Comprehensive analysis of bacteriocins in Streptococcus 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.

The two-component regulatory systems GraRS and SrrAB mediate Staphylococcus aureus susceptibility to Pep5 produced by clinical isolate of Staphylococcus epidermidis

Staphylococcus aureus is a common bacterium on the skin and in the nose that sometimes causes severe illness. Bacteriocins, antimicrobial peptides, or proteins produced by bacteria are candidates for the treatment of S. aureus infection. In this study, we found that a clinical Staphylococcus epidermidis strain, KSE112, produced the lantibiotic Pep5, which showed anti-S. aureus activity. The complete nucleotide sequence of the Pep5-encoding plasmid was determined. Several S. aureus two-component regulatory systems (TCSs) are known to be involved in bacteriocin susceptibility. Therefore, susceptibility tests were performed using TCS-inactivated S. aureus mutants to determine which TCS is responsible for Pep5 susceptibility; the ΔgraRS mutant exhibited increased susceptibility to Pep5, while the ΔsrrAB mutant exhibited decreased susceptibility. GraRS is known to regulate dltABCD and mprF in concert with vraFG, and Pep5 susceptibility was significantly increased in the ΔdltABCD, ΔmprF, and ΔvraFG mutants. Regarding the ΔsrrAB mutant, cross-resistance to aminoglycosides was observed. As aminoglycoside activity is known to be affected by aerobic respiration, we focused on qoxABCD and cydAB, which are quinol oxidase genes that are necessary for aerobic respiration and have downregulated the expression in the ΔsrrAB mutant. We constructed ΔqoxABCD and ΔcydAB mutants and found that qoxABCD inactivation decreased susceptibility to Pep5 and aminoglycosides. These results indicate that reduced aerobic respiration due to the reduced qoxABCD expression in the ΔsrrAB mutant decreased Pep5 activity.IMPORTANCEThe emergence of drug-resistant bacteria, including MRSA, is a severe health problem worldwide. Thus, the development of novel antimicrobial agents, including bacteriocins, is needed. In this report, we found a Pep5-producing strain with anti-S. aureus activity. We determined the complete sequence of the Pep5-encoding plasmid for the first time. However, in S. aureus, GraRS and its effectors conferred decreased susceptibility to Pep5. We also revealed that another TCS, SrrAB, affects susceptibility Pep5 and other lantibiotics by controlling aerobic respiration. In our study, we investigated the efficacy of Pep5 against S. aureus and other Gram-positive bacteria and revealed that respiratory constancy regulated by TCS is required for the antimicrobial activity of nisin, nukacin, and Pep5. These findings provide important information for the clinical application of bacteriocins and suggest that they have different properties among similar pore-forming lantibiotics.

Ursoricin, a bacteriocin of Streptococcus ursoris, has potent activity against methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci

The emergence of drug-resistant bacteria, particularly methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE), has increased the need to discover novel antimicrobial agents that are effective against these species. Here, we describe the identification and purification of the mutacin BHT-B-like gene locus and bacteriocin peptide from Streptococcus ursoris, which is closely related to Streptococcus ratti; hence, we named this bacteriocin ursoricin. Ursoricin is a cationic, chromosome-encoded peptide that has potent antimicrobial effects against Gram-positive pathogens, including MRSA and VRE, with minimum inhibitory concentrations in the micromolar range. Ursoricin also inhibits the biofilm formation of high biofilm-forming S. aureus. Antibacterial activity was retained after treatment at 100°C for 60 min at a pH range of 3-9 and was partially reduced by treatment with proteinase K for 2 h (63% residual activity). The potent anti-MRSA, anti-VRE, and antibiofilm effects of ursoricin suggest that it is a possible candidate for the treatment of MRSA, VRE, and biofilm-associated infections.

IMPORTANCE

The emergence of multidrug-resistant bacteria worldwide has posed a significant public health threat and economic burdens that make the identification and development of novel antimicrobial agents urgent. Bacteriocins are promising new agents that exhibit antibacterial activity against a wide range of human pathogens. In this study, we report that the bacteriocin produced by Streptococcus ursoris showed good antibacterial activity against a wide range of Staphylococcus aureus and enterococcus strains, particularly methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and high biofilm-forming S. aureus. Interestingly, this bacteriocin had a stronger effect on S. aureus than on Staphylococcus epidermidis, which is a major commensal bacterium in human skin; this result is important when considering the disturbance of bacterial flora, especially on the skin, mediated by the application of antibacterial agents.

