Transcriptional and metabolic effects of glucose on Streptococcus pneumoniae sugar metabolism

Carbon source-dependent capsule thickness regulation in Streptococcus pneumoniae

Background

The polysaccharide capsule of Streptococcus pneumoniae plays a major role in virulence, adherence to epithelial cells, and overall survival of the bacterium in the human host. Galactose, mannose, and N-acetylglucosamine (GlcNAc) are likely to be relevant for metabolization in the nasopharynx, while glucose is the primary carbon source in the blood. In this study, we aim to further the understanding of the influence of carbon sources on pneumococcal growth, capsule biosynthesis, and subsequent adherence potential.

Methods

We tested the growth behavior of clinical wild-type and capsule knockout S. pneumoniae strains, using galactose, GlcNAc, mannose, and glucose as carbon source for growth. We measured capsule thickness and quantified capsule precursors by fluorescein isothiocyanate (FITC)-dextran exclusion assays and 31P-nuclear magnetic resonance measurements, respectively. We also performed epithelial adherence assays using Detroit 562 cells and performed a transcriptome analysis (RNA sequencing).

Results

We observed a reduced growth in galactose, mannose, and GlcNAc compared to growth in glucose and found capsular size reductions in mannose and GlcNAc compared to galactose and glucose. Additionally, capsular precursor measurements of uridine diphosphate-(UDP)-glucose and UDP-galactose showed less accumulation of precursors in GlcNAc or mannose than in glucose and galactose, indicating a possible link with the received capsular thickness measurements. Epithelial adherence assays showed an increase in adherence potential for a pneumococcal strain, when grown in mannose compared to glucose. Finally, transcriptome analysis of four clinical isolates revealed not only strain specific but also common carbon source-specific gene expression.

Conclusion

Our findings may indicate a careful adaption of the lifestyle of S. pneumoniae according to the monosaccharides encountered in the respective human niche.

Identification of Streptococcus pneumoniae genes associated with hypothiocyanous acid tolerance through genome-wide screening

Streptococcus pneumoniae is a commensal bacterium and invasive pathogen that causes millions of deaths worldwide. The pneumococcal vaccine offers limited protection, and the rise of antimicrobial resistance will make treatment increasingly challenging, emphasizing the need for new antipneumococcal strategies. One possibility is to target antioxidant defenses to render S. pneumoniae more susceptible to oxidants produced by the immune system. Human peroxidase enzymes will convert bacterial-derived hydrogen peroxide to hypothiocyanous acid (HOSCN) at sites of colonization and infection. Here, we used saturation transposon mutagenesis and deep sequencing to identify genes that enable S. pneumoniae to tolerate HOSCN. We identified 37 genes associated with S. pneumoniae HOSCN tolerance, including genes involved in metabolism, membrane transport, DNA repair, and oxidant detoxification. Single-gene deletion mutants of the identified antioxidant defense genes sodA, spxB, trxA, and ahpD were generated and their ability to survive HOSCN was assessed. With the exception of ΔahpD, all deletion mutants showed significantly greater sensitivity to HOSCN, validating the result of the genome-wide screen. The activity of hypothiocyanous acid reductase or glutathione reductase, known to be important for S. pneumoniae tolerance of HOSCN, was increased in three of the mutants, highlighting the compensatory potential of antioxidant systems. Double deletion of the gene encoding glutathione reductase and sodA sensitized the bacteria significantly more than single deletion. The HOSCN defense systems identified in this study may be viable targets for novel therapeutics against this deadly pathogen. IMPORTANCE Streptococcus pneumoniae is a human pathogen that causes pneumonia, bacteremia, and meningitis. Vaccination provides protection only against a quarter of the known S. pneumoniae serotypes, and the bacterium is rapidly becoming resistant to antibiotics. As such, new treatments are required. One strategy is to sensitize the bacteria to killing by the immune system. In this study, we performed a genome-wide screen to identify genes that help this bacterium resist oxidative stress exerted by the host at sites of colonization and infection. By identifying a number of critical pneumococcal defense mechanisms, our work provides novel targets for antimicrobial therapy.

