to be associated with the maintenance of competence or found to possess a cin-box-like element upstream of their coding gene. Most notable among the non-responsive genes were those encoding the molecular chaperones DnaK, GroEL and GroES, which are considered to be up-regulated by sessile growth. Analysis of the rest of the proteome indicated that a number of cellular functions associated with carbon uptake and cell division were down-regulated. The data obtained were consistent with the hypothesis that a reduction in the general growth rate of mature biofilms of S. mutans in a neutral pH environment is associated with the maintenance of transformation without the concomitant stress response observed during the transient state of competence in bacterial batch cultures.

Protein expression by Streptococcus mutans during initial stage of biofilm formation

Cells growing on surfaces in biofilms exhibit properties distinct from those of planktonic cells, such as increased resistance to biocides and antimicrobial agents. In spite of increased interest in biofilms, very little is known about alterations in cell physiology that occur upon attachment of cells to a surface. In this study we have investigated the changes induced in the protein synthesis by contact of Streptococcus mutans with a surface. Log-phase planktonic cells of S. mutans were allowed to adhere to a glass slide for 2 h in the presence of a (14)C-amino acid mixture. Nonadhered cells were washed away, and the adhered cells were removed by sonication. The proteins were extracted from the nonadhered planktonic and the adhered biofilm cells and separated by two-dimensional gel electrophoresis followed by autoradiography and image analysis. Image analysis revealed that the relative rate of synthesis of 25 proteins was enhanced and that of 8 proteins was diminished > or =1.3-fold in the biofilm cells. Proteins of interest were identified by mass spectrometry and computer-assisted protein sequence analysis. Of the 33 proteins associated with the adhesion response, all but 10 were identified by mass spectrometry and peptide mass fingerprinting. The most prominent change in adhered cells was the increase in relative synthesis of enzymes involved in carbohydrate catabolism indicating that a redirection in protein synthesis towards energy generation is an early response to contact with and adhesion to a surface.

Effect of acid stress on the physiology of biofilm cells of Streptococcus mutans

Streptococcus mutans is a component of the dental plaque biofilm and an important aetiological agent in dental caries. Although this organism growing in the suspended (planktonic) state has been well characterized, relatively little is known about its physiology in biofilms, particularly in the acidic environments associated with caries development. The authors determined the effect of biofilm age (1-5 days) and cell density on selected metabolic properties under conditions of glucose limitation in a biofilm-chemostat at pH 7.5 and compared these baseline values with those of 3 day biofilms subjected to acid stress. Biofilm cell biomass more than doubled over the 5 day experimental period under baseline conditions, with the glycolytic rate, glucose uptake, glucose-PTS (phosphotransferase system) activity and protein synthesis maximum at 1-2 days. DNA and RNA synthesis increased for the first 3 days before decreasing in the 5 day biofilms, while H(+)/ATPase activity was higher in 5 day biofilms than 1 day biofilms, with overall activity 5-13-fold higher per cell unit than in the associated planktonic cells. Glucose pulsing (50 mM final concentration) for three consecutive days without pH control for 5 h (pH 4.39+/-0.02) resulted in a progressive decrease in planktonic cell numbers; however, the rate of acid formation and glucose utilization in the chemostat by these cells increased per cell unit. Assays for carbohydrate metabolism in the latter cells showed increased activity, as did an assay for H(+)/ATPase (8-fold); however, DNA, RNA and protein synthesis were repressed (0.3-0.7-fold). Although the 3 day biofilm viable cell counts declined by 51 % on glucose pulsing, all the physiological parameters measured by cell unit increased in activity, with notable increases in RNA and protein synthesis (4.6-7.6-fold). The results indicate that the maintenance of intracellular pH homeostasis is the basis of the enhanced physiological status and acid tolerance of biofilm cells.