Probiotic Effect of Streptococcus dentisani on Oral Pathogens: An In Vitro Study

Probiotics, including Streptococcus dentisani, have been proposed as an alternative to re-establish the ecology of the oral cavity and inhibit the formation of pathogenic biofilms. The main objective of this work was to assess the probiotic ability of S. dentisani against Streptococcus mutans, Streptococcus mitis, and Candida albicans biofilms. The ability of the strains to form a monospecies biofilm and the probiotic potential of S. dentisani using the competition, exclusion, and displacement strategies were determined. All strains were moderate biofilm producers. The ability of S. dentisani to compete with and exclude S. mutans and S. mitis during biofilm formation was not significant. However, S. dentisani significantly reduced pathologic streptococcal biofilms using the displacement strategy. Also S. dentisani reduced the formation of the C. albicans biofilm mainly through competition and displacement. In vitro, S. dentisani exhibited probiotic potential to reduce the formation of potentially pathogenic biofilms. Further investigation is required to understand the biofilm-inhibiting mechanisms exhibited by this probiotic strain.

Nattokinase, a Subtilisin-like Alkaline-Serine Protease, Reduces Mutacin Activity by Inactivating the Competence-Stimulating Peptide in Streptococcus mutans

Streptococcus mutans is a major cariogenic organism because of its ability to form biofilms on tooth surfaces. Bacteriocins produced by S. mutans (known as mutacins) are indirect pathogenic factors that play a role in the persistence of this microbe in the oral environment. Nattokinase, a subtilisin-like alkaline serine protease, potently inhibits biofilm formation without affecting S. mutans growth. However, effective strategies utilizing nattokinase to control mutacin production by S. mutans are lacking. In this study, we evaluated the effect of nattokinase on mutacin activity in 46 strains of S. mutans with different mutacin genotypes isolated from the dental plaques of pediatric patients with caries. Nattokinase reduced the activity of mutacin against oral streptococci at a concentration of 1 mg/mL in all clinical isolates. Furthermore, nattokinase reduced the expression of non-lantibiotic mutacin structural genes (nlmABCD) and inactivated the extracellular competence-stimulating peptide involved in comDE activation, which regulates non-lantibiotic mutacin gene expression. These results suggest that nattokinase may reduce the virulence of S. mutans and could potentially be used as a new caries-preventive agent as an alternative to conventional drug treatments.

Opposing genetic polymorphisms of two ABC transporters contribute to the variation of nukacin resistance in Streptococcus mutans

Streptococcus mutans is a cariogenic bacterium that produces a variety of bacteriocins and retains resistance to these bacteriocins. In this study, we investigated the susceptibility of 127 S. mutans strains to nukacins produced by Staphylococcus spp., which are commensal bacteria in humans. We detected diverse susceptibilities among strains. Nineteen strains had a disrupted LctF (type I), which is responsible for nukacin susceptibility, whereas the remaining 108 strains had an intact LctF (type II) and displayed resistance to nukacins. However, the type I strains still showed resistance to nukacins to some extent. Interestingly, 18/19 (94.7%) type I strains carried a mukA-T locus, which is related to the synthesis of mutacin K8, and mukFEG, an ABC transporter. In contrast, among type II strains, only 6/108 strains (5.6%) had both the mukA-T locus and mukFEG, 19/108 strains (17.6%) carried only mukFEG, and 83/108 strains (76.9%) harbored neither mukA-T nor mukFEG. We also found that MukF had two variants: 305 amino acids (type α) and 302 amino acids (type β). All type I strains showed a type α (MukFα), whereas most type II strains with mukFEG (22/25 strains) had a type β (MukFβ). Then, we constructed a mukFEG-deletion mutant complemented with MukFαEG or MukFβEG and found that only MukFαEG was involved in nukacin resistance. The nukacin resistance capability of type II-LctFEG was stronger than that of MukFαEG. In conclusion, we identified a novel nukacin resistance factor, MukFEG, and either LctFEG or MukFEG was active in most strains via genetic polymorphisms depending on mukA-T genes.

IMPORTANCE

Streptococcus mutans is an important pathogenic bacterium not only for dental caries but also for systemic diseases. S. mutans is known to produce a variety of bacteriocins and to retain resistance these bacteriocins. In this study, two ABC transporters, LctFEG and MukFEG, were implicated in nukacin resistance and each ABC transporter has two subtypes, active and inactive. Of the two ABC transporters, only one ABC transporter was always resistant, while the other ABC transporter was inactivated by genetic mutation. Interestingly, this phenomenon was defined by the presence or absence of the mutacin K8 synthesis gene region, one of the bacteriocins of S. mutans. This suggests that the resistance acquisition is tightly controlled in each strain. This study provides important evidence that the insertion of bacteriocin synthesis genes is involved in the induction of genetic polymorphisms and suggests that bacteriocin synthesis genes may play an important role in bacterial evolution.

Biological Activities of Virgin Coconut and Virgin Olive Oil Mixture against Oral Primary Colonizers: An In Vitro Study

AIM AND BACKGROUND

This study aimed to explore the potential synergistic interaction of virgin coconut oil (VCO) and virgin olive oil (VOO) mixture against Streptococcus sanguinis, Streptococcus mutans, and Lactobacillus casei in a single and mixture species through the minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), antiadherence, and antibiofilm activities.

