Construction and characterization of isogenic mutants of Streptococcus mutans deficient in major surface protein antigen P1 (I/II)

Illuminating the oral microbiome and its host interactions: tools and approaches for molecular microbiology studies

Advancements in DNA sequencing technologies within the last decade have stimulated an unprecedented interest in the human microbiome, largely due the broad diversity of human diseases found to correlate with microbiome dysbiosis. As a direct consequence of these studies, a vast number of understudied and uncharacterized microbes have been identified as potential drivers of mucosal health and disease. The looming challenge in the field is to transition these observations into defined molecular mechanistic studies of symbiosis and dysbiosis. In order to meet this challenge, many of these newly identified microbes will need to be adapted for use in experimental models. Consequently, this review presents a comprehensive overview of the molecular microbiology tools and techniques that have played crucial roles in genetic studies of the bacteria found within the human oral microbiota. Here, we will use specific examples from the oral microbiome literature to illustrate the biology supporting these techniques, why they are needed in the field, and how such technologies have been implemented. It is hoped that this information can serve as a useful reference guide to help catalyze molecular microbiology studies of the many new understudied and uncharacterized species identified at different mucosal sites in the body.

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.

Microorganisms: crucial players of smokeless tobacco for several health attributes

Global consumption of smokeless tobacco (SLT) reached 300 million users worldwide majorly from middle-income countries. More than 4000 chemical compounds represent it as one of the noxious consumable products by humans. Besides toxicants/carcinogens, the heavy microbial load on smokeless tobacco further keeps human health at higher risk. Several of these inhabitant microbes participate in biofilm formation and secrete endotoxin/mycotoxins and proinflammatory-like molecules, leading to several oral diseases. Tobacco-associated bacteria exhibit their role in tobacco-specific nitrosamines (TSNAs) formation and acetaldehyde production; both are well-documented carcinogens. Moreover, tobacco exhibits the potential to alter the oral microbiome and induce dysbiotic conditions that lead to the onset of several oral and systemic diseases. Traditional cultivation approaches of microbiology provide partial information of microbial communities of a habitat; therefore, microbiomics has now been employed to study the metagenomes of entire microbial communities. In the past 5 years, few NGS-based investigations have revealed that SLT harbors four dominant phyla (Firmicutes, Proteobacteria, Actinobacteria, and Bacteroidetes) dominating Bacillus spp. and/or Pseudomonas spp. However, functional characterization of their genetic elements will be a more informative attribute to understand the correlation between inhabitant microbial diversity and their relatedness concerning abundance and diseases. This review provides an update on the microbial diversity of SLT and its associated attributes in human health. KEY POINTS: • Heavy microbial load on smokeless tobacco alarms for poor oral hygiene. • Inhabitant microorganisms of SLT participate in TSNA and biofilm formation. • SLTs alter the oral microbiome and causes oral dysbiosis.

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

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

Characterization of an intermolecular quaternary interaction between discrete segments of the Streptococcus mutans adhesin P1 by NMR spectroscopy

Cell surface-localized P1 adhesin (aka Antigen I/II or PAc) of the cariogenic bacterium Streptococcus mutans mediates sucrose-independent adhesion to tooth surfaces. Previous studies showed that P1's C-terminal segment (C123, AgII) is also liberated as a separate polypeptide, contributes to cellular adhesion, interacts specifically with intact P1 on the cell surface, and forms amyloid fibrils. Identifying how C123 specifically interacts with P1 at the atomic level is essential for understanding related virulence properties of S. mutans. However, with sizes of ~ 51 and ~ 185 kDa, respectively, C123 and full-length P1 are too large to achieve high-resolution data for full structural analysis by NMR. Here, we report on biologically relevant interactions of the individual C3 domain with A3VP1, a polypeptide that represents the apical head of P1 as it is projected on the cell surface. Also evaluated are C3's interaction with C12 and the adhesion-inhibiting monoclonal antibody (MAb) 6-8C. NMR titration experiments with ¹⁵ N-enriched C3 demonstrate its specific binding to A3VP1. Based on resolved C3 assignments, two binding sites, proximal and distal, are identified. Complementary NMR titration of A3VP1 with a C3/C12 complex suggests that binding of A3VP1 occurs on the distal C3 binding site, while the proximal site is occupied by C12. The MAb 6-8C binding interface to C3 overlaps with that of A3VP1 at the distal site. Together, these results identify a specific C3-A3VP1 interaction that serves as a foundation for understanding the interaction of C123 with P1 on the bacterial surface and the related biological processes that stem from this interaction. DATABASE: BMRB submission code: 27935.

Infection Function of Adhesin-Like Protein ALP609 from Spiroplasma melliferum CH-1

Spiroplasma melliferum is the causative agent of spiroplasmosis in honeybees. During infection, adhesion of spiroplasmas to the host cells through adhesion factors is a crucial step. In this study, we identified an adhesin-like protein (ALP609) in S. melliferum CH-1 and investigated its role in the infection. To determine whether ALP609 is an adhesion factor, we performed indirect immunofluorescence microscopy to visualize its adhesion properties. Subsequently, an infection model of S. melliferum CH-1 was established using primary midgut cells of Apis mellifera to examine the adhesion and invasion of spiroplasma using anti-ALP609 antibodies inhibition assays and competition assays with recombinant ALP609 in vitro. We found that anti-ALP609 antibodies could inhibit the adhesion and invasion of spiroplasma to the midgut cells of A. mellifera and reduce midgut cell invasion on increased exposure to recombinant ALP609. To the best of our knowledge, this is the first report identifying adhesion-related factors in S. melliferum. Our results suggested that ALP609 is an adhesin-like protein critical for invasion of S. melliferum CH-1 into midgut cells of A. mellifera.

Effects of sub-minimum inhibitory concentrations of lemon essential oil on the acid tolerance and biofilm formation of Streptococcus mutans

OBJECTIVES

Lemon essential oil (LEO) is a kind of secondary metabolite from lemon peels and has been found to inhibit cariogenic bacteria for decades. However, its effects on main cariogenic virulence factors are rarely reported. The present study aimed to investigate the effects of sub-minimum inhibitory concentrations (sub-MICs) of LEO on the acid tolerance and biofilm formation of Streptococcus mutans (S. mutans) and preliminarily reveal the possible underlying mechanisms.

DESIGNS

Effects of LEO on the acid tolerance and biofilm formation of S. mutans were investigated by the broth dilution method and crystal violet staining method respectively. The expression of luxS, srtA and spaP gene was also determined to explore the underlying mechanism. In addition, Tea polyphenols (TP), a major natural inhibitor of cariogenic virulence factors, and limonene (LIM), the major component of LEO, were selected as comparisons to evaluate the effects of LEO.

RESULTS

Sub-MICs of LEO, LIM and TP exhibited a dose-dependent inhibition of growth of S. mutans at pH ranging from 4.0 to 7.0. The formation of S. mutans biofilm was remarkably inhibited and the inhibitory rates of LEO, LIM and TP were 97.87%, 94.88% and 96.01% respectively at 1/2 MIC. Similarly, a down-regulation was observed in the expression of luxS, srtA and spaP gene at sub-MIC levels.

CONCLUSIONS

Effects of LEO were similar or slightly stronger than LIM and TP, suggesting that LEO might represent a novel, natural anticarious agent that inhibited the specific genes associated with bacterial acid tolerance and biofilm formation without necessarily affecting the growth of oral bacteria.

