RegG, a CcpA homolog, participates in regulation of amylase-binding protein A gene (abpA) expression in Streptococcus gordonii

Flavorless vs. Flavored Electronic Cigarette-Generated Aerosol and E-Liquid on the Growth of Common Oral Commensal Streptococci

Introduction

Electronic cigarette (ECIG) use or vaping has become popular globally. While the question “Is vaping safer than smoking?” continues, it is becoming clearer that one of the most dangerous components of E-liquids are the flavorings. Since the oral cavity is the first anatomical site to be assaulted by ECIG aerosol, the aim of this study is to test the hypothesis that flavored ECIG aerosols or E-liquids pose a more detrimental effect on the growth of commensal oral streptococcal bacteria compared to flavorless aerosols or E-liquids.

Methods

Kirby Bauer assays and 24-h planktonic growth curves were used to compare the effects of flavorless vs. flavored (tobacco, menthol, cinnamon, strawberry and blueberry) ECIG-generated aerosols and E-liquids on the growth of four common strains of oral commensal bacteria (Streptococcus gordonii, Streptococcus intermedius, Streptococcus mitis and Streptococcus oralis).

Results

Kirby Bauer assays revealed inhibition of growth for all bacteria tested when exposed to 100% menthol, cinnamon or strawberry flavors. In contrast, 5% flavor in E-liquid had no effect. When exposed to 100 puffs of ECIG-generated aerosol ± flavors (≈ 0.05% flavor in brain heart infusion media) or an equivalent amount of E-liquid ± flavors, twenty-four hour planktonic growth curves indicated no effect on growth for all streptococci tested. Subsequent twenty-four hour planktonic growth curves testing the effects of E-liquid ± flavors (0.0625, 0.125, 0.25, 0.3125, 0.625, and 1.25% flavor in brain heart infusion media) revealed dose-dependent inhibition of growth, particularly for menthol, cinnamon and strawberry), for all bacteria tested.

Conclusion

These results support the hypothesis that flavored E-liquids are more detrimental to the growth of oral commensal bacteria than unflavored E-liquids. The streptococci tested in this study are early colonizers and part of the foundation of oral biofilms and dental plaque. Disturbances in the composition and growth of these primary colonizers is crucial to the development of a healthy dental plaque and host-bacteria interactions. E-liquids and their aerosols containing flavoring agents alter the growth of these bacteria. Such perturbations of pioneering oral communities pose a potential risk to the health of the oral cavity and, ultimately, health in general.

Dynamics of the Streptococcus gordonii Transcriptome in Response to Medium, Salivary α-Amylase, and Starch

Streptococcus gordonii, a primary colonizer of the tooth surface, interacts with salivary α-amylase via amylase-binding protein A (AbpA). This enzyme hydrolyzes starch to glucose, maltose, and maltodextrins that can be utilized by various oral bacteria for nutrition. Microarray studies demonstrated that AbpA modulates gene expression in response to amylase, suggesting that the amylase-streptococcal interaction may function in ways other than nutrition. The goal of this study was to explore the role of AbpA in gene regulation through comparative transcriptional profiling of wild-type KS1 and AbpA(-) mutant KS1ΩabpA under various environmental conditions. A portion of the total RNA isolated from mid-log-phase cells grown in 5% CO2 in (i) complex medium with or without amylase, (ii) defined medium (DM) containing 0.8% glucose with/without amylase, and (iii) DM containing 0.2% glucose and amylase with or without starch was reverse transcribed to cDNA and the rest used for RNA sequencing. Changes in the expression of selected genes were validated by quantitative reverse transcription-PCR. Maltodextrin-associated genes, fatty acid synthesis genes and competence genes were differentially expressed in a medium-dependent manner. Genes in another cluster containing a putative histidine kinase/response regulator, peptide methionine sulfoxide reductase, thioredoxin protein, lipoprotein, and cytochrome c-type protein were downregulated in KS1ΩabpA under all of the environmental conditions tested. Thus, AbpA appears to modulate genes associated with maltodextrin utilization/transport and fatty acid synthesis. Importantly, in all growth conditions AbpA was associated with increased expression of a potential two-component signaling system associated with genes involved in reducing oxidative stress, suggesting a role in signal transduction and stress tolerance.