MATERIALS AND METHODS

The broth microdilution technique was used to individually determine the MIC of both oils and an oil mixture (in the ratio of 1:1) in a 96-well microtiter plate. As for the MBC, the subcultured method was used. The fractional inhibitory concentration index (ΣFIC) was determined to identify the interaction types between both oils. The oil mixture at its MIC was then tested on its antibiofilm and antiadherence effect.

RESULTS

The MIC of the oil mixture against the tested microbiota was 50-100%. The oil mixture was bactericidal at 100% concentration for all the mentioned microbes except S. mutans. The ΣFIC value was 2 to 4, indicating that the VCO and VOO acted additively against the microbiota. Meanwhile, the oil mixture at MIC (50% for S. sanguinis and L. casei; 100% for S. mutans and mixture species) exhibited antiadherence and antibiofilm activity toward the microbiota in mixture species.

CONCLUSION

The oil mixture possesses antibacterial, antibiofilm, and antiadherence properties toward the tested microbiota, mainly at 50-100% concentration of oil mixture. There was no synergistic interaction found between VCO and VOO.

CLINICAL SIGNIFICANCE

Children and individuals with special care may benefit from using the oil mixture, primarily to regulate the biofilm formation and colonization of the bacteria. Furthermore, the oil mixture is natural and nontoxic compared to chemical-based oral healthcare products. How to cite this article: Ng YM, Sockalingam SNMP, Shafiei Z, et al. Biological Activities of Virgin Coconut and Virgin Olive Oil Mixture against Oral Primary Colonizers: An In Vitro Study. J Contemp Dent Pract 2024;25(3):260-266.

Ancient Genomes From Bronze Age Remains Reveal Deep Diversity and Recent Adaptive Episodes for Human Oral Pathobionts

Ancient microbial genomes can illuminate pathobiont evolution across millenia, with teeth providing a rich substrate. However, the characterization of prehistoric oral pathobiont diversity is limited. In Europe, only preagricultural genomes have been subject to phylogenetic analysis, with none compared to more recent archaeological periods. Here, we report well-preserved microbiomes from two 4,000-year-old teeth from an Irish limestone cave. These contained bacteria implicated in periodontitis, as well as Streptococcus mutans, the major cause of caries and rare in the ancient genomic record. Despite deriving from the same individual, these teeth produced divergent Tannerella forsythia genomes, indicating higher levels of strain diversity in prehistoric populations. We find evidence of microbiome dysbiosis, with a disproportionate quantity of S. mutans sequences relative to other oral streptococci. This high abundance allowed for metagenomic assembly, resulting in its first reported ancient genome. Phylogenetic analysis indicates major postmedieval population expansions for both species, highlighting the inordinate impact of recent dietary changes. In T. forsythia, this expansion is associated with the replacement of older lineages, possibly reflecting a genome-wide selective sweep. Accordingly, we see dramatic changes in T. forsythia's virulence repertoire across this period. S. mutans shows a contrasting pattern, with deeply divergent lineages persisting in modern populations. This may be due to its highly recombining nature, allowing for maintenance of diversity through selective episodes. Nonetheless, an explosion in recent coalescences and significantly shorter branch lengths separating bacteriocin-carrying strains indicate major changes in S. mutans demography and function coinciding with sugar popularization during the industrial period.

Evaluation of Antibacterial and Anti-oxidant Activities of Suaeda monoica Extract for Its Potential Application

BACKGROUND

 An assessment of Suaeda monoica extract's antimicrobial and antioxidant properties was undertaken in light of its possible application as an oral care product. The maintenance of optimal dental health is just as important as overall wellness. Food particles become trapped in the mouth cavity, making it easy for oral bacteria to infect.

AIM

 The study sought to ascertain the antibacterial and antioxidant properties of salt marsh Suaeda monoica extract.

MATERIALS AND METHODS

 Leaves of Suaeda monoica, collected, dried and powdered, were dissolved in 70% methanol and the extract of 25-100 μg/ml was analyzed for antioxidant activity through total antioxidant assay, 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, and total reducing power. Suaeda monoica antibacterial activity was also performed and the minimum inhibitory concentration was determined for 75 μg/ml, 100μg/ml, and 150 μg/ml concentrations and tetracycline in 10mcg/disc as a control against three different oral pathogens: Staphylococcus mutans, Streptococcus aureus, and Klebsiella spp.

RESULTS

 At varying concentrations of 75 mg/ml to 150 mg/ml, Suaeda monoica extracts are efficacious with varying concentrations against the investigated bacterial strains. In the present study, in the DPPH assay, total reducing power, and total antioxidant activity assay, there was an increase in inhibitory percentage as the concentration increased from 25-100 µg/ml, showing maximum inhibition at 100 µg/ml concentration.