Streptococcus mutans SpaP binds to RadD of Fusobacterium nucleatum ssp. polymorphum

Adhesin-mediated bacterial interspecies interactions are important elements in oral biofilm formation. They often occur on a species-specific level, which could determine health or disease association of a biofilm community. Among the key players involved in these processes are the ubiquitous fusobacteria that have been recognized for their ability to interact with numerous different binding partners. Fusobacterial interactions with Streptococcus mutans, an important oral cariogenic pathogen, have previously been described but most studies focused on binding to non-mutans streptococci and specific cognate adhesin pairs remain to be identified. Here, we demonstrated differential binding of oral fusobacteria to S. mutans. Screening of existing mutant derivatives indicated SpaP as the major S. mutans adhesin specific for binding to Fusobacterium nucleatum ssp. polymorphum but none of the other oral fusobacteria tested. We inactivated RadD, a known adhesin of F. nucleatum ssp. nucleatum for interaction with a number of gram-positive species, in F. nucleatum ssp. polymorphum and used a Lactococcus lactis heterologous SpaP expression system to demonstrate SpaP interaction with RadD of F. nucleatum ssp. polymorphum. This is a novel function for SpaP, which has mainly been characterized as an adhesin for binding to host proteins including salivary glycoproteins. In conclusion, we describe an additional role for SpaP as adhesin in interspecies adherence with RadD-SpaP as the interacting adhesin pair for binding between S. mutans and F. nucleatum ssp. polymorphum. Furthermore, S. mutans attachment to oral fusobacteria appears to involve species- and subspecies-dependent adhesin interactions.

Flagellin-rPAc vaccine inhibits biofilm formation but not proliferation of S. mutans

As the main etiologic bacterium of dental caries, Streptococcus mutans (S. mutans) has been considered as the primary object of vaccine research. We previously constructed a recombinant flagellin-rPAc fusion protein (KF-rPAc) that consists of an alanine-rich region to proline-rich region fragment of PAc (rPAc) from S. mutans and flagellin KF from E.coli K12 strain. Intranasal (i.n) immunization of KF-rPAc could induce high level of rPAc-specific antibody responses and offer robust protection against dental caries. In caries development, biofilm formation was considered as the necessary process involved. As PAc possesses other activities besides affecting adherence of S. mutans to salivary glycoproteins, we wondered whether rPAc-specific antibody responses induced by KF-rPAc could inhibit biofilm formation. Hence, in the present study, a simple and convenient in vitro biofilm model of S. mutans was constructed without saliva pre-coated. Both serum and saliva from KF-rPAc immunized rats significantly inhibited biofilm formation. Moreover, with the presence of serum or saliva, the biofilm formation is negatively correlated with the level of rPAc-specific antibody, and positively correlated with caries scores in rat. Moreover, in immunized mice, the level of rPAc-specific antibody also negatively correlated with the biofilm formation. Unlike ampicillin, serum of KF-rPAc immunized mice only inhibited biofilm formation but not proliferation. All together, we discovered that besides the well known blocking adherence of S. mutans to salivary glycoproteins by rPAc-specific antibody, flagellin-rPAc vaccine could also protects tooth from caries by inhibiting biofilm structure formation in between bacteria.

Collagen-binding proteins of Streptococcus mutans and related streptococci

The ability of Streptococcus mutans to interact with collagen through the expression of collagen-binding proteins (CBPs) bestows this oral pathogen with an alternative to the sucrose-dependent mechanism of colonization classically attributed to caries development. Based on the abundance and distribution of collagen throughout the human body, stringent adherence to this molecule grants S. mutans with the opportunity to establish infection at different host sites. Surface proteins, such as SpaP, WapA, Cnm and Cbm, have been shown to bind collagen in vitro, and it has been suggested that these molecules play a role in colonization of oral and extra-oral tissues. However, robust collagen binding is not achieved by all strains of S. mutans, particularly those that lack Cnm or Cbm. These observations merit careful dissection of the contribution from these different CBPs towards tissue colonization and virulence. In this review, we will discuss the current understanding of mechanisms used by S. mutans and related streptococci to colonize collagenous tissues, and the possible contribution of CBPs to infections in different sites of the host.

Specific binding of a naturally occurring amyloidogenic fragment of Streptococcus mutans adhesin P1 to intact P1 on the cell surface characterized by solid state NMR spectroscopy

The P1 adhesin (aka Antigen I/II or PAc) of the cariogenic bacterium Streptococcus mutans is a cell surface-localized protein involved in sucrose-independent adhesion and colonization of the tooth surface. The immunoreactive and adhesive properties of S. mutans suggest an unusual functional quaternary ultrastructure comprised of intact P1 covalently attached to the cell wall and interacting with non-covalently associated proteolytic fragments thereof, particularly the ~57-kDa C-terminal fragment C123 previously identified as Antigen II. S. mutans is capable of amyloid formation when grown in a biofilm and P1 is among its amyloidogenic proteins. The C123 fragment of P1 readily forms amyloid fibers in vitro suggesting it may play a role in the formation of functional amyloid during biofilm development. Using wild-type and P1-deficient strains of S. mutans, we demonstrate that solid state NMR (ssNMR) spectroscopy can be used to (1) globally characterize cell walls isolated from a Gram-positive bacterium and (2) characterize the specific binding of heterologously expressed, isotopically-enriched C123 to cell wall-anchored P1. Our results lay the groundwork for future high-resolution characterization of the C123/P1 ultrastructure and subsequent steps in biofilm formation via ssNMR spectroscopy, and they support an emerging model of S. mutans colonization whereby quaternary P1-C123 interactions confer adhesive properties important to binding to immobilized human salivary agglutinin.

Molecule Targeting Glucosyltransferase Inhibits Streptococcus mutans Biofilm Formation and Virulence

Dental plaque biofilms are responsible for numerous chronic oral infections and cause a severe health burden. Many of these infections cannot be eliminated, as the bacteria in the biofilms are resistant to the host's immune defenses and antibiotics. There is a critical need to develop new strategies to control biofilm-based infections. Biofilm formation in Streptococcus mutans is promoted by major virulence factors known as glucosyltransferases (Gtfs), which synthesize adhesive extracellular polysaccharides (EPS). The current study was designed to identify novel molecules that target Gtfs, thereby inhibiting S. mutans biofilm formation and having the potential to prevent dental caries. Structure-based virtual screening of approximately 150,000 commercially available compounds against the crystal structure of the glucosyltransferase domain of the GtfC protein from S. mutans resulted in the identification of a quinoxaline derivative, 2-(4-methoxyphenyl)-N-(3-{[2-(4-methoxyphenyl)ethyl]imino}-1,4-dihydro-2-quinoxalinylidene)ethanamine, as a potential Gtf inhibitor. In vitro assays showed that the compound was capable of inhibiting EPS synthesis and biofilm formation in S. mutans by selectively antagonizing Gtfs instead of by killing the bacteria directly. Moreover, the in vivo anti-caries efficacy of the compound was evaluated in a rat model. We found that the compound significantly reduced the incidence and severity of smooth and sulcal-surface caries in vivo with a concomitant reduction in the percentage of S. mutans in the animals' dental plaque (P < 0.05). Taken together, these results represent the first description of a compound that targets Gtfs and that has the capacity to inhibit biofilm formation and the cariogenicity of S. mutans.

Variation of expression defects in cell surface 190-kDa protein antigen of Streptococcus mutans

Streptococcus mutans, which consists of four serotypes, c, e, f, and k, possesses a 190-kDa cell surface protein antigen (PA) for initial tooth adhesion. We used Western blot analysis to determine PA expression in 750 S. mutans isolates from 150 subjects and found a significantly higher prevalence of the isolates with PA expression defects in serotypes f and k compared to serotypes c and e. Moreover, the defect patterns could be classified into three types; no PA expression on whole bacterial cells and in their supernatant samples (Type N1), PA expression mainly seen in supernatant samples (Type N2), and only low expression of PA in the samples of whole bacterial cells (Type W). The underlying reasons for the defects were mutations in the gene encoding PA as well as in the transcriptional processing of this gene for Type N1, defects in the sortase gene for Type N2, and low mRNA expression of PA for Type W. Since cellular hydrophobicity and phagocytosis susceptibility of the PA-defective isolates were significantly lower than those of the normal expression isolates, the potential implication of such defective isolates in systemic diseases involving bacteremia other than dental caries was suggested. Additionally, multilocus sequence typing was utilized to characterize S. mutans clones that represented a proportion of isolates with PA defects of 65-100%. Therefore, we described the molecular basis for variation defects in PA expression of S. mutans. Furthermore, we also emphasized the strong association between PA expression defects and serotypes f and k as well as the clonal relationships among these isolates.