Taking the starch out of oral biofilm formation: molecular basis and functional significance of salivary α-amylase binding to oral streptococci

α-Amylase-binding streptococci (ABS) are a heterogeneous group of commensal oral bacterial species that comprise a significant proportion of dental plaque microfloras. Salivary α-amylase, one of the most abundant proteins in human saliva, binds to the surface of these bacteria via specific surface-exposed α-amylase-binding proteins. The functional significance of α-amylase-binding proteins in oral colonization by streptococci is important for understanding how salivary components influence oral biofilm formation by these important dental plaque species. This review summarizes the results of an extensive series of studies that have sought to define the molecular basis for α-amylase binding to the surface of the bacterium as well as the biological significance of this phenomenon in dental plaque biofilm formation.

Effect of starch and amylase on the expression of amylase-binding protein A in Streptococcus gordonii

Streptococcus gordonii is a common oral commensal bacterial species in tooth biofilm (dental plaque) and specifically binds to salivary amylase through the surface exposed amylase-binding protein A (AbpA). When S. gordonii cells are pretreated with amylase, amylase bound to AbpA facilitates growth with starch as a primary nutrition source. The goal of this study was to explore possible regulatory effects of starch, starch metabolites and amylase on the expression of S. gordonii AbpA. An amylase ligand-binding assay was used to assess the expression of AbpA in culture supernatants and on bacterial cells from S. gordonii grown in defined medium supplemented with 1% starch, 0.5 mg ml(-1) amylase, with starch and amylase together, or with various linear malto-oligosaccharides. Transcription of abpA was determined by reverse transcription quantitative polymerase chain reaction. AbpA was not detectable in culture supernatants containing either starch alone or amylase alone. In contrast, the amount of AbpA was notably increased when starch and amylase were both present in the medium. The expression of abpA was significantly increased (P < 0.05) following 40 min of incubation in defined medium supplemented with starch and amylase. Similar results were obtained in the presence of maltose and other short-chain malto-oligosacchrides. These results suggest that the products of starch hydrolysis produced from the action of salivary α-amylase, particularly maltose and maltotriose, up-regulate AbpA expression in S. gordonii.

CcpA regulates biofilm formation and competence in Streptococcus gordonii

Streptococcus gordonii is an important member of the oral biofilm community. As an oral commensal streptococcus, S. gordonii is considered beneficial in promoting biofilm homeostasis. CcpA is known as the central regulator of carbon catabolite repression in Gram-positive bacteria and is also involved in the control of virulence gene expression. To further establish the role of CcpA as central regulator in S. gordonii, the effect of CcpA on biofilm formation and natural competence of S. gordonii was investigated. These phenotypic traits have been suggested to be important to oral streptococci in coping with environmental stress. Here we demonstrate that a CcpA mutant was severely impaired in its biofilm-forming ability, showed a defect in extracellular polysaccharide production and reduced competence. The data suggest that CcpA is involved in the regulation of biofilm formation and competence development in S. gordonii.

Environmental influences on competitive hydrogen peroxide production in Streptococcus gordonii

Streptococcus gordonii is an important member of the oral biofilm. One of its phenotypic traits is the production of hydrogen peroxide (H2O2). H2O2 is an antimicrobial component produced by S. gordonii that is able to antagonize the growth of cariogenic Streptococcus mutans. Strategies that modulate H2O2 production in the oral cavity may be useful as a simple therapeutic mechanism to improve oral health, but little is known about the regulation of H2O2 production. The enzyme responsible for H2O2 production is pyruvate oxidase, encoded by spxB. The functional studies of spxB expression and SpxB abundance presented in this report demonstrate a strong dependence on environmental oxygen tension and carbohydrate availability. Carbon catabolite repression (CCR) modulates spxB expression carbohydrate dependently. Catabolite control protein A (CcpA) represses spxB expression by direct binding to the spxB promoter, as shown by electrophoretic mobility shift assays (EMSA). Promoter mutation studies revealed the requirement of two catabolite-responsive elements (CRE) for CcpA-dependent spxB regulation, as evaluated by spxB expression and phenotypic H2O2 production assays. Thus, molecular mechanisms for the control of S. gordonii spxB expression are presented for the first time, demonstrating the possibility of manipulating H2O2 production for increased competitive fitness.