CONCLUSION

 The results of the investigation show that Suaeda monoica has significant antibacterial and antioxidant activity in a concentration-dependent manner and can be potentially used as an oral care agent after it is assessed for clinical use.

Selective Bacteriocins: A Promising Treatment for Staphylococcus aureus Skin Infections Reveals Insights into Resistant Mutants, Vancomycin Resistance, and Cell Wall Alterations

The emergence of antibiotic-resistant S. aureus has become a major public health concern, necessitating the discovery of new antimicrobial compounds. Given that the skin microbiome plays a critical role in the host defence against pathogens, the development of therapies that target the interactions between commensal bacteria and pathogens in the skin microbiome offers a promising approach. Here, we report the discovery of two bacteriocins, cerein 7B and cerein B4080, that selectively inhibit S. aureus without affecting S. epidermidis, a commensal bacterium on the skin. Our study revealed that exposure of S. aureus to these bacteriocins resulted in mutations in the walK/R two-component system, leading to a thickening of the cell wall visible by transmission electron microscopy and subsequent decreased sensitivity to vancomycin. Our findings prompt a nuanced discussion of the potential of those bacteriocins for selective targeting of S. aureus on the skin, given the emergence of resistance and co-resistance with vancomycin. The idea put forward implies that by preserving commensal bacteria, selective compounds could limit the emergence of resistance in pathogenic cells by promoting competition with remaining commensal bacteria, ultimately reducing chronical infections and limiting the spread of antibiotic resistance.

Role of Metabolic Adaptation of Streptococcus suis to Host Niches in Bacterial Fitness and Virulence

Streptococcus suis, both a common colonizer of the porcine upper respiratory tract and an invasive pig pathogen, successfully adapts to different host environments encountered during infection. Whereas the initial infection mainly occurs via the respiratory tract, in a second step, the pathogen can breach the epithelial barrier and disseminate within the whole body. Thereby, the pathogen reaches other organs such as the heart, the joints, or the brain. In this review, we focus on the role of S. suis metabolism for adaptation to these different in vivo host niches to encounter changes in nutrient availability, host defense mechanisms and competing microbiota. Furthermore, we highlight the close link between S. suis metabolism and virulence. Mutants deficient in metabolic regulators often show an attenuation in infection experiments possibly due to downregulation of virulence factors, reduced resistance to nutritive or oxidative stress and to phagocytic activity. Finally, metabolic pathways as potential targets for new therapeutic strategies are discussed. As antimicrobial resistance in S. suis isolates has increased over the last years, the development of new antibiotics is of utmost importance to successfully fight infections in the future.

Different CprABC amino acid sequences affect nisin A susceptibility in Clostridioides difficile isolates

Clinical isolates of Clostridioides difficile sometimes exhibit multidrug resistance and cause diarrhea after antibiotic administration. Metronidazole and vancomycin are often used as therapeutic agents, but resistance to these antibiotics has been found clinically. Therefore, the development of alternative antimicrobial agents is needed. Nisin A, produced by Lactococcus lactis, has been demonstrated to be effective against C. difficile infection. In this study, we evaluated the susceptibility of 11 C. difficile clinical isolates to nisin A and found that they could be divided into 2 groups: high and low susceptibility. Since CprABC and DltDABC, which are responsible for nisin A efflux and cell surface charge, respectively, have been reported to be related to nisin A susceptibility, we investigated the expression of cprA and dltA among the 11 strains. cprA expression in all strains was induced by nisin A, but dltA expression was not. The expression levels of both genes did not correlate with nisin A susceptibility in these clinical isolates. To evaluate cell surface charge, we performed a cytochrome C binding assay and found no relationship between charge and nisin A susceptibility. Then, we determined the whole genome sequence of each clinical isolate and carried out phylogenetic analysis. The 11 isolates separated into two major clusters, which were consistent with the differences in nisin A susceptibility. Furthermore, we found common differences in several amino acids in the sequences of CprA, CprB, and CprC between the two clusters. Therefore, we speculated that the different amino acid sequences of CprABC might be related to nisin A susceptibility. In addition, C. difficile strains could be divided in the same two groups based on susceptibility to epidermin and mutacin III, which are structurally similar to nisin A. These results suggest that genotypic variations in C. difficile strains confer different susceptibilities to bacteriocins.