Binding forces of Streptococcus mutans P1 adhesin

Streptococcus mutans is a Gram-positive oral bacterium that is a primary etiological agent associated with human dental caries. In the oral cavity, S. mutans adheres to immobilized salivary agglutinin (SAG) contained within the salivary pellicle on the tooth surface. Binding to SAG is mediated by cell surface P1, a multifunctional adhesin that is also capable of interacting with extracellular matrix proteins. This may be of particular importance outside of the oral cavity as S. mutans has been associated with infective endocarditis and detected in atherosclerotic plaque. Despite the biomedical importance of P1, its binding mechanisms are not completely understood. In this work, we use atomic force microscopy-based single-molecule and single-cell force spectroscopy to quantify the nanoscale forces driving P1-mediated adhesion. Single-molecule experiments show that full-length P1, as well as fragments containing only the P1 globular head or C-terminal region, binds to SAG with relatively weak forces (∼50 pN). In contrast, single-cell analyses reveal that adhesion of a single S. mutans cell to SAG is mediated by strong (∼500 pN) and long-range (up to 6000 nm) forces. This is likely due to the binding of multiple P1 adhesins to self-associated gp340 glycoproteins. Such a cooperative, long-range character of the S. mutans-SAG interaction would therefore dramatically increase the strength and duration of cell adhesion. We also demonstrate, at single-molecule and single-cell levels, the interaction of P1 with fibronectin and collagen, as well as with hydrophobic, but not hydrophilic, substrates. The binding mechanism (strong forces, cooperativity, broad specificity) of P1 provides a molecular basis for its multifunctional adhesion properties. Our methodology represents a valuable approach to probe the binding forces of bacterial adhesins and offers a tractable methodology to assess anti-adhesion therapy.

Identification of a supramolecular functional architecture of Streptococcus mutans adhesin P1 on the bacterial cell surface

P1 (antigen I/II) is a sucrose-independent adhesin of Streptococcus mutans whose functional architecture on the cell surface is not fully understood. S. mutans cells subjected to mechanical extraction were significantly diminished in adherence to immobilized salivary agglutinin but remained immunoreactive and were readily aggregated by fluid-phase salivary agglutinin. Bacterial adherence was restored by incubation of postextracted cells with P1 fragments that contain each of the two known adhesive domains. In contrast to untreated cells, glutaraldehyde-treated bacteria gained reactivity with anti-C-terminal monoclonal antibodies (mAbs), whereas epitopes recognized by mAbs against other portions of the molecule were masked. Surface plasmon resonance experiments demonstrated the ability of apical and C-terminal fragments of P1 to interact. Binding of several different anti-P1 mAbs to unfixed cells triggered release of a C-terminal fragment from the bacterial surface, suggesting a novel mechanism of action of certain adherence-inhibiting antibodies. We also used atomic force microscopy-based single molecule force spectroscopy with tips bearing various mAbs to elucidate the spatial organization and orientation of P1 on living bacteria. The similar rupture lengths detected using mAbs against the head and C-terminal regions, which are widely separated in the tertiary structure, suggest a higher order architecture in which these domains are in close proximity on the cell surface. Taken together, our results suggest a supramolecular organization in which additional P1 polypeptides, including the C-terminal segment originally identified as antigen II, associate with covalently attached P1 to form the functional adhesive layer.

SMU.746-SMU.747, a putative membrane permease complex, is involved in aciduricity, acidogenesis, and biofilm formation in Streptococcus mutans

Dental caries induced by Streptococcus mutans is one of the most prevalent chronic infectious diseases worldwide. The pathogenicity of S. mutans relies on the bacterium's ability to colonize tooth surfaces and survive a strongly acidic environment. We performed an ISS1 transposon mutagenesis to screen for acid-sensitive mutants of S. mutans and identified an SMU.746-SMU.747 gene cluster that is needed for aciduricity. SMU.746 and SMU.747 appear to be organized in an operon and encode a putative membrane-associated permease. SMU.746- and SMU.747-deficient mutants showed a reduced ability to grow in acidified medium. However, the short-term or long-term acid survival capacity and F1F0 ATPase activity remained unaffected in the mutants. Furthermore, deletion of both genes did not change cell membrane permeability and the oxidative and heat stress responses. Growth was severely affected even with slight acidification of the defined medium (pH 6.5). The ability of the mutant strain to acidify the defined medium during growth in the presence of glucose and sucrose was significantly reduced, although the glycolysis rate was only slightly affected. Surprisingly, deletion of the SMU.746-SMU.747 genes triggered increased biofilm formation in low-pH medium. The observed effects were more striking in a chemically defined medium. We speculate that the SMU.746-SMU.747 complex is responsible for amino acid transport, and we discuss its possible role in colonization and survival in the oral environment.

Role of sortase in Streptococcus mutans under the effect of nicotine

Streptococcus mutans is a common Gram-positive bacterium and plays a significant role in dental caries. Tobacco and/or nicotine have documented effects on S. mutans growth and colonization. Sortase A is used by many Gram-positive bacteria, including S. mutans, to facilitate the insertion of certain cell surface proteins, containing an LPXTGX motif such as antigen I/II. This study examined the effect of nicotine on the function of sortase A to control the physiology and growth of S. mutans using wild-type S. mutans NG8, and its isogenic sortase-defective and -complemented strains. Briefly, the strains were treated with increasing amounts of nicotine in planktonic growth, biofilm metabolism, and sucrose-induced and saliva-induced antigen I/II-dependent biofilm formation assays. The strains exhibited no significant differences with different concentrations of nicotine in planktonic growth assays. However, they had significantly increased (P≤0.05) biofilm metabolic activity (2- to 3-fold increase) as the concentration of nicotine increased. Furthermore, the sortase-defective strain was more sensitive metabolically to nicotine than the wild-type or sortase-complemented strains. All strains had significantly increased sucrose-induced biofilm formation (2- to 3-fold increase) as a result of increasing concentrations of nicotine. However, the sortase-defective strain was not able to make as much sucrose- and saliva-induced biofilm as the wild-type NG8 did with increasing nicotine concentrations. These results indicated that nicotine increased metabolic activity and sucrose-induced biofilm formation. The saliva-induced biofilm formation assay and qPCR data suggested that antigen I/II was upregulated with nicotine but biofilm was not able to be formed as much as wild-type NG8 without functional sortase A.

Potential high virulence for infective endocarditis in Streptococcus mutans strains with collagen-binding proteins but lacking PA expression

OBJECTIVE

Streptococcus mutans, an aetiologic agent of dental caries, is a pathogen for infective endocarditis (IE). We investigated strains that express collagen-binding proteins (CBPs) with further classification based on expression of the 190-kDa protein antigen (PA).

METHOD

Zeta-potential values of strains TW871 (CBP+/PA+) and MT8148 (CBP-/PA+), and their respective PA-defective mutant strains TW871PD (CBP+/PA-) and MT8148PD (CBP-/PA-), were analysed, as were their adhesion to and invasion of human umbilical vein endothelial cells (HUVECs). The distribution of strains from the oral cavities of 200 healthy individuals was analysed for CBP and/or PA expression and the strains were characterised for their adhesion and invasion properties.

RESULTS

TW871PD and MT8148PD showed significantly lower zeta-potential values than TW871 and MT8148, respectively. Collagen-binding rates were significantly higher for TW871PD than for TW871 but nearly negligible for MT8148 and MT8148PD. The adhesion and invasion rates of HUVECs were significantly higher for TW871PD than for TW871 and significantly higher for TW871 than for MT8148 and MT8148PD. The prevalence of CBP+ strains was ~10% and ~3% in the case of CBP+/PA- strains. Analyses of 200 clinical strains showed the CBP+/PA- group to have higher adhesion and invasion rates than other groups.

CONCLUSIONS

CBP+/PA- S. mutans strains, despite their low distribution frequency, may be highly virulent for infective endocarditis.