The EIIABMan phosphotransferase system permease regulates carbohydrate catabolite repression in Streptococcus gordonii

Commensal oral streptococci play critical roles in oral biofilm formation and promote dental health by competing with, and antagonizing the growth of, pathogenic organisms, such as Streptococcus mutans. Efficient utilization of the spectrum of carbohydrates in the oral cavity by commensal streptococci is essential for their persistence, and yet very little is known about the regulation of carbohydrate catabolism by these organisms. Carbohydrate catabolite repression (CCR) in the abundant oral commensal Streptococcus gordonii strain DL-1 was investigated using the exo-β-D-fructosidase gene (fruA) and a fructose/mannose sugar:phosphotransferase (PTS) enzyme II operon (levDEFG) as model systems. Functional studies confirmed the predicted roles of FruA and LevD in S. gordonii. ManL, the AB domain of a fructose/mannose-type enzyme II PTS permease, contributed to utilization of glucose, mannose, galactose, and fructose and exerted primary control over CCR of the fruA and levD operons. Unlike in S. mutans, ManL-dependent CCR was not sugar specific, and galactose was very effective at eliciting CCR in S. gordonii. Inactivation of the apparent ccpA homologue of S. gordonii actually enhanced CCR of fruA and levD, an effect likely due to its demonstrated role in repression of manL expression. Thus, there are some similarities and fundamental differences in CCR control mechanisms between the oral pathogen S. mutans and the oral commensal S. gordonii that may eventually be exploited to enhance the competitiveness of health-associated commensals in oral biofilms.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Interspecies signaling between Veillonella atypica and Streptococcus gordonii requires the transcription factor CcpA

Streptococcus gordonii and Veillonella atypica, two early-colonizing members of the dental plaque biofilm, participate in a relationship that results in increased transcription of the S. gordonii gene amyB, encoding an alpha-amylase. We show that the transcription factor CcpA is required for this interspecies interaction.

Sucrose metabolism contributes to in vivo fitness of Streptococcus pneumoniae

We characterized two sucrose-metabolizing systems -sus and scr- and describe their roles in the physiology and virulence of Streptococcus pneumoniae in murine models of carriage and pneumonia. The sus and scr systems are regulated by LacI family repressors SusR and ScrR respectively. SusR regulates an adjacent ABC transporter (susT1/susT2/susX) and sucrose-6-phosphate (S-6-P) hydrolase (susH). ScrR controls an adjacent PTS transporter (scrT), fructokinase (scrK) and second S-6-P hydrolase (scrH). sus and scr play niche-specific roles in virulence. The susH and sus locus mutants are attenuated in the lung, but dispensable in nasopharyngeal carriage. Conversely, the scrH and scr locus mutants, while dispensable in the lung, are attenuated for nasopharyngeal colonization. The scrH/susH double mutant is more attenuated than scrH in the nasopharynx, indicating SusH can substitute in this niche. Both systems are sucrose-inducible, with ScrH being the major in vitro hydrolase. The scrH/susH mutant does not grow on sucrose indicating that sus and scr are the only sucrose-metabolizing systems in S. pneumoniae. We propose a model describing hierarchical regulation of the scr and sus systems by the putative inducer, S-6-P. The transport and metabolism of sucrose or a related disaccharide thus contributes to S. pneumoniae colonization and disease.

Comparison of transformation protocols in Streptococcus gordonii and evaluation of native promoter strength using a multiple-copy plasmid

An active area of research in the development of Streptococcus gordonii for use as a bacterial commensal vector involves the identification and utilization of strong promoters for high-level expression of heterologous products. Escherichia coli plasmid vectors containing different streptococcal promoters often fail to become established in E. coli for unknown reasons. Therefore, it is desirable at times to transform S. gordonii, which is naturally competent, with small quantities of nascently ligated DNA without using E. coli first to amplify or screen the product. By comparing the efficiency of two methods used to induce competence in S. gordonii, it was shown that the use of a synthetic competence stimulating peptide substantially enhanced plasmid uptake by S. gordonii. We amplified the amylase-binding protein (abpA) promoter from the S. gordonii genome and, using a synthetic peptide to induce competence, directly introduced plasmid DNA containing this promoter into S. gordonii as an unamplified product of ligation. This plasmid facilitated abundant secretion of a heterologous product by S. gordonii. By assessing the levels of heterologous product secreted by two plasmid constructs, it was possible to evaluate the relative strength of two native promoters.