TaxiBGC: a Taxonomy-Guided Approach for Profiling Experimentally Characterized Microbial Biosynthetic Gene Clusters and Secondary Metabolite Production Potential in Metagenomes

Biosynthetic gene clusters (BGCs) in microbial genomes encode bioactive secondary metabolites (SMs), which can play important roles in microbe-microbe and host-microbe interactions. Given the biological significance of SMs and the current profound interest in the metabolic functions of microbiomes, the unbiased identification of BGCs from high-throughput metagenomic data could offer novel insights into the complex chemical ecology of microbial communities. Currently available tools for predicting BGCs from shotgun metagenomes have several limitations, including the need for computationally demanding read assembly, predicting a narrow breadth of BGC classes, and not providing the SM product. To overcome these limitations, we developed taxonomy-guided identification of biosynthetic gene clusters (TaxiBGC), a command-line tool for predicting experimentally characterized BGCs (and inferring their known SMs) in metagenomes by first pinpointing the microbial species likely to harbor them. We benchmarked TaxiBGC on various simulated metagenomes, showing that our taxonomy-guided approach could predict BGCs with much-improved performance (mean F1 score, 0.56; mean PPV score, 0.80) compared with directly identifying BGCs by mapping sequencing reads onto the BGC genes (mean F1 score, 0.49; mean PPV score, 0.41). Next, by applying TaxiBGC on 2,650 metagenomes from the Human Microbiome Project and various case-control gut microbiome studies, we were able to associate BGCs (and their SMs) with different human body sites and with multiple diseases, including Crohn's disease and liver cirrhosis. In all, TaxiBGC provides an in silico platform to predict experimentally characterized BGCs and their SM production potential in metagenomic data while demonstrating important advantages over existing techniques. IMPORTANCE Currently available bioinformatics tools to identify BGCs from metagenomic sequencing data are limited in their predictive capability or ease of use to even computationally oriented researchers. We present an automated computational pipeline called TaxiBGC, which predicts experimentally characterized BGCs (and infers their known SMs) in shotgun metagenomes by first considering the microbial species source. Through rigorous benchmarking techniques on simulated metagenomes, we show that TaxiBGC provides a significant advantage over existing methods. When demonstrating TaxiBGC on thousands of human microbiome samples, we associate BGCs encoding bacteriocins with different human body sites and diseases, thereby elucidating a possible novel role of this antibiotic class in maintaining the stability of microbial ecosystems throughout the human body. Furthermore, we report for the first time gut microbial BGC associations shared among multiple pathologies. Ultimately, we expect our tool to facilitate future investigations into the chemical ecology of microbial communities across diverse niches and pathologies.

Exploiting Conserved Quorum Sensing Signals in Streptococcus mutans and Streptococcus pneumoniae

Bacterial species of the Streptococcus genera are considered either commensal bacteria or potential pathogens, according to their metabolic evolution and production of quorum sensing (QS)-controlled virulence factors. S. mutans, in particular, has become one of the best-studied examples of bacteria that are able to get along or cheat commensal species, even of the same genera. S. mutans and S. pneumoniae share homolog QS pathways and a competence stimulating peptide (CSP) for regulating bacteriocin production. Intriguingly, the abundance of S. pneumoniae and S. mutans alternates in complex microbial communities, thus opening the role for the fratricide communication of homolog QS systems. Since the inhibition of the QS has been proposed in treating bacterial infections, in this study, we designed and synthesized analogs of S. pneumoniae CSP with precise residual modifications. We reported that S. pneumoniae CSP analogs reduced the expression of genes involved in the QS of S. mutans and biofilm formation without affecting bacterial growth. The CSP analogs inhibited bacteriocin production in S. mutans, as reported by co-cultures with commensal bacteria of the oral cavity. The peptide CSP1AA, bearing substitutions in the residues involved in QS receptor recognition and activation, reported the most significant quorum-quenching activities. Our findings provide new insights into specific chemical drivers in the CSP sequences controlling the interconnection between S. mutans and S. pneumoniae. We think that the results reported in this study open the way for new therapeutic interventions in controlling the virulence factors in complex microbial communities such as the oral microbiota.

A DNA-Damage Inducible Gene Promotes the Formation of Antibiotic Persisters in Response to the Quorum Sensing Signaling Peptide in Streptococcus mutans

Bacteria use quorum sensing (QS) to communicate with each other via secreted small autoinducers produced by individuals. QS allows bacteria to display a unified response that benefits the species during adaptation to environment, colonization, and defense against competitors. In oral streptococci, the CSP-ComDE QS is an inducible DNA damage repair system that is pivotal for bacterial survival. In the oral pathogen Streptococcus mutans, the QS system positively influences the formation of antibiotic persisters, cells that can survive antibiotic attack by entering a non-proliferative state. We recently identified a novel gene, pep299, that is activated in the persister cell fraction induced by QS. In this study, we focused our investigation on the role of pep299, a gene encoding a bacteriocin-like peptide, in the formation of antibiotic persisters. Mutant Δ299, unable to produce Pep299, showed a dramatic reduction in the number of stress-induced persisters. Using a co-culture assay, we showed that cells overproducing pep299 induced the formation of persisters in the mutant, suggesting that Pep299 was actively secreted and detected by neighboring cells. Cells exposed to DNA damage conditions activated the gene expression of pep299. Interestingly, our results suggested that the pep299 gene was also involved in the regulation of a QS-inducible toxin−antitoxin system. Our study suggests that the pep299 gene is at the core of the triggered persistence phenotype in S. mutans, allowing cells to transition into a state of reduced metabolic activity and antibiotic tolerance.