Curcumin inhibits the Sortase A activity of the Streptococcus mutans UA159

Streptococcus mutans (S. mutans) forms part of the commensal microflora and is deemed to be the major pathogen responsible for the generation of dental caries. The enzyme, sortase A enzyme, modulates the surface properties and cariogenicity of S. mutans. Curcumin has been reported to be an inhibitor of Staphylococcus aureus sortase A. In this study, inhibition of a purified S. mutans UA159 sortase A by curcumin was evaluated. Curcumin exerted strong inhibitory activity with a half maximal inhibitory concentration (IC50) of 10.2 ± 0.7 μM which was lower than the minimum inhibitory concentration of 175 μM and the minimum bactericidal concentration of 350 μM. These results indicated that curcumin is a S. mutans UA159 sortase A inhibitor and therefore represents as a promising anticaries agent.

Curcumin reduces Streptococcus mutans biofilm formation by inhibiting sortase A activity

BACKGROUND

Sortase A is an enzyme responsible for the covalent attachment of Pac proteins to the cell wall in Streptococcus mutans. It has been shown to play a role in modulating the surface properties and the biofilm formation and influence the cariogenicity of S. mutans. Curcumin, an active ingredient of turmeric, was reported to be an inhibitor for Staphylococcus aureus sortase A. The aim of this study was to investigate the inhibitory ability of curcumin against S. mutans sortase A and the effect of curcumin for biofilm formation.

METHODS

The antimicrobial activity of the curcumin to the S. mutans and inhibitory ability of the curcumin against the purified sortase A in vitro were detected. Western-blot and real-time PCR were used to analysis the sortase A mediated Pac protein changes when the S. mutans was cultured with curcumin. The curcumin on the S. mutans biofilm formation was determined by biofilm formation analysis.

RESULTS

Curcumin can inhibit purified S. mutans sortase A with a half-maximal inhibitory concentration (IC50) of (10.2±0.7)μmol/l, which is lower than minimum inhibitory concentration (MIC) of 175μmol/l. Curcumin (15μmol/l) was found to release the Pac protein to the supernatant and reduce S. mutans biofilm formation.

CONCLUSIONS

These results indicated that curcumin is an S. mutans sortase A inhibitor and has promising anti-caries characteristics through an anti-adhesion-mediated mechanism.

Isolation and characterization of an antibacterial biflavonoid from an African chewing stick Garcinia kola Heckel (Clusiaceae)

ETHNOPHARMACOLOGICAL RELEVANCE

The use of African chewing sticks in maintaining oral health is widely practiced in African countries. It has been reported that chewing stick users have a lower rate of dental caries and a better general oral health than non-users. It is generally thought that the beneficial effect of chewing stick is attributed to the mechanical cleansing effect and antimicrobial substances present in the stick. However, the active antimicrobial substances remain uncharacterized.

AIM OF THE STUDY

To provide a scientific basis for the anti-caries effect of African chewing sticks, the authors purify an active antibacterial compound from Garcinia kola Heckel, a Nigerian chewing stick and examined the antibacterial activity of this compound against the cariogenic bacterium Streptococcus mutans.

MATERIALS AND METHODS

Methanol extract was prepared from Garcinia kola and was further fractionated by solvent extractions. Silica gel chromatography was used to purify the antibacterial compound from the active fraction. The identity of the purified compound was determined by NMR analysis. The antibacterial activity of the purified compound was examined by standard microbiological assays.

RESULTS

The antibacterial activity was found in the ether fraction and the active compound was isolated and determined to be a biflavonoid named GB1. GB1 was active against Streptococcus mutans and other oral bacteria with minimum inhibitory concentration (MIC) values of 32-64μg/ml. The basis for the antibacterial effect of GB1 was investigated using Streptococcus mutans as the target. At 256μg/ml, GB1 exhibited some bacteriocidal activity against Streptococcus mutans and induced the aggregation of Streptococcus mutans. GB1 has no apparent effects on protein synthesis and DNA synthesis but inhibited glucose uptake and utilization by Streptococcus mutans suggesting that GB1 exerts its antibacterial effect by inhibiting metabolism. GB1 also inhibited the formation of water-insoluble glucan by the extracellular glucosyltransferases from Streptococcus mutans in a dose-dependent manner. Streptococcus mutans did not develop resistance to GB1 upon subculturing in the presence of sub-MIC level of the biflavonoid.

CONCLUSION

The antibacterial effect and glucan synthesis-inhibition property of this biflavonoid may account for some of the beneficial effects reported in the chewing stick users.

The AgI/II family adhesin AspA is required for respiratory infection by Streptococcus pyogenes

Streptococcus pyogenes (GAS) is a human pathogen that causes pharyngitis and invasive diseases such as toxic shock syndrome and sepsis. The upper respiratory tract is the primary reservoir from which GAS can infect new hosts and cause disease. The factors involved in colonisation are incompletely known however. Previous evidence in oral streptococci has shown that the AgI/II family proteins are involved. We hypothesized that the AspA member of this family might be involved in GAS colonization. We describe a novel mouse model of GAS colonization of the nasopharynx and lower respiratory tract to elucidate these interactions. We used two clinical M serotypes expressing AspA, and their aspA gene deletant isogenic mutants in experiments using adherence assays to respiratory epithelium, macrophage phagocytosis and neutrophil killing assays and in vivo models of respiratory tract colonisation and infection. We demonstrated the requirement for AspA in colonization of the respiratory tract. AspA mutants were cleared from the respiratory tract and were deficient in adherence to epithelial cells, and susceptible to phagocytosis. Expression of AspA in the surrogate host Lactococcus lactis protected bacteria from phagocytosis. Our results suggest that AspA has an essential role in respiratory infection, and may function as a novel anti-phagocytic factor.

An intramolecular interaction involving the N terminus of a streptococcal adhesin affects its conformation and adhesive function

BACKGROUND

P1 is an adhesin on the surface of Streptococcus mutans.

RESULTS

Destroying the high affinity interaction between the N and C termini of S. mutans P1 creates a non-adherent phenotype.

CONCLUSION

The N terminus facilitates proper folding, function, and stability within recombinant P1.

SIGNIFICANCE

The relationship between folding, maturation, and cell surface assembly is critical to understanding the P1 mechanism of action. The adhesin P1 is localized on the surface of the oral pathogen Streptococcus mutans and facilitates an interaction with the glycoprotein complex salivary agglutinin that is comprised primarily of the scavenger receptor gp340. Recent crystal structures of P1 display an unusual structure in which the protein folds back upon itself to form an elongated hybrid helical stalk with a globular head at the apex and a globular C-terminal region at the base. The N terminus of P1 has not yet been characterized. In this report we describe the contribution of an interaction between the N-terminal and C-terminal portions of the protein that is required for proper function of P1 on the surface of S. mutans. Utilizing recombinant N-terminal and C-terminal fragments, we employed isothermal titration calorimetry and native gel electrophoresis to demonstrate that these fragments form a high affinity and stable complex in solution. Furthermore, circular dichroism and surface plasmon resonance measurements indicated that the N-terminal fragment contributes to the folding and increases the functionality of the C-terminal fragment in trans. Finally, we utilized circular dichroism, surface plasmon resonance, and differential scanning calorimetry to show that an N-terminal 106-amino acid segment within P1 contributes to the proper folding and function of the full-length recombinant molecule and increases the stability of its elongated hybrid helical stalk.

Functional amyloid formation by Streptococcus mutans

Dental caries is a common infectious disease associated with acidogenic and aciduric bacteria, including Streptococcus mutans. Organisms that cause cavities form recalcitrant biofilms, generate acids from dietary sugars and tolerate acid end products. It has recently been recognized that micro-organisms can produce functional amyloids that are integral to biofilm development. We now show that the S. mutans cell-surface-localized adhesin P1 (antigen I/II, PAc) is an amyloid-forming protein. This conclusion is based on the defining properties of amyloids, including binding by the amyloidophilic dyes Congo red (CR) and Thioflavin T (ThT), visualization of amyloid fibres by transmission electron microscopy and the green birefringent properties of CR-stained protein aggregates when viewed under cross-polarized light. We provide evidence that amyloid is present in human dental plaque and is produced by both laboratory strains and clinical isolates of S. mutans. We provide further evidence that amyloid formation is not limited to P1, since bacterial colonies without this adhesin demonstrate residual green birefringence. However, S. mutans lacking sortase, the transpeptidase enzyme that mediates the covalent linkage of its substrates to the cell-wall peptidoglycan, including P1 and five other proteins, is not birefringent when stained with CR and does not form biofilms. Biofilm formation is inhibited when S. mutans is cultured in the presence of known inhibitors of amyloid fibrillization, including CR, Thioflavin S and epigallocatechin-3-gallate, which also inhibited ThT uptake by S. mutans extracellular proteins. Taken together, these results indicate that S. mutans is an amyloid-forming organism and suggest that amyloidogenesis contributes to biofilm formation by this oral microbe.