Loss of penicillin tolerance by inactivating the carbon catabolite repression determinant CcpA in Streptococcus gordonii

OBJECTIVES

Antibiotic tolerance is a phenomenon allowing bacteria to withstand drug-induced killing. Here, we studied a penicillin-tolerant mutant of Streptococcus gordonii (Tol1), which was shown to be deregulated in the expression of the arginine deiminase operon (arc). arc was not directly responsible for tolerance, but is controlled by the global regulator CcpA. Therefore, we sought whether CcpA might be implicated in tolerance.

METHODS

The ccpA gene was characterized and subsequently inactivated by PCR ligation mutagenesis in both the susceptible wild-type (WT) and Tol1. The minimal inhibitory concentration and time-kill curves for the strains were determined and the outcome of penicillin treatment in experimental endocarditis assessed.

RESULTS

ccpA sequence and expression were similar between the WT and Tol1 strains. In killing assays, the WT lost 3.5 +/- 0.6 and 5.3 +/- 0.6 log(10) cfu/mL and Tol1 lost 0.4 +/- 0.2 and 1.4 +/- 0.9 log(10) cfu/mL after 24 and 48 h of penicillin exposure, respectively. Deletion of ccpA almost totally restored Tol1 kill susceptibility (loss of 2.5 +/- 0.7 and 4.9 +/- 0.7 log(10) cfu/mL at the same endpoints). In experimental endocarditis, penicillin treatment induced a significant reduction in vegetation bacterial densities between Tol1 (4.1 log(10) cfu/g) and Tol1DeltaccpA (2.4 log(10) cfu/g). Restitution of ccpA re-established the tolerant phenotype both in vitro and in vivo.

CONCLUSIONS

CcpA, a global regulator of the carbon catabolite repression system, is implicated in penicillin tolerance both in vitro and in vivo. This links antibiotic survival to bacterial sugar metabolism. However, since ccpA sequence and expression were similar between the WT and Tol1 strains, other factors are probably involved in tolerance.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Catabolite control protein A (CcpA) contributes to virulence and regulation of sugar metabolism in Streptococcus pneumoniae

We characterized the role of catabolite control protein A (ccpA) in the physiology and virulence of Streptococcus pneumoniae. S. pneumoniae has a large percentage of its genome devoted to sugar uptake and metabolism, and therefore, regulation of these processes is likely to be crucial for fitness in the nasopharynx and may play a role during invasive disease. In many bacteria, carbon catabolite repression (CCR) is central to such regulation, influencing hierarchical sugar utilization and growth rates. CcpA is the major transcriptional regulator in CCR in several gram-positive bacteria. We show that CcpA functions in CCR of lactose-inducible beta-galactosidase activity in S. pneumoniae. CCR of maltose-inducible alpha-glucosidase, raffinose-inducible alpha-galactosidase, and cellobiose-inducible beta-glucosidase is unaffected in the ccpA strain, suggesting that other regulators, possibly redundant with CcpA, control these systems. The ccpA strain is severely attenuated for nasopharyngeal colonization and lung infection in the mouse, establishing its role in fitness on these mucosal surfaces. Comparison of the cell wall fraction of the ccpA and wild-type strains shows that CcpA regulates many proteins in this compartment that are involved in central and intermediary metabolism, a subset of which are required for survival and multiplication in vivo. Both in vitro and in vivo defects were complemented by providing ccpA in trans. Our results demonstrate that CcpA, though not a global regulator of CCR in S. pneumoniae, is required for colonization of the nasopharynx and survival and multiplication in the lung.