Efficiency of Antimicrobial Peptides Against Multidrug-Resistant Staphylococcal Pathogens

Antibiotics play a vital role in saving millions of lives from fatal infections; however, the inappropriate use of antibiotics has led to the emergence and propagation of drug resistance worldwide. Multidrug-resistant bacteria represent a significant challenge to treating infections due to the limitation of available antibiotics, necessitating the investigation of alternative treatments for combating these superbugs. Under such circumstances, antimicrobial peptides (AMPs), including human-derived AMPs and bacteria-derived AMPs (so-called bacteriocins), are considered potential therapeutic drugs owing to their high efficacy against infectious bacteria and the poor ability of these microorganisms to develop resistance to them. Several staphylococcal species including Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus haemolyticus, and Staphylococcus saprophyticus are commensal bacteria and known to cause many opportunistic infectious diseases. Methicillin-resistant Staphylococci, especially methicillin-resistant S. aureus (MRSA), are of particular concern among the critical multidrug-resistant infectious Gram-positive pathogens. Within the past decade, studies have reported promising AMPs that are effective against MRSA and other methicillin-resistant Staphylococci. This review discusses the sources and mechanisms of AMPs against staphylococcal species, as well as their potential to become chemotherapies for clinical infections caused by multidrug-resistant staphylococci.

Gene Rearrangement and Modification of Immunity Factors Are Correlated with the Insertion of Bacteriocin Cassettes in Streptococcus mutans

Bacteriocins have been applied in the food industries and have become promising next-generation antibiotics. Some bacteria produce bacteriocins and possess immunity factors for self-protection. Nisin A, a bacteriocin produced by Lactococcus lactis, shows broad-spectrum activity. However, the evolution and cross-resistance ability of the immunity factors in some species results in reduced susceptibility to bacteriocins. Here, we investigated the elements responsible for nisin A resistance in Streptococcus mutans and their contribution to mutacins (bacteriocins produced by S. mutans) resistance. We classified the nisin A-resistance regions into six types based on the different combinations of 3 immunity factors, mutFEG, nsrX, and mutHIJ, and the presence of mutacin synthesis operon upstream of mutF. Data shows that NsrX effectively acts against nisin A but not mutacins, while the newly identified ABC transporter MutHIJ acts against three mutacins but not nisin A. Three types of MutFEG are identified based on their amino acid sequences: α (in Nsr-types C and D-I), β (in Nsr-types B and d-III), and γ (in Nsr-type E). MutFEG-α strongly contributes to mutacin I resistance, while MutFEG-β and MutFEG-γ strongly contribute to mutacin III, IIIb, and nisin A resistance. Additionally, mutFEG-like structures could be found in various streptococcal species isolated from the oral cavity of humans, chimpanzees, monkeys, bears, and hamsters. Our findings suggest that immunity factors rearrange and adapt in the presence of bacteriocins and could be transferred among closely related species, thus altering the bacterial competition within the microflora.

IMPORTANCEStreptococcus mutans is an important organism of oral microbiota and associated with dental caries and systemic diseases such as stroke and endocarditis. They produce bacteriocins known as mutacins to compete with other oral bacteria and possess immune factors for self-protection. We found that the nisin A and mutacins resistance patterns correlated with the immunity components and MutFEG variants, and the genetic difference was driven by the insertion of mutacin-synthesis cassettes. Our study provides an understanding of the development of bacteriocin resistance among streptococcal species, which may alter the bacterial interaction and ecology within the oral biofilm.

mucG, mucH, and mucI Modulate Production of Mutanocyclin and Reutericyclins in Streptococcus mutans B04Sm5

Streptococcus mutans is considered a primary etiologic agent of dental caries, which is the most common chronic infectious disease worldwide. S. mutans B04Sm5 was recently shown to produce reutericyclins and mutanocyclin through the muc biosynthetic gene cluster and to utilize reutericyclins to inhibit the growth of neighboring commensal streptococci. In this study, examination of S. mutans and muc phylogeny suggested evolution of an ancestral S. mutans muc into three lineages within one S. mutans clade and then horizontal transfer of muc to other S. mutans clades. The roles of the mucG and mucH transcriptional regulators and the mucI transporter were also examined. mucH was demonstrated to encode a transcriptional activator of muc. mucH deletion reduced production of mutanocyclin and reutericyclins and eliminated the impaired growth and inhibition of neighboring streptococci phenotypes, which are associated with reutericyclin production. ΔmucG had increased mutanocyclin and reutericyclin production, which impaired growth and increased the ability to inhibit neighboring streptococci. However, deletion of mucG also caused reduced expression of mucD, mucE, and mucI. Deletion of mucI reduced mutanocyclin and reutericylin production but enhanced growth, suggesting that mucI may not transport reutericyclin as its homolog does in Limosilactobacillus reuteri. Further research is needed to determine the roles of mucG and mucI and to identify any cofactors affecting the activity of the mucG and mucH regulators. Overall, this study provided pangenome and phylogenetic analyses that serve as a resource for S. mutans research and began elucidation of the regulation of reutericyclins and mutanocyclin production in S. mutans. IMPORTANCE S. mutans must be able to outcompete neighboring organisms in its ecological niche in order to cause dental caries. S. mutans B04Sm5 inhibited the growth of neighboring commensal streptococci through production of reutericyclins via the muc biosynthetic gene cluster. In this study, an S. mutans pangenome database and updated phylogenetic tree were generated that will serve as valuable resources for the S. mutans research community and that provide insights into the carriage and evolution of S. mutans muc. The MucG and MucH regulators, and the MucI transporter, were shown to modulate production of reutericyclins and mutanocyclin. These genes also affected the ability of S. mutans to inhibit neighboring commensals, suggesting that they may play a role in S. mutans virulence.