Contribution of a Streptococcus mutans antigen expressed by a Salmonella vector vaccine in dendritic cell activation

A Salmonella vector vaccine expressing the saliva-binding region (SBR) of the adhesin AgI/II of Streptococcus mutans has been shown to induce a mixed Th1/Th2 anti-SBR immune response in mice and to require Toll-like receptor 2 (TLR2), TLR4, and MyD88 signaling for the induction of mucosal anti-SBR antibody responses. Since dendritic cells (DC) are critical in innate and adaptive immunity, the present study assessed the role of SBR expression by the vector vaccine in DC activation. Bone marrow-derived DC from wild-type and TLR2, TLR4, and MyD88 knockout mice were stimulated with Salmonella vector BRD509, the SBR-expressing Salmonella vector vaccine BRD509(pSBRT7), or SBR protein, and the DC responses to different stimuli were compared by assessing costimulatory molecule expression, cytokine production, and signaling pathways. The DC response to both BRD509(pSBRT7) and BRD509 was dependent mainly on TLR4. BRD509(pSBRT7) and BRD509 induced upregulation of CD80, CD86, CD40, and major histocompatibility complex class II (MHC II) expression. Lower levels of interleukin-10 (IL-10) and IL-12p40 were produced by BRD509(pSBRT7)-stimulated DC than by BRD509-stimulated DC. Furthermore, BRD509(pSBRT7)-stimulated DC showed decreased p38 phosphorylation compared to that induced by DC stimulated with BRD509. However, BRD509(pSBRT7)-treated DC produced a higher level of IL-6 than BRD509-stimulated cells. The low IL-12p40 and high IL-6 cytokine profile expressed by BRD509(pSBRT7)-stimulated DC may represent a shift toward a Th2 response, as suggested by the increased expression in Jagged-1. These results provide novel evidence that a heterologous protein expressed by a Salmonella vector vaccine can differentially affect DC activation.

Functional characterization of a lipoprotein-encoding operon in Campylobacter jejuni

BACKGROUND

Bacterial lipoproteins have important functions in bacterial pathogenesis and physiology. In Campylobacter jejuni, a major foodborne pathogen causing gastroenteritis in humans, the majority of lipoproteins have not been functionally characterized. Previously, we showed by DNA microarray that CmeR, a transcriptional regulator repressing the expression of the multidrug efflux pump CmeABC, modulates the expression of a three-gene operon (cj0089, cj0090, and cj0091) encoding a cluster of lipoproteins in C. jejuni.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, we characterized the function and regulation of the cj0089-cj0090-cj0091 operon. In contrast to the repression of cmeABC, CmeR activates the expression of the lipoprotein genes and the regulation is confirmed by immunoblotting using anti-Cj0089 and anti-Cj0091 antibodies. Gel mobility shift assay showed that CmeR directly binds to the promoter of the lipoprotein operon, but the binding is much weaker compared with the promoter of cmeABC. Analysis of different cellular fractions indicated that Cj0089 was associated with the inner membrane, while Cj0091 was located on the outer membrane. Inactivation of cj0091, but not cj0089, significantly reduced the adherence of C. jejuni to INT 407 cells in vitro, indicating that Cj0091 has a function in adherence. When inoculated into chickens, the Cj0091 mutant also showed a defect in early colonization of the intestinal tract, suggesting that Cj0091 contributes to Campylobacter colonization in vivo. It was also shown that Cj0091 was produced and immunogenic in chickens that were naturally infected by C. jejuni.

CONCLUSION/SIGNIFICANCE

These results indicate that the lipoprotein operon is subject to direct regulation by CmeR and that Cj0091 functions as an adhesion mechanism in C. jejuni and contributes to Campylobacter colonization of the intestinal tract in animal hosts.

A mechanism for extremely weak SpaP-expression in Streptococcus mutans strain Z1

Background

Streptococcus mutans surface-protein antigen (SpaP, PAc, or antigen I/II) has been well known to play an important role in initial attachment to tooth surfaces. However, strains with weak SpaP-expression were recently reported to be found in natural populations of S. mutans. The S. mutans gbpC-negative strain Z1, which we previously isolated from saliva and plaque samples, apparently expresses relatively low levels of SpaP protein compared to S. mutans strains MT8148 or UA159.

Objective

To elucidate the mechanism for weak SpaP-expression in this strain, the spaP gene region in strain Z1 was amplified by polymerase chain reaction (PCR) and analyzed.

Methods

Allelic exchange mutants between strains Z1 and UA159 involving the spaP gene region were constructed. The SpaP protein expressed in the mutants was detected with Coomasie Brilliant Blue (CBB)-staining and Western blot analysis following SDS-PAGE.

Results

The 4689 bp spaP gene coding sequence for Z1 appeared to be intact. In contrast, a 20 bp nucleotide sequence appeared to be deleted from the region immediately upstream from the Z1 spaP gene when compared to the same region in UA159. The 216 bp and 237 bp intergenic fragments upstream from the spaP gene, respectively, from Z1 and UA159 were isolated, modified, and transformed into the other strain by allelic replacement. The resultant UA159-promoter region-mutant exhibited extremely weak SpaP-expression similar to that of strain Z1 and the Z1 complemented mutant expressed Spa protein levels like that of strain UA159.

Conclusion

These results suggest that weak SpaP-expression in strain Z1 resulted from a 20 bp-deletion in the spaP gene promoter region.

Role of the cell wall microenvironment in expression of a heterologous SpaP-S1 fusion protein by Streptococcus gordonii

The charge density in the cell wall microenvironment of Gram-positive bacteria is believed to influence the expression of heterologous proteins. To test this, the expression of a SpaP-S1 fusion protein, consisting of the surface protein SpaP of Streptococcus mutans and a pertussis toxin S1 fragment, was studied in the live vaccine candidate bacterium Streptococcus gordonii. Results showed that the parent strain PM14 expressed very low levels of SpaP-S1. By comparison, the dlt mutant strain, which has a mutation in the dlt operon preventing d-alanylation of the cell wall lipoteichoic acids, and another mutant strain, OB219(pPM14), which lacks the LPXTG major surface proteins SspA and SspB, expressed more SpaP-S1 than the parent. Both the dlt mutant and the OB219(pPM14) strain had a more negatively charged cell surface than PM14, suggesting that the negative charged cell wall played a role in the increase in SpaP-S1 production. Accordingly, the addition of Ca(2+), Mg(2+), and K(+), presumably increasing the positive charge of the cell wall, led to a reduction in SpaP-S1 production, while the addition of bicarbonate resulted in an increase in SpaP-S1 production. The level of SpaP-S1 production could be correlated with the level of PrsA, a peptidyl-prolyl cis/trans isomerase, in the cells. PrsA expression appears to be regulated by the cell envelope stress two-component regulatory system LiaSR. The results collectively indicate that the charge density of the cell wall microenvironment can modulate heterologous SpaP-S1 protein expression in S. gordonii and that this modulation is mediated by the level of PrsA, whose expression is regulated by the LiaSR two-component regulatory system.