Genomic variation in Streptococcus mutans: deletions affecting the multiple pathways of β-glucoside metabolism

The genome of Streptococcus mutans UA159 contains two phospho-beta-glucosidase genes, bglA and celA, which occur in operon-like arrangements along with genes for components of phosphotransferase transport systems and a third phospho-beta-glucosidase encoded by the arb gene, which does not have its own associated transport system but relies on uptake by the bgl or cel systems. Targeted inactivation of each of the phospho-beta-glucosidase genes revealed that bglA is involved in aesculin hydrolysis, celA is essential for utilisation of cellobiose, amygdalin, gentobiose and salicin, and arb is required for utilisation of arbutin. Inactivation of genes for the phosphotransferase systems revealed an overlap of specificity for transport of beta-glucosides and also indicated that further, unidentified transport systems exist. The cel and arb genes are subject to catabolite repression by glucose, but the regM gene is not essential for catabolite repression. Screening a collection of isolates of S. mutans revealed strains with deletions affecting the msm, bgl and/or cel operons. The phenotypes of these strains could largely be explained on the basis of the results obtained from the knockout mutants of S. mutans UA159 but also indicated the existence of other pathways apparently absent from UA159. The extensive genetic and phenotypic variation found in beta-glucoside metabolism indicates that there may be extensive heterogeneity in the species.

Role of RegM, a homologue of the catabolite repressor protein CcpA, in the virulence of Streptococcus pneumoniae

As part of a study of virulence gene regulation in Streptococcus pneumoniae, we have identified a gene encoding a homologue of the staphylococcal catabolite control protein CcpA in the pneumococcal genome sequence. The pneumococcal protein, designated RegM, has significant similarity to members of the LacI/GalR family of bacterial regulatory proteins. S. pneumoniae D39 derivatives with insertion-duplication or deletion mutations in regM were significantly attenuated in virulence with respect to the wild-type strain. In defined media containing either sucrose or lactose as sole carbon sources, the in vitro growth rates of D39 and the regM mutants were essentially the same. However, in the presence of galactose the regM mutants grew significantly faster than the wild-type strain, whereas growth rates were significantly lower in the presence of glucose or maltose. These data are consistent with the involvement of regM in the catabolism of carbohydrates in S. pneumoniae. RegM was a repressor of both alpha-glucosidase and beta-galactosidase activities in S. pneumoniae, but unlike the situation in certain other bacteria, it does not mediate the repression of these enzymes by glucose. The observed attenuation in virulence was not attributable to poorer growth of the regM mutants in mouse blood ex vivo, but nevertheless, the mutants were rapidly cleared from the blood of infected mice in vivo. The regM mutation had no apparent impact on expression of several confirmed pneumococcal virulence proteins, but studies employing a lacZ transcriptional fusion construct indicated that mutation of regM resulted in a significant reduction in transcription of the capsular polysaccharide biosynthesis locus (cps). Thus, regM is the first gene outside of the cps locus to be implicated in regulation of capsular gene expression.

Role of Streptococcus gordonii amylase-binding protein A in adhesion to hydroxyapatite, starch metabolism, and biofilm formation

Interactions between bacteria and salivary components are thought to be important in the establishment and ecology of the oral microflora. alpha-Amylase, the predominant salivary enzyme in humans, binds to Streptococcus gordonii, a primary colonizer of the tooth. Previous studies have implicated this interaction in adhesion of the bacteria to salivary pellicles, catabolism of dietary starches, and biofilm formation. Amylase binding is mediated at least in part by the amylase-binding protein A (AbpA). To study the function of this protein, an erythromycin resistance determinant [erm(AM)] was inserted within the abpA gene of S. gordonii strains Challis and FAS4 by allelic exchange, resulting in abpA mutant strains Challis-E1 and FAS4-E1. Comparison of the wild-type and mutant strains did not reveal any significant differences in colony morphology, biochemical metabolic profiles, growth in complex or defined media, surface hydrophobicity, or coaggregation properties. Scatchard analysis of adhesion isotherms demonstrated that the wild-type strains adhered better to human parotid-saliva- and amylase-coated hydroxyapatite than did the AbpA mutants. In contrast, the mutant strains bound to whole-saliva-coated hydroxyapatite to a greater extent than did the wild-type strains. While the wild-type strains preincubated with purified salivary amylase grew well in defined medium with potato starch as the sole carbohydrate source, the AbpA mutants did not grow under the same conditions even after preincubation with amylase. In addition, the wild-type strain produced large microcolonies in a flow cell biofilm model, while the abpA mutant strains grew much more poorly and produced relatively small microcolonies. Taken together, these results suggest that AbpA of S. gordonii functions as an adhesin to amylase-coated hydroxyapatite, in salivary-amylase-mediated catabolism of dietary starches and in human saliva-supported biofilm formation by S. gordonii.