The antimicrobial and antibiofilm effects of three herbal extracts on Streptococcus mutans compared with Chlorhexidine 0.2% (in vitro study)

There is a special focus on using natural materials and herbal plants to prevent dental caries. Previous studies showed that some herbal plants have antimicrobial effects on oral pathogens. Thus we investigated the antimicrobial effects of three herbal extracts (Carum copticum, Phlomis bruguieri, and Marrubium parviflorum) on the growth of Streptococcus mutans, as the most important bacteria causing dental caries. First, plant methanolic extracts were prepared. Then, to evaluate the antimicrobial activity of the three herbal extracts, the agar well diffusion method and MIC were performed. The biofilm formation was carried out using a broth dilution method with 2% glucose-supplemented BHIS in sterile 96-well microplates. Serial dilutions (50, 25, 12.5, 6.25, 3.12 mg/ml) of extracts were prepared. Next, a 0.5 McFarland Suspension of S. mutans was added to wells. The inhibitory effect on biofilm formation was measured by the ELISA reader apparatus. The assay was repeated three times, and the average was calculated as 3. The results were compared with those of Chlorhexidine 0.2%. Carum copticum showed a better effect in the agar well diffusion method than others. MIC of the extracts of Carum coptimum, Phlomis bruguieri, and Marrubium parviflorum were 3.12, 6.25, and 12.5 mg/ml, respectively. Overall, the highest activity belonged to Carum copticum extract. For the anti-biofilm effect, the OD values of Carum copticum and Marrubium parviflorum were significantly different from that of Phlomis bruguieri. Although all of the methanolic herbal extracts can inhibit S. mutans growth and remove the biofilm, the effect of Carum copticum was better than Phlomis bruguieri and Marrubium parviflorum. Further studies are recommended to indicate how these extracts perform against the bacteria.

Tetramic Acids Mutanocyclin and Reutericyclin A, Produced by Streptococcus mutans Strain B04Sm5 Modulate the Ecology of an in vitro Oral Biofilm

The human oral microbiome consists of diverse microbes actively communicating and interacting through a variety of biochemical mechanisms. Dental caries is a major public health issue caused by fermentable carbohydrate consumption that leads to dysbiosis of the oral microbiome. Streptococcus mutans is a known major contributor to caries pathogenesis, due to its exceptional ability to form biofilms in the presence of sucrose, as well as to its acidophilic lifestyle. S. mutans can also kill competing bacteria, which are typically health associated, through the production of bacteriocins and other small molecules. A subset of S. mutans strains encode the muc biosynthetic gene cluster (BGC), which was recently shown to produce the tetramic acids, mutanocyclin and reutericyclins A, B, and C. Reutericyclin A displayed strong antimicrobial activity and mutanocyclin appeared to be anti-inflammatory; however the effect of these compounds, and the carriage of muc by S. mutans, on the ecology of the oral microbiota is not known, and was examined here using a previously developed in vitro biofilm model derived from human saliva. While reutericyclin significantly inhibited in vitro biofilm formation and acid production at sub-nanomolar concentrations, mutanocyclin did not present any activity until the high micromolar range. 16S rRNA gene sequencing revealed that reutericyclin drastically altered the biofilm community composition, while mutanocyclin showed a more specific effect, reducing the relative abundance of cariogenic Limosilactobacillus fermentum. Mutanocyclin or reutericyclin produced by the S. mutans strains amended to the community did not appear to affect the community in the same way as the purified compounds, although the results were somewhat confounded by the differing growth rates of the S. mutans strains. Regardless of the strain added, the addition of S. mutans to the in vitro community significantly increased the abundance of S. mutans and Veillonella infantium, only. Overall, this study illustrates that reutericyclin A and mutanocyclin do impact the ecology of a complex in vitro oral biofilm; however, further research is needed to determine the extent to which the production of these compounds affects the virulence of S. mutans.