Identification of a novel bacteriocin regulatory system in Streptococcus mutans

Recently, we described the function of an uncharacterized two-gene regulatory system consisting of a LytTR family transcription regulator and a putative membrane protein, which we referred to as the hdrRM operon. In this study, we determined that the HdrRM system controls the expression of an analogous uncharacterized regulatory system annotated as SMU.2080 and SMU.2081. Like hdrRM, the SMU.2080-2081 operon encodes a LytTR family transcription regulator and putative membrane protein, which we now refer to as BrsR and BrsM respectively. Examination of the regulatory mechanism of the BrsRM system suggests that BrsM serves to antagonize the function of the transcription regulator BrsR. Further analyses of the regulatory role of BrsR determined that it functions as a transcription activator for a variety of bacteriocins and bacteriocin-related genes. In vitro electromobility shift assays confirmed that BrsR binds to the promoter regions of several bacteriocin genes and requires the presence of a LytTR family consensus direct repeat in order to stably bind DNA. In addition, we identified a novel regulatory scheme in which both the HdrRM and BrsRM systems coregulate each other and ultimately determine whether bacteriocin production will inhibit competitor organisms or result in lethality to the producer.

Elongated fibrillar structure of a streptococcal adhesin assembled by the high-affinity association of alpha- and PPII-helices

Streptococcus mutans antigen I/II (AgI/II) is a cell surface-localized protein adhesin that interacts with salivary components within the salivary pellicle. AgI/II contributes to virulence and has been studied as an immunological and structural target, but a fundamental understanding of its underlying architecture has been lacking. Here we report a high-resolution (1.8 A) crystal structure of the A(3)VP(1) fragment of S. mutans AgI/II that demonstrates a unique fibrillar form (155 A) through the interaction of two noncontiguous regions in the primary sequence. The A(3) repeat of the alanine-rich domain adopts an extended alpha-helix that intertwines with the P(1) repeat polyproline type II (PPII) helix to form a highly extended stalk-like structure heretofore unseen in prokaryotic or eukaryotic protein structures. Velocity sedimentation studies indicate that full-length AgI/II that contains three A/P repeats extends over 50 nanometers in length. Isothermal titration calorimetry revealed that the high-affinity association between the A(3) and P(1) helices is enthalpically driven. Two distinct binding sites on AgI/II to the host receptor salivary agglutinin (SAG) were identified by surface plasmon resonance (SPR). The current crystal structure reveals that AgI/II family proteins are extended fibrillar structures with the number of alanine- and proline-rich repeats determining their length.

Induction of neutralizing antibodies in mice immunized with an amino-terminal polypeptide of Streptococcus mutans P1 protein produced by a recombinant Bacillus subtilis strain

The oral pathogen Streptococcus mutans expresses a surface protein, P1, which interacts with the salivary pellicle on the tooth surface or with fluid-phase saliva, resulting in bacterial adhesion or aggregation, respectively. P1 is a target of protective immunity. Its N-terminal region has been associated with adhesion and aggregation functions and contains epitopes recognized by efficacious antibodies. In this study, we used Bacillus subtilis, a gram-positive expression host, to produce a recombinant N-terminal polypeptide of P1 (P1(39-512)) derived from the S. mutans strain UA159. Purified P1(39-512) reacted with an anti-full-length P1 antiserum as well as one raised against intact S. mutans cells, indicating preserved antigenicity. Immunization of mice with soluble and heat-denatured P1(39-512) induced antibodies that reacted specifically with native P1 on the surface of S. mutans cells. The anti-P1(39-512) antiserum was as effective at blocking saliva-mediated aggregation of S. mutans cells and better at blocking bacterial adhesion to saliva-coated plastic surfaces compared with the anti-full-length P1 antiserum. In addition, adsorption of the anti-P1 antiserum with P1(39-512) eliminated its ability to block the adhesion of S. mutans cells to abiotic surfaces. The present results indicate that P1(39-512), expressed and purified from a recombinant B. subtilis strain, maintains important immunological features of the native protein and represents an additional tool for the development of anticaries vaccines.

Serotype classification of Streptococcus mutans and its detection outside the oral cavity

Streptococcus mutans, generally known as a major pathogen of dental caries, is also a possible causative agent of bacteremia and infective endocarditis. S. mutans is classified into serotypes c, e, f and k based on the chemical composition of serotype-specific polysaccharides, with approximately 70-80% of strains found in the oral cavity classified as serotype c, followed by e (approximately 20%), and f and k (less than 5% each). Serotype k was recently designated as a novel serotype and shown to possess unique features, the most prominent being a defect of the glucose side chain in serotype-specific rhamnose-glucose polymers, which is related to a higher incidence of detection in cardiovascular specimens, owing to phagocytosis resistance. Molecular analyses of cardiovascular specimens showed a high detection frequency for S. mutans DNA, among which the detection rate for serotype k was quite high. These findings suggest that serotype k S. mutans possibly has a high level of virulence for systemic diseases.

Gene cloning and characterization of Streptococcus intermedius fimbriae involved in saliva-mediated aggregation

Streptococcus intermedius, an oral commensal and a cause of systemic pyogenic disease, expresses fimbriae. To identify the gene(s) encoding these fimbriae, we used a serum raised against purified fimbriae to screen libaries of recombinant lambda phages. The cloned gene cluster encoding S. intermedius fimbriae, (saliva-mediated aggregation and adherence-associated fimbriae), contained 4 ORFs, i.e. a putative ribonulease (Saf1), a putative adhesin (Saf2), the main pilus subunit (Saf3) and a sortase C (SrtC). Escherichia coli strains harboring recombinant phages and plasmids carrying the saf3 gene produced a 55kDa protein recognized by the antifimbriae serum. Saf3 contains an N-terminal signal sequence and a C-terminal cell-wall-anchoring motif LPXTG. Among strains of the Streptococcus anginosus group, only serotype g and untypable strains were found to contain the saf3 gene, to possess the fimbrial antigen and to exhibit saliva-mediated aggregation. Knockout mutants made by insertion of an erythromycin resistance gene into saf3 did not produce fimbrial structures or fimbrial antigens and did not participate in saliva-mediated aggregation. The adherent activity of mutants toward plastic wells coated with salivary agglutinin was about 65% that of the parental strain, and the reaction depended on calcium. There was no significant difference in adherence to hydroxyapatite beads pretreated with salivary agglutinin between the parental and mutant strains. These results demonstrated that Saf3 is associated with aggregation, and suggested that other molecule(s) are associated with adherence of S. intermedius.

Beneficial immunomodulation by Streptococcus mutans anti-P1 monoclonal antibodies is Fc independent and correlates with increased exposure of a relevant target epitope

We showed previously that deliberate immunization of BALB/c mice with immune complexes (IC) of the cariogenic bacterium Streptococcus mutans and mAbs against its surface adhesin P1 results in changes in the specificity and isotype of elicited anti-P1 Abs. Depending on the mAb, changes were beneficial, neutral, or detrimental, as measured by the ability of the serum from immunized mice to inhibit bacterial adherence to human salivary agglutinin by a BIAcore surface plasmon resonance assay. The current study further defined changes in the host response that result from immunization with IC containing beneficial mAbs, and evaluated mechanisms by which beneficial immunomodulation could occur in this system. Immunomodulatory effects varied depending upon genetic background, with differing results in C57BL/6 and BALB/c mice. Desirable effects following IC immunization were observed in the absence of activating FcRs in BALB/c Fcer1g transgenic mice. mAb F(ab')(2) mediated desirable changes similar to those observed using intact IgG. Sera from IC-immunized BALB/c mice that were better able to inhibit bacterial adherence demonstrated an increase in Abs able to compete with an adherence-inhibiting anti-P1 mAb, and binding of a beneficial immumomodulatory mAb to S. mutans increased exposure of that epitope. Consistent with a mechanism involving a mAb-mediated structural alteration of P1 on the cell surface, immunization with truncated P1 derivatives lacking segments that contribute to recognition by beneficial immunomodulatory mAbs resulted in an improvement in the ability of elicited serum Abs to inhibit bacterial adherence compared with immunization with the full-length protein.