Inoculation with Lentilactobacillus buchneri alone or in combination with Lentilactobacillus hilgardii modifies gene expression, fermentation profile, and starch digestibility in high-moisture corn

Inoculants combining Lentilactobacillus buchneri and Lentilactobacillus hilgardii have been shown to improve the aerobic stability of high-moisture corn (HMC) and whole-plant corn silage, but the mode of action of this co-inoculation remains to be elucidated. This study used metatranscriptomics to evaluate the effects of inoculation with L. buchneri alone or combined with L. hilgardii on the bacterial community, gene expression, fermentation profile, and starch digestibility in HMC. High-moisture corn not inoculated (Control) or inoculated with L. buchneri NCIMB 40788 (LB) or L. buchneri NCIMB 40788 combined with L. hilgardii CNCM-I-4785 (Combo) was ensiled in mini silo bags for 30, 60, 120, and 180 days. The fermentation profile was evaluated at all time points. Metatranscriptomics was performed on samples collected on day 120. Combo had a greater alpha diversity richness index of contigs than LB and Control, and inoculation with Combo and LB modified the beta-diversity of contigs compared to Control. Out of 69 genes of interest, 20 were differentially expressed in LB compared to Control and 25 in Combo compared to Control. Of those differently expressed genes, 16 (10 of which were associated with carbohydrate metabolism and six with amino acid metabolism) were differently expressed in both LB and Combo compared to Control, and all those genes were upregulated in the inoculated silages. When we compared Combo and LB, we found seven genes expressed differently, four associated with carbohydrate metabolism and downregulated in Combo, and three associated with amino acid metabolism and upregulated in Combo. At day 120, the inoculated silages had more culturable lactic acid bacteria, higher Lactobacillus relative abundance, and lower Leuconostoc relative abundance than Control. The concentration of acetic acid remained low throughout ensiling in Control, but in LB and Combo, it increased up to day 60 and remained stable from day 60 to 180. The 1,2-propanediol was only detected in LB and Combo. Inoculation did not affect the concentration of starch, but starch digestibility was greater in Combo than in Control. Inoculation of HMC with Combo modified the gene expression and fermentation profile compared to Control and LB, improving starch digestibility compared to uninoculated HMC.

Airway metabolic profiling during Streptococcus pneumoniae infection identifies branched chain amino acids as signatures of upper airway colonisation

Streptococcus pneumoniae is a leading cause of community-acquired pneumonia and bacteraemia and is capable of remarkable phenotypic plasticity, responding rapidly to environmental change. Pneumococcus is a nasopharyngeal commensal, but is responsible for severe, acute infections following dissemination within-host. Pneumococcus is adept at utilising host resources, but the airways are compartmentalised and those resources are not evenly distributed. Challenges and opportunities in metabolite acquisition within different airway niches may contribute to the commensal-pathogen switch when pneumococcus moves from nasopharynx into lungs. We used NMR to characterise the metabolic landscape of the mouse airways, in health and during infection. Using paired nasopharynx and lung samples from naïve animals, we identified fundamental differences in metabolite bioavailability between airway niches. Pneumococcal pneumonia was associated with rapid and dramatic shifts in the lung metabolic environment, whilst nasopharyngeal carriage led to only modest change in upper airway metabolite profiles. NMR spectra derived from the nasopharynx of mice infected with closely-related pneumococcal strains that differ in their colonisation potential could be distinguished from one another using multivariate dimensionality reduction methods. The resulting models highlighted that increased branched-chain amino acid (BCAA) bioavailability in nasopharynx is a feature of infection with the high colonisation potential strain. Subsequent analysis revealed increased expression of BCAA transport genes and increased intracellular concentrations of BCAA in that same strain. Movement from upper to lower airway environments is associated with shifting challenges in metabolic resource allocation for pneumococci. Efficient biosynthesis, liberation or acquisition of BCAA is a feature of adaptation to nasopharyngeal colonisation.

The role of pneumococcal extracellular vesicles on the pathophysiology of the kidney disease hemolytic uremic syndrome