Complete sequences of epidermin and nukacin encoding plasmids from oral-derived Staphylococcus epidermidis and their antibacterial activity

Staphylococcus epidermidis is a commensal bacterium in humans. To persist in the bacterial flora of the host, some bacteria produce antibacterial factors such as the antimicrobial peptides known as bacteriocins. In this study, we tried to isolate bacteriocin-producing S. epidermidis strains. Among 150 S. epidermidis isolates from the oral cavities of 287 volunteers, we detected two bacteriocin-producing strains, KSE56 and KSE650. Complete genome sequences of the two strains confirmed that they carried the epidermin-harboring plasmid pEpi56 and the nukacin IVK45-like-harboring plasmid pNuk650. The amino acid sequence of epidermin from KSE56 was identical to the previously reported sequence, but the epidermin synthesis-related genes were partially different. The prepeptide amino acid sequences of nukacin KSE650 and nukacin IVK45 showed one mismatch, but both mature peptides were entirely similar. pNuk650 was larger and had an additional seven ORFs compared to pIVK45. We then investigated the antibacterial activity of the two strains against several skin and oral bacteria and found their different activity patterns. In conclusion, we report the complete sequences of 2 plasmids coding for bacteriocins from S. epidermidis, which were partially different from those previously reported. Furthermore, this is the first report to show the complete sequence of an epidermin-carrying plasmid, pEpi56.

KATSUMATA et al.Comprehensive characterization of sortase A-dependent surface proteins in Streptococcus mutansComprehensive characterization of sortase A-dependent surface proteins in Streptococcus mutans

Streptococcus mutans, a cariogenic pathogen, adheres to the tooth surface and forms a biofilm. Bacterial cell surface proteins are associated with adherence to substrates. Sortase A (SrtA) mediates the localization of proteins with an LPXTG motif-containing proteins to the cell surface by covalent binding to peptidoglycan. In S. mutans UA159, 6 SrtA-dependent proteins, SpaP, WapA, WapE, DexA, FruA, and GbpC, were identified. Although some of these proteins were characterized, a comprehensive analysis of the 6 proteins has not been reported. In this study, we constructed mutants deficient in each of these proteins and the SrtA-deficient mutant. The SrtA-deficient mutant showed drastically decreased binding to salivary components, biofilm formation, bacterial coaggregation activity, hydrophobicity, and cellular matrix binding (collagen type I, fibronectin, and laminin). The SpaP-deficient mutant showed significantly reduced binding to salivary components and partially increased coaggregation with Porphyromonas gingivalis, and decreased hydrophobicity, and collagen binding. The WapA-deficient mutant showed slightly decreased coaggregation with Fusobacterium nucleatum. Although the SrtA-deficient mutant showed drastically altered phenotypes, all SrtA-dependent protein-deficient mutants, except the SpaP-deficient mutant, did not show considerable alterations in binding to salivary components. These results indicate that the 6 proteins may coordinately contribute to these activities. In addition, using genomic data of 125 S. mutans strains, we compared the amino acid sequences of each surface protein and found many variations among strains, which may affect the phenotype of cell surface proteins in S. mutans. This article is protected by copyright. All rights reserved.

Genotypic and phenotypic characterization of Streptococcus mutans strains isolated from patients with dental caries

Background and Objectives

The oral cavity harbors numerous Streptococcus mutans strains which display remarkable genotypic and phenotypic diversity. This study evaluated the genotypic and phenotypic diversity of 209 S. mutans strains isolated from 336 patients with dental caries and compared with the universal reference strain, UA159.

Materials and Methods

Selective cultivation on mitis-salivaries-bacitracin agar and species-specific polymerase chain reaction (PCR) was carried out to isolate and identify the 209 S. mutans isolates from 336 patients with dental caries. Arbitrarily primed polymerase chain reaction (AP-PCR), PCR amplification of specific gene, acid production and biofilm formation capacity were performed to evaluate the genotypic and phenotypic variation. Student's t-test and Chi-square test were used for analysis of variables and a probability (P) of <0.05 was considered as significant.

Results

Our study revealed a high degree of genotypic and phenotypic variability among the clinical strains. We observed significant differences in colony morphology, generation time, biofilm formation, and acid production while growing in culture medium. All the clinical isolates were able to lower pH while growing in Todd-Hewitt broth. Consistent with phenotypic variations, we also observed genotypic variation by AP-PCR and gene specific PCR. AP-PCR analysis suggested that most of the patients with dental caries have distinct type of S. mutans strains. Genes related to various two component systems were highly conserved among the isolated strains, however, bacteriocin encoding genes such as nlmAB, nlmC were absent in nearly half of the clinical isolates.

Conclusion

Our results support that S. mutans clinical isolates have wide genotypic diversity and show variation in growth kinetics, acid production, acid tolerance and biofilm formation capacity and indicates the presence of diverse mechanism to initiate and establish the biofilm lifestyle which leads to tooth decay.