Overcoming codon-usage bias in heterologous protein expression in Streptococcus gordonii

One of the limitations facing the development of Streptococcus gordonii into a successful vaccine vector is the inability of this bacterium to express high levels of heterologous proteins. In the present study, we have identified 12 codons deemed as rare codons in S. gordonii and seven other streptococcal species. tRNA genes encoding 10 of the 12 rare codons were cloned into a plasmid. The plasmid was transformed into strains of S. gordonii expressing the fusion protein SpaP/S1, the anti-complement receptor 1 (CR1) single-chain variable fragment (scFv) antibody, or the Toxoplasma gondii cyclophilin C18 protein. These three heterologous proteins contained high percentages of amino acids encoded by rare codons. The results showed that the production of SpaP/S1, anti-CR1 scFv and C18 increased by 2.7-, 120- and 10-fold, respectively, over the control strains. In contrast, the production of the streptococcal SpaP protein without the pertussis toxin S1 fragment was not affected by tRNA gene supplementation, indicating that the increased production of SpaP/S1 protein was due to the ability to overcome the limitation caused by rare codons required for the S1 fragment. The increase in anti-CR1 scFv production was also observed in Streptococcus mutans following tRNA gene supplementation. Collectively, the findings in the present study demonstrate for the first time, to the best of our knowledge, that codon-usage bias exists in Streptococcus spp. and the limitation of heterologous protein expression caused by codon-usage bias can be overcome by tRNA supplementation.

Adhesion determinants of the Streptococcus species

Streptococci are clinically important Gram-positive bacteria that are capable to cause a wide variety of diseases in humans and animals. Phylogenetic analyses based on 16S rRNA sequences of the streptococcal species reveal a clustering pattern, reflecting, with a few exceptions, their pathogenic potential and ecological preferences. Microbial adhesion to host tissues is the initial critical event in the pathogenesis of most infections. Streptococci use multiple adhesins to attach to the epithelium, and their expression is regulated in response to environmental and growth conditions. Bacterial adhesins recognize and bind cell surface molecules and extracellular matrix components through specific domains that for certain adhesin families have been well defined and found conserved across the streptococcal species. In this review, we present the different streptococcal adhesin families categorized on the basis of their adhesive properties and structural characteristics, and, when available, we focus the attention on conserved functional domains.

A spontaneous lactate dehydrogenase deficient mutant of Streptococcus rattus for use as a probiotic in the prevention of dental caries

AIMS

To study the ability of daily applications of Streptococcus rattus strain JH145 to affect the numbers of an implanted Streptococcus mutans strain in a rat model.

METHODS AND RESULTS

A spontaneous L(+)-lactate dehydrogenase (LDH)-deficient mutant of Streptococcus rattus, JH146, was isolated by screening on selective medium and compared with a previously isolated spontaneous LDH deficient strain, JH145. Both strains were shown to have single base pair deletion mutations in the structural gene (ldh) for LDH, and reversion frequencies were approximately the same. Animals treated once daily with >or=10(6) CFU (colony forming units) of JH145 showed a statistically significant decrease in the proportion of implanted S. mutans to total cultivable bacteria in oral swab samples. The rate of decrease in S. mutans levels was dose-dependent. No adverse effects were observed by in-life observation of treated animals, and histopathological, haematological and blood chemistry analyses were unremarkable.

CONCLUSIONS

The results presented indicate that daily application of JH145, a naturally occurring LDH-deficient variant of S. rattus, can compete with S. mutans for its habitat on the tooth surface.

SIGNIFICANCE AND IMPACT OF THE STUDY

S. rattus JH145 has potential as a probiotic for use in the prevention of dental caries.

Role of Clp proteins in expression of virulence properties of Streptococcus mutans

Mutational analysis revealed that members of the Clp system, specifically the ClpL chaperone and the ClpXP proteolytic complex, modulate the expression of important virulence attributes of Streptococcus mutans. Compared to its parent, the DeltaclpL strain displayed an enhanced capacity to form biofilms in the presence of sucrose, had reduced viability, and was more sensitive to acid killing. The DeltaclpP and DeltaclpX strains displayed several phenotypes in common: slow growth, tendency to aggregate in culture, reduced autolysis, and reduced ability to grow under stress, including acidic pH. Unexpectedly, the DeltaclpP and DeltaclpX mutants were more resistant to acid killing and demonstrated enhanced viability in long-term survival assays. Biofilm formation by the DeltaclpP and DeltaclpX strains was impaired when grown in glucose but enhanced in sucrose. In an animal study, the average number of S. mutans colonies recovered from the teeth of rats infected with the DeltaclpP or DeltaclpX strain was slightly lower than that of the parent strain. In Bacillus subtilis, the accumulation of the Spx global regulator, a substrate of ClpXP, has accounted for the DeltaclpXP phenotypes. Searching the S. mutans genome, we identified two putative spx genes, designated spxA and spxB. The inactivation of either of these genes bypassed phenotypes of the clpP and clpX mutants. Western blotting demonstrated that Spx accumulates in the DeltaclpP and DeltaclpX strains. Our results reveal that the proteolysis of ClpL and ClpXP plays a role in the expression of key virulence traits of S. mutans and indicates that the underlying mechanisms by which ClpXP affect virulence traits are associated with the accumulation of two Spx orthologues.

Protein antigen in serotype k Streptococcus mutans clinical isolates

Streptococcus mutans, a major pathogen of dental caries and infective endocarditis, is classified into serotypes c, e, f, and k, with serotype k strains recently reported to be frequently detected in persons with infective endocarditis. Thus, we hypothesized that common properties associated with infective endocarditis are present in those strains. Fifty-six oral S. mutans strains, including 11 serotype k strains, were analyzed. Western blotting analysis revealed expression of the 3 types of glucosyltransferases in all strains, while expression of the approximately 190-kDa cell-surface protein (PA) was absent in 12 strains, among which the prevalence of serotype k (7/12) was significantly high. Furthermore, cellular hydrophobicity and phagocytosis susceptibility were lower in the group of serotype k strains. These results indicate that the absence of PA expression, low cellular hydrophobicity, and phagocytosis susceptibility are common bacterial properties associated with serotype k strains, which may be associated with virulence for infective endocarditis.

Streptococcus mutans SMU.623c codes for a functional, metal-dependent polysaccharide deacetylase that modulates interactions with salivary agglutinin

The genome sequence of the oral pathogen Streptococcus mutans predicts the presence of two putative polysaccharide deacetylases. The first, designated PgdA in this paper, shows homology to the catalytic domains of peptidoglycan deacetylases from Streptococcus pneumoniae and Listeria monocytogenes, which are both thought to be involved in the bacterial defense mechanism against human mucosal lysozyme and are part of the CAZY family 4 carbohydrate esterases. S. mutans cells in which the pgdA gene was deleted displayed a different colony texture and a slightly increased cell surface hydrophobicity and yet did not become hypersensitive to lysozyme as shown previously for S. pneumoniae. To understand this apparent lack of activity, the high-resolution X-ray structure of S. mutans PgdA was determined; it showed the typical carbohydrate esterase 4 fold, with metal bound in a His-His-Asp triad. Analysis of the protein surface showed that an extended groove lined with aromatic residues is orientated toward the active-site residues. The protein exhibited metal-dependent de-N-acetylase activity toward a hexamer of N-acetylglucosamine. No activity was observed toward shorter chitooligosaccharides or a synthetic peptidoglycan tetrasaccharide. In agreement with the lysozyme data this would suggest that S. mutans PgdA does not act on peptidoglycan but on an as-yet-unidentified polysaccharide within the bacterial cell surface. Strikingly, the pgdA-knockout strain showed a significant increase in aggregation/agglutination by salivary agglutinin, in agreement with this gene acting as a deacetylase of a cell surface glycan.

Molecular interactions of alanine-rich and proline-rich regions of cell surface protein antigen c in Streptococcus mutans

INTRODUCTION

Streptococcus mutans has been implicated as a primary causative agent of dental caries in humans, and its cell surface protein antigen c (PAc) is known to be associated with sucrose-independent adhesion to tooth surfaces. PAc is composed of several domains, including an N-terminal signal sequence, an alanine-rich repeat region (A-region), a proline-rich repeat region (P-region), and an anchor region.