Streptococcus pneumoniae-induced hemolytic uremic syndrome (Sp-HUS) is a kidney disease characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. This disease is frequently underdiagnosed and its pathophysiology is poorly understood. In this work, we compared clinical strains, isolated from infant Sp-HUS patients, with a reference pathogenic strain D39, for host cytotoxicity and further explored the role of Sp-derived extracellular vesicles (EVs) in the pathogenesis of an HUS infection. In comparison with the wild-type strain, pneumococcal HUS strains caused significant lysis of human erythrocytes and increased the release of hydrogen peroxide. Isolated Sp-HUS EVs were characterized by performing dynamic light-scattering microscopy and proteomic analysis. Sp-HUS strain released EVs at a constant concentration during growth, yet the size of the EVs varied and several subpopulations emerged at later time points. The cargo of the Sp-HUS EVs included several virulence factors at high abundance, i.e., the ribosomal subunit assembly factor BipA, the pneumococcal surface protein A, the lytic enzyme LytC, several sugar utilization, and fatty acid synthesis proteins. Sp-HUS EVs strongly downregulated the expression of the endothelial surface marker platelet endothelial cell adhesion molecule-1 and were internalized by human endothelial cells. Sp-HUS EVs elicited the release of pro-inflammatory cytokines (interleukin [IL]-1β, IL-6) and chemokines (CCL2, CCL3, CXCL1) by human monocytes. These findings shed new light on the overall function of Sp-EVs, in the scope of infection-mediated HUS, and suggest new avenues of research for exploring the usefulness of Sp-EVs as therapeutic and diagnostic targets. IMPORTANCE Streptococcus pneumoniae-associated hemolytic uremic syndrome (Sp-HUS) is a serious and underdiagnosed deadly complication of invasive pneumococcal disease. Despite the introduction of the pneumococcal vaccine, cases of Sp-HUS continue to emerge, especially in children under the age of 2. While a lot has been studied regarding pneumococcal proteins and their role on Sp-HUS pathophysiology, little is known about the role of extracellular vesicles (EVs). In our work, we isolate and initially characterize EVs from a reference pathogenic strain (D39) and a strain isolated from a 2-year-old patient suffering from Sp-HUS. We demonstrate that despite lacking cytotoxicity toward human cells, Sp-HUS EVs are highly internalized by endothelial cells and can trigger cytokine and chemokine production in monocytes. In addition, this work specifically highlights the distinct morphological characteristics of Sp-HUS EVs and their unique cargo. Overall, this work sheds new light into potentially relevant players contained in EVs that might elucidate about pneumococcal EVs biogenesis or pose as interesting candidates for vaccine design.

Analysis and modification of central carbon metabolism in Hypsizygus marmoreus for improving mycelial growth performance and fruiting body yield

Hypsizygus marmoreus is one of the main industrially cultivated varieties of edible fungi, with a delicious taste and high nutritional value. However, the long harvest period of 130-150 days greatly limits its large-scale expansion. This study aimed to investigate the effects of central carbon metabolism (CCM) on the mycelial growth performance and fruiting body formation of H. marmoreus. Nine edible fungi with different harvest periods were collected and used to evaluate their intracellular carbon metabolic differences in the CCM, which revealed that the imbalanced distribution of intracellular carbon metabolic levels in the CCM of H. marmoreus might be one of the key factors resulting in a slow mycelial growth rate and a long harvest period. Further analysis by three strategies, including metabolomics, adaptation of different carbon sources, and chemical interference, confirmed that low carbon flux into the pentose phosphate pathway (PPP) limited the supply of raw materials, reduced power, and thus influenced the mycelial growth of H. marmoreus. Furthermore, four transformants with increased expression levels of glucose-6-phosphate dehydrogenase (G6PDH), a key rate-limiting enzyme in the PPP of H. marmoreus, were developed and showed more extracellular soluble protein secretion and higher sugar assimilation rates, as well as improved mycelial growth rates in bottle substrate mixtures. Finally, cultivation experiments indicated that the maturation periods of the fruiting body with ~4-5 days in advance and the maximum fruiting body yield of 574.8 g per bag with an increase of 7.4% were achieved by improving the G6PDH expression level of the PPP in H. marmoreus. This study showed that CCM played an important role in the mycelial growth and development of H. marmoreus, which provided new insights for future advancements in cultivating and breeding edible fungi.

StkP- and PhpP-Mediated Posttranslational Modifications Modulate the S. pneumoniae Metabolism, Polysaccharide Capsule, and Virulence

Pneumococcal Ser/Thr kinase (StkP) and its cognate phosphatase (PhpP) play a crucial role in bacterial cytokinesis. However, their individual and reciprocal metabolic and virulence regulation-related functions have yet to be adequately investigated in encapsulated pneumococci. Here, we demonstrate that the encapsulated pneumococcal strain D39-derived D39ΔPhpP and D39ΔStkP mutants displayed differential cell division defects and growth patterns when grown in chemically defined media supplemented with glucose or nonglucose sugars as the sole carbon source. Microscopic and biochemical analyses supported by RNA-seq-based global transcriptomic analyses of these mutants revealed significantly down- and upregulated polysaccharide capsule formation and cps2 genes in D39ΔPhpP and D39ΔStkP mutants, respectively. While StkP and PhpP individually regulated several unique genes, they also participated in sharing the regulation of the same set of differentially regulated genes. Cps2 genes were reciprocally regulated in part by the StkP/PhpP-mediated reversible phosphorylation but independent of the MapZ-regulated cell division process. StkP-mediated dose-dependent phosphorylation of CcpA proportionately inhibited CcpA-binding to Pcps2A, supporting increased cps2 gene expression and capsule formation in D39ΔStkP. While the attenuation of the D39ΔPhpP mutant in two mouse infection models corroborated with several downregulated capsules-, virulence-, and phosphotransferase systems (PTS)-related genes, the D39ΔStkP mutant with increased amounts of polysaccharide capsules displayed significantly decreased virulence in mice compared to the D39 wild-type, but more virulence compared to D39ΔPhpP. NanoString technology-based inflammation-related gene expression and Meso Scale Discovery-based multiplex chemokine analysis of human lung cells cocultured with these mutants confirmed their distinct virulence phenotypes. StkP and PhpP may, therefore, serve as critical therapeutic targets.

Targeting NAD+ regeneration enhances antibiotic susceptibility of Streptococcus pneumoniae during invasive disease

Anaerobic bacteria are responsible for half of all pulmonary infections. One such pathogen is Streptococcus pneumoniae (Spn), a leading cause of community-acquired pneumonia, bacteremia/sepsis, and meningitis. Using a panel of isogenic mutants deficient in lactate, acetyl-CoA, and ethanol fermentation, as well as pharmacological inhibition, we observed that NAD(H) redox balance during fermentation was vital for Spn energy generation, capsule production, and in vivo fitness. Redox balance disruption in fermentation pathway-specific fashion substantially enhanced susceptibility to killing in antimicrobial class-specific manner. Blocking of alcohol dehydrogenase activity with 4-methylpyrazole (fomepizole), an FDA-approved drug used as an antidote for toxic alcohol ingestion, enhanced susceptibility of multidrug-resistant Spn to erythromycin and reduced bacterial burden in the lungs of mice with pneumonia and prevented the development of invasive disease. Our results indicate fermentation enzymes are de novo targets for antibiotic development and a novel strategy to combat multidrug-resistant pathogens.

Combining the In Silico and In Vitro Assays to Identify Strobilanthes cusia Kuntze Bioactives against Penicillin-Resistant Streptococcus pneumoniae

Leaves of Strobilanthes cusia Kuntze (S. cusia) are a widely used alexipharmic Traditional Chinese Medicine (TCM) in southern China for the prevention of cold and respiratory tract infectious diseases. One of the most common bacterial pathogens in the respiratory tract is the gram-positive bacterium Streptococcus pneumoniae. The antibiotic resistance of colonized S. pneumoniae makes it a more serious threat to public health. In this study, the leaves of S. cusia were found to perform antibacterial effects on the penicillin-resistant S. pneumoniae (PRSP). Confocal assay and Transmission Electron Microscopy (TEM) monitored the diminished cell wall integrity and capsule thickness of the PRSP with treatment. The following comparative proteomics analysis revealed that the glycometabolism-related pathways were enriched for the differentially expressed proteins between the samples with treatment and the control. To further delve into the specific single effective compound, the bio-active contents of leaves of S. cusia were analyzed by UPLC-UV-ESI-Q-TOF/MS, and 23 compounds were isolated for anti-PRSP screening. Among them, Tryptanthrin demonstrated the most promising effect, and it possibly inhibited the N-glycan degradation proteins, as suggested by reverse docking analysis in silico and further experimental verification by the surface plasmon resonance assay (SPR). Our study provided a research foundation for applications of the leaves of S. cusia as a TCM, and supplied a bio-active compound Tryptanthrin as a candidate drug skeleton for infectious diseases caused by the PRSP.

Uncovering the link between the SpnIII restriction modification system and LuxS in Streptococcus pneumoniae meningitis isolates

Streptococcus pneumoniae is capable of randomly switching their genomic DNA methylation pattern between six distinct bacterial subpopulations (A-F) via recombination of a type 1 restriction-modification locus, spnIII. These pneumococcal subpopulations exhibit phenotypic changes which favor carriage or invasive disease. In particular, the spnIIIB allele has been associated with increased nasopharyngeal carriage and the downregulation of the luxS gene. The LuxS/AI-2 QS system represent a universal language for bacteria and has been linked to virulence and biofilm formation in S. pneumoniae. In this work, we have explored the link between spnIII alleles, the luxS gene and virulence in two clinical pneumococcal isolates from the blood and cerebrospinal fluid (CSF) of one pediatric meningitis patient. The blood and CSF strains showed different virulence profiles in mice. Analysis of the spnIII system of these strains recovered from the murine nasopharynx showed that the system switched to different alleles commensurate with the initial source of the isolate. Of note, the blood strain showed high expression of spnIIIB allele, previously linked with less LuxS protein production. Importantly, strains with deleted luxS displayed different phenotypic profiles compared to the wildtype, but similar to the strains recovered from the nasopharynx of infected mice. This study used clinically relevant S. pneumoniae strains to demonstrate that the regulatory network between luxS and the type 1 restriction-modification system play a key role in infections and may support different adaptation to specific host niches.

Glucose levels affect MgaSpn regulation on the virulence and adaptability of Streptococcus pneumoniae

Streptococcus pneumoniae can regulate virulence gene expression by sensing environmental changes, which is key to its pathogenicity. The global transcription regulator MgaSpn of Streptococcus pneumoniae regulates virulence genes expression by directly binding to the promoter regions, but its role in response to different environments remains unclear. In this study, we found that glucose levels could affect phosphocholine content, which was mediated by MgaSpn. MgaSpn can also alter its anti-phagocytosis ability, depending on the availability of glucose. In addition, transcriptome analysis of wild-type D39s in low and high glucose concentrations revealed that MgaSpn was also involved in the regulation of carbon metabolism inhibition (carbon catabolite repression; CCR) and translation processes, which made S. pneumoniae highly competitive in fluctuating environments. In conclusion, MgaSpn is closely related to the virulence and environmental adaptability of Streptococcus pneumoniae.

Exogenous glycogen utilization effects the transcriptome and pathogenicity of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that causes severe infections in humans and the swine industry. Acquisition and utilization of available carbon sources from challenging host environments is necessary for bacterial pathogens to ensure growth and proliferation. Glycogen is abundant in mammalian body and may support the growth of SS2 during infection in hosts. However, limited information is known about the mechanism between the glycogen utilization and host adaptation of SS2. Here, the pleiotropic effects of exogenous glycogen on SS2 were investigated through transcriptome sequencing. Analysis of transcriptome data showed that the main basic metabolic pathways, especially the core carbon metabolism pathways and virulence-associated factors, of SS2 responded actively to glycogen induction. Glycogen induction led to the perturbation of the glycolysis pathway and citrate cycle, but promoted the pentose phosphate pathway and carbohydrate transport systems. Extracellular glycogen utilization also promoted the mixed-acid fermentation in SS2 rather than homolactic fermentation. Subsequently, apuA, a gene encoding the unique bifunctional amylopullulanase for glycogen degradation, was deleted from the wild type and generated the mutant strain ΔapuA. The pathogenicity details of the wild type and ΔapuA cultured in glucose and glycogen were investigated and compared. Results revealed that the capsule synthesis or bacterial morphology were not affected by glycogen incubation or apuA deletion. However, extracellular glycogen utilization significantly enhanced the hemolytic activity, adhesion and invasion ability, and lethality of SS2. The deletion of apuA also impaired the pathogenicity of bacteria cultured in glucose, indicating that ApuA is indeed an important virulence factor. Our results revealed that exogenous glycogen utilization extensively influenced the expression profile of the S. suis genome. Based on the transcriptome response, exogenous glycogen utilization promoted the carbon adaption and pathogenicity of SS2.

Mechanistic Insights into the Impact of Air Pollution on Pneumococcal Pathogenesis and Transmission

Streptococcus pneumoniae (the pneumococcus) is the leading cause of pneumonia and bacterial meningitis. A number of recent studies indicate an association between the incidence of pneumococcal disease and exposure to air pollution. Although the epidemiological evidence is substantial, the underlying mechanisms by which the various components of air pollution (particulate matter and gases such as NO2 and SO2) can increase susceptibility to pneumococcal infection are less well understood. In this review, we summarize the various effects air pollution components have on pneumococcal pathogenesis and transmission; exposure to air pollution can enhance host susceptibility to pneumococcal colonization by impairing the mucociliary activity of the airway mucosa, reducing the function and production of key antimicrobial peptides, and upregulating an important pneumococcal adherence factor on respiratory epithelial cells. Air pollutant exposure can also impair the phagocytic killing ability of macrophages, permitting increased replication of S. pneumoniae. In addition, particulate matter has been shown to activate various extra- and intracellular receptors of airway epithelial cells, which may lead to increased proinflammatory cytokine production. This increases recruitment of innate immune cells, including macrophages and neutrophils. The inflammatory response that ensues may result in significant tissue damage, thereby increasing susceptibility to invasive disease, because it allows S. pneumoniae access to the underlying tissues and blood. This review provides an in-depth understanding of the interaction between air pollution and the pneumococcus, which has the potential to aid the development of novel treatments or alternative strategies to prevent disease, especially in areas with high concentrations of air pollution.

Gene Expression Regulation in Airway Pathogens: Importance for Otitis Media

Otitis media (OM) is an inflammatory disorder in the middle ear. It is mainly caused by viruses or bacteria associated with the airways. Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis are the three main pathogens in infection-related OM, especially in younger children. In this review, we will focus upon the multifaceted gene regulation mechanisms that are well-orchestrated in S. pneumoniae, H. influenzae, and M. catarrhalis during the course of infection in the middle ear either in experimental OM or in clinical settings. The sophisticated findings from the past 10 years on how the othopathogens govern their virulence phenotypes for survival and host adaptation via phase variation- and quorum sensing-dependent gene regulation, will be systematically discussed. Comprehensive understanding of gene expression regulation mechanisms employed by pathogens during the onset of OM may provide new insights for the design of a new generation of antimicrobial agents in the fight against bacterial pathogens while combating the serious emergence of antimicrobial resistance.

Anatomical site-specific carbohydrate availability impacts Streptococcus pneumoniae virulence and fitness during colonization and disease

Streptococcus pneumoniae (Spn) colonizes the nasopharynx asymptomatically but can also cause severe life-threatening disease. Importantly, stark differences in carbohydrate availability exist between the nasopharynx and invasive disease sites, such as the bloodstream, which most likely impact Spn's behavior. Herein, using chemically-defined media (CDM) supplemented with physiological levels of carbohydrates, we examined how anatomical-site specific carbohydrate availability impacted Spn physiology and virulence. Spn grown in CDM modeling the nasopharynx (CDM-N) had reduced metabolic activity, slower growth rate, demonstrated mixed acid fermentation with marked H₂O₂ production, and were in a carbon-catabolite repression (CCR)-derepressed state versus Spn grown in CDM modeling blood (CDM-B). Using RNA-seq, we determined the transcriptome for Spn WT and its isogenic CCR deficient mutant in CDM-N and CDM-B. Genes with altered expression as a result of changes in carbohydrate availability or catabolite control protein deficiency, respectively, were primarily involved in carbohydrate metabolism, but also encoded for established virulence determinants such polysaccharide capsule and surface adhesins. We confirmed that anatomical site-specific carbohydrate availability directly influenced established Spn virulence traits. Spn grown in CDM-B formed shorter chains, produced more capsule, were less adhesive, and were more resistant to macrophage killing in an opsonophagocytosis assay. Moreover, growth of Spn in CDM-N or CDM-B prior to the challenge of mice impacted relative fitness in a colonization and invasive disease model, respectively. Thus, anatomical site-specific carbohydrate availability alters Spn physiology and virulence, in turn promoting anatomical-site specific fitness.

Carbon Source-Dependent Changes of the Structure of Streptococcus pneumoniae Capsular Polysaccharide with Serotype 6F

The structure of the exopolysaccharide capsule of Streptococcus pneumoniae is defined by the genetic arrangement of the capsule operon allowing the unequivocal identification of the pneumococcal serotype. Here, we investigated the environment-dependent composition of the polysaccharide structure of S. pneumoniae serotype 6F. When grown in a chemically defined medium (CDM) with glucose versus galactose, the exopolysaccharide capsule of the serotype 6F strains reveals a ratio of 1/0.6 or 1/0.3 for galactose/glucose in the capsule by ¹H-NMR analyses, respectively. Increased production of the capsule precursor UDP-glucose has been identified by ³¹P-NMR in CDM with glucose. Flow cytometric experiments using monoclonal antibodies showed decreased labelling of Hyp6AG4 (specific for serotype 6A) antibodies when 6F is grown in glucose as compared to galactose, which mirrors the 1H-NMR results. Whole-genome sequencing analyses of serotype 6F isolates suggested that the isolates evolved during two different events from serotype 6A during the time when the 13-valent pneumococcal conjugate vaccine (PCV-13) was introduced. In conclusion, this study shows differences in the capsular structure of serotype 6F strains using glucose as compared to galactose as the carbon source. Therefore, 6F strains may show slightly different polysaccharide composition while colonizing the human nasopharynx (galactose rich) as compared to invasive locations such as the blood (glucose rich).

A molecular link between cell wall biosynthesis, translation fidelity, and stringent response in Streptococcus pneumoniae

Survival in the human host requires bacteria to respond to unfavorable conditions. In the important Gram-positive pathogen Streptococcus pneumoniae, cell wall biosynthesis proteins MurM and MurN are tRNA-dependent amino acyl transferases which lead to the production of branched muropeptides. We demonstrate that wild-type cells experience optimal growth under mildly acidic stressed conditions, but ΔmurMN strain displays growth arrest and extensive lysis. Furthermore, these stress conditions compromise the efficiency with which alanyl-tRNAAla synthetase can avoid noncognate mischarging of tRNAAla with serine, which is toxic to cells. The observed growth defects are rescued by inhibition of the stringent response pathway or by overexpression of the editing domain of alanyl-tRNAAla synthetase that enables detoxification of tRNA misacylation. Furthermore, MurM can incorporate seryl groups from mischarged Seryl-tRNAAlaUGC into cell wall precursors with exquisite specificity. We conclude that MurM contributes to the fidelity of translation control and modulates the stress response by decreasing the pool of mischarged tRNAs. Finally, we show that enhanced lysis of ΔmurMN pneumococci is caused by LytA, and the murMN operon influences macrophage phagocytosis in a LytA-dependent manner. Thus, MurMN attenuates stress responses with consequences for host-pathogen interactions. Our data suggest a causal link between misaminoacylated tRNA accumulation and activation of the stringent response. In order to prevent potential corruption of translation, consumption of seryl-tRNAAla by MurM may represent a first line of defense. When this mechanism is overwhelmed or absent (ΔmurMN), the stringent response shuts down translation to avoid toxic generation of mistranslated/misfolded proteins.

Modulation of quorum sensing-associated virulence in bacteria: carbohydrate as a key factor

Quorum sensing (QS) is a method of inter-cellular communication that permits bacteria to dispense information about cell density and to synchronize the gene expression accordingly. Gram-positive and Gram-negative bacteria utilize distinct quorum sensing mechanisms for effective pathogenesis. Virulence factor production by pathogenic bacteria is one of the important traits that is under the control of QS. A growing body of evidence has indicated the role of the nutritional environment notably by carbohydrates in dictating the QS-associated virulence gene regulation. The modulation of QS by carbohydrates mitigates the survival and establishment of the pathogen within its host which in turn leads to an increase in morbidity and mortality. This mini-review throws light on the predilection of pathogenic bacteria to rapidly regulate its QS-linked virulence gene expression based on the changing nutrient levels that assist them in prospering within diverse niches.

Cadmium stress dictates central carbon flux and alters membrane composition in Streptococcus pneumoniae

Metal ion homeostasis is essential for all forms of life. However, the breadth of intracellular impacts that arise upon dysregulation of metal ion homeostasis remain to be elucidated. Here, we used cadmium, a non-physiological metal ion, to investigate how the bacterial pathogen, Streptococcus pneumoniae, resists metal ion stress and dyshomeostasis. By combining transcriptomics, metabolomics and metalloproteomics, we reveal that cadmium stress dysregulates numerous essential cellular pathways including central carbon metabolism, lipid membrane biogenesis and homeostasis, and capsule production at the transcriptional and/or functional level. Despite the breadth of cellular pathways susceptible to metal intoxication, we show that S. pneumoniae is able to maintain viability by utilizing cellular pathways that are predominately metal-independent, such as the pentose phosphate pathway to maintain energy production. Collectively, this work provides insight into the cellular processes impacted by cadmium and how resistance to metal ion toxicity is achieved in S. pneumoniae.

Neville et al. investigate how Streptococcus pneumoniae mitigates metal ion stress. Despite cadmium induced dysregulation of central carbon metabolism and lipid membrane homeostasis, they find that S. pneumoniae can remain viable by selectively utilizing predominately metal-independent cellular pathways. This study provides insights into how bacteria overcome metal ion toxicity.

Role of the pyruvate metabolic network on carbohydrate metabolism and virulence in Streptococcus pneumoniae

Streptococcus pneumoniae is a major human pathogen that must adapt to unique nutritional environments in several host niches. The pneumococcus can metabolize a range of carbohydrates that feed into glycolysis ending in pyruvate, which is catabolized by several enzymes. We investigated how the pneumococcus utilizes these enzymes to metabolize different carbohydrates and how this impacts survival in the host. Loss of ldh decreased bacterial burden in the nasopharynx and enhanced bacteremia in mice. Loss of spxB, pdhC or pfl2 decreased bacteremia and increased host survival. In glucose or galactose, loss of ldh increased capsule production, whereas loss of spxB and pdhC reduced capsule production. The pfl2 mutant exhibited reduced capsule production only in galactose. In glucose, pyruvate was metabolized primarily by LDH to generate lactate and NAD⁺ and by SpxB and PDHc to generate acetyl-CoA. In galactose, pyruvate metabolism was shunted toward acetyl-CoA production. The majority of acetyl-CoA generated by PFL was used to regenerate NAD⁺ with a subset used in capsule production, while the acetyl-CoA generated by SpxB and PDHc was utilized primarily for capsule biosynthesis. These data suggest that the pneumococcus can alter flux of pyruvate metabolism dependent on the carbohydrate present to succeed in distinct host niches.

In vivo dual RNA-seq reveals that neutrophil recruitment underlies differential tissue tropism of Streptococcus pneumoniae

Streptococcus pneumoniae is a genetically diverse human-adapted pathogen commonly carried asymptomatically in the nasopharynx. We have recently shown that a single nucleotide polymorphism (SNP) in the raffinose pathway regulatory gene rafR accounts for a difference in the capacity of clonally-related strains to cause localised versus systemic infection. Using dual RNA-seq, we show that this SNP affects expression of bacterial genes encoding multiple sugar transporters, and fine-tunes carbohydrate metabolism, along with extensive rewiring of host transcriptional responses to infection, particularly expression of genes encoding cytokine and chemokine ligands and receptors. The data predict a crucial role for differential neutrophil recruitment (confirmed by in vivo neutrophil depletion and IL-17 neutralization) indicating that early detection of bacteria by the host in the lung environment is crucial for effective clearance. Thus, dual RNA-seq provides a powerful tool for understanding complex host-pathogen interactions and reveals how a single bacterial SNP can drive differential disease outcomes.

Minhas, Aprianto et al. apply dual RNA seq to a set of related Streptococcus pneumoniae strains to find that differential neutrophil recruitment explains different tissue tropism of these strains. This study highlights the power of dual RNA-seq in investigating how a single bacterial SNP determines the host’s disease outcomes.

The N-Acetylglucosaminidase LytB of Streptococcus pneumoniae Is Involved in the Structure and Formation of Biofilms

The N-acetylglucosaminidase LytB of Streptococcus pneumoniae is involved in nasopharyngeal colonization and is responsible for cell separation at the end of cell division; thus, ΔlytB mutants form long chains of cells. This paper reports the construction and properties of a defective pneumococcal mutant producing an inactive LytB protein (LytBE585A). It is shown that an enzymatically active LytB is required for in vitro biofilm formation, as lytB mutants (either ΔlytB or producing the inactive LytBE585A) are incapable of forming substantial biofilms, despite that extracellular DNA is present in the biofilm matrix. Adding small amounts (0.5 to 2.0 μg/ml) of exogenous LytB or some LytB constructs restored the biofilm-forming capacity of lytB mutants to wild-type levels. The LytBE585A mutant formed biofilm more rapidly than ΔlytB mutants in the presence of LytB. This suggests that the mutant protein acted in a structural role, likely through the formation of complexes with extracellular DNA. The chain-dispersing capacity of LytB allowed the separation of daughter cells, presumably facilitating the formation of microcolonies and, finally, of biofilms. A role for the possible involvement of LytB in the synthesis of the extracellular polysaccharide component of the biofilm matrix is also discussed.IMPORTANCE It has been previously accepted that biofilm formation in S. pneumoniae must be a multigenic trait because the mutation of a single gene has led to only to partial inhibition of biofilm production. In the present study, however, evidence that the N-acetylglucosaminidase LytB is crucial in biofilm formation is provided. Despite the presence of extracellular DNA, strains either deficient in LytB or producing a defective LytB enzyme formed only shallow biofilms.

Host-glycan metabolism is regulated by a species