METHODS

To investigate the functions of each domain, an A-region-deficient mutant strain of S. mutans was constructed, and recombinant PAc and A- and P-region proteins were also constructed. The interactions of each domain with the recombinant proteins were analyzed using surface plasmon resonance spectroscopy with a biomolecular interaction analyzing system.

RESULTS

The A-region-deficient mutant strain showed the lowest levels of adherence to saliva-coated hydroxyapatite. Furthermore, findings in an immunoblot assay indicated that the A-region protein reacted strongly with proline-rich proteins in saliva, while the recombinant P-region protein interacted more quickly with PAc than the recombinant A-region protein.

CONCLUSION

These results suggest that the A-region has a strong relationship with adhesion to tooth surfaces, while the P-region has a high affinity for PAc.

Characteristics of biofilm formation by Streptococcus mutans in the presence of saliva

Interactions between salivary agglutinin and the adhesin P1 of Streptococcus mutans contribute to bacterial aggregation and mediate sucrose-independent adherence to tooth surfaces. We have examined biofilm formation by S. mutans UA159, and derivative strains carrying mutations affecting the localization or expression of P1, in the presence of fluid-phase or adsorbed saliva or salivary agglutinin preparations. Whole saliva- and salivary agglutinin-induced aggregation of S. mutans was adversely affected by the loss of P1 and sortase (SrtA) but not by the loss of trigger factor (RopA). Fluid-phase salivary agglutinin and, to a lesser extent, immobilized agglutinin inhibited biofilm development by S. mutans in the absence of sucrose, and whole saliva was more effective at decreasing biofilm formation than salivary agglutinin. Inhibition of biofilm development by salivary agglutinin was differently influenced by particular mutations, with the P1-deficient strain displaying a greater inhibition of biofilm development than the SrtA- or RopA-deficient strains. As expected, biofilm-forming capacities of all strains in the presence of salivary preparations were markedly enhanced in the presence of sucrose, although biofilm formation by the mutants was less efficient than that by the parental strain. Aeration strongly inhibited biofilm development, and the presence of salivary components did not restore biofilm formation in aerated conditions. The results disclose a potent ability of salivary constituents to moderate biofilm formation by S. mutans through P1-dependent and P1-independent pathways.

Scavenger receptor gp340 aggregates group A streptococci by binding pili

Group A streptococci (GAS) are the most frequent cause of bacterial pharyngitis. The first obstacle to GAS colonization of the pharynx is saliva. As well as forming a physical barrier, saliva contains components of innate and acquired immunity. Previous work has shown that saliva induces bacterial aggregation, which may serve as a clearance mechanism. As the aggregation of some oral streptococci in saliva is mediated by long proteinaceous appendages, we hypothesized that pili of GAS might behave similarly. Wild-type GAS M1 strain SF370 aggregated in saliva, while pilus-defective mutants did not. Similarly, heterologous expression of diverse GAS pili on the surface of Lactococcus lactis induced aggregation in saliva, while control strains were unaffected. Further studies revealed that aggregating bacteria bound salivary component gp340. Purified gp340 aggregated wild-type GAS and L. lactis expressing GAS pili, but not control strains. GAS pilus-defective mutants were abrogated in gp340 binding and aggregation. Furthermore, gp340-mediated aggregation reduced bacterial adhesion to human epithelial cells, suggesting a role in host defence.

Effects of antibodies against a fusion protein consisting of parts of cell surface protein antigen and glucosyltransferase of Streptococcus sobrinus on cell adhesion of mutans streptococci

BACKGROUND/AIMS

The cell surface protein antigen (PAg) and glucosyltransferases (GTFs) produced by Streptococcus sobrinus are considered to be major colonization factors of the organism.

METHODS

We constructed a fusion gene encoding a protein composed of the alanine-rich region of PAg (PAgA) and the glucan-binding domain (GB) of GTF-I, which catalyzes the synthesis of water-insoluble glucan in S. sobrinus. The fusion protein PAgA-GB was purified from cell extracts of Escherichia coli harboring the fusion gene, and antibodies against the fusion protein were prepared in rabbits.

RESULTS

In the presence of sucrose, the antibody against PAgA-GB significantly inhibited the adhesion of both S. sobrinus MT8145 and Streptococcus mutans Xc to saliva-coated hydroxyapatite beads, and the inhibitory effect on S. sobrinus was stronger than that on S. mutans. In the absence of sucrose, the antibody against PAgA-GB significantly inhibited the adhesion of both S. sobrinus and S. mutans, however the inhibitory effect on S. sobrinus was unexpectedly weaker than that on S. mutans. A similar result was observed with the antibody against the intact recombinant PAg protein (rPAg), while the same antibody reacted more strongly against S. sobrinus than against S. mutans cells.

CONCLUSION

Taken together, these results show that the antibody against S. sobrinus GTF-I may be useful for effective inhibition of the sucrose-dependent adhesion of S. sobrinus. However, PAg of S. sobrinus may not function primarily as a receptor for acquired pellicles, and other cell surface proteins may be involved in the sucrose-independent adhesion of S. sobrinus.

Modification of an effector strain for replacement therapy of dental caries to enable clinical safety trials

AIMS

To construct a genetically modified strain of Streptococcus mutans for dental caries prevention. The strain has significantly reduced cariogenicity owing to a deletion of the entire open reading frame for lactate dehydrogenase, and has excellent colonization potential through the production of a natural antibiotic called mutacin 1140. For use in human clinical trials, additional mutations were introduced to enable rapid elimination of the strain in case of adverse side effects and to increase genetic stability.

METHODS

Deletion mutations were introduced into the dal gene for d-alanine biosynthesis and the comE gene for genetic transformation. The resulting strain, A2JM, was tested for dependence on exogenous d-alanine and its ability to be eradicated from colonized rats. The strain was also tested for its ability to exchange DNA with another strain of S. mutans in in vitro and in vivo models.

CONCLUSIONS

A2JM was completely dependent on exogenous d-alanine, but could colonize the oral cavity of rats in low numbers in the absence of dietary d-alanine. Results indicated that A2JM can scavenge d-alanine from other plaque bacteria. Lowering of the total oral bacterial load through daily application of chlorhexidine enabled virtually complete eradication of A2JM. The introduction of the comE gene did not significantly decrease the transformability of A2JM in in vitro or in vivo models. The addition of a deletion in the comE gene does, nonetheless, provide additional safety as it has a very low reversion frequency.

SIGNIFICANCE AND IMPACT OF THE STUDY

Based on the safety and efficacy profiles established in vitro and in animal models, A2JM appears suitable for safe use in human clinical trials.

Manganese affects Streptococcus mutans virulence gene expression

BACKGROUND/AIMS

Studies of trace metals in drinking water and tooth enamel have suggested a caries-promoting potential for manganese (Mn). Additionally, Mn has been shown to be essential for the expression of mutans streptococci virulence factors such as the glucan-binding lectin (GBL) of Streptococcus sobrinus. The Streptococcus mutans glucan-binding protein (Gbp) GbpC is the functional analogue of the S. sobrinus GBL. S. mutans Gbps have been shown to contribute to biofilm architecture and virulence. This study was undertaken to examine the effects of Mn on the transcription of genes encoding S. mutans Gbps, including gbpC, along with other critical S. mutans virulence genes.

METHODS

Microarray analyses suggested the potential for an Mn effect on Gbp genes. Further investigation of the Mn effects on selected genes was undertaken by performing Northern blots, Western blots, and RT-PCR under conditions of planktonic and biofilm growth in Mn-depleted media or in media containing 50 mircoM Mn.

RESULTS

Mn resulted in increased expression of gbpC and gtfB, and decreased expression of wapA, in both planktonic and biofilm cultures. The expression levels of gbpA and gbpD were also decreased in the presence of Mn, but only in biofilms. The expression of gtfC was increased in the presence of Mn only in planktonic cultures. The spaP gene was expressed more highly in Mn-supplemented planktonic cultures but less in Mn-supplemented biofilms.

CONCLUSION

Mn availability affects the expression of multiple S. mutans genes involved in adhesion and biofilm formation. Furthermore, these effects depend on the growth state of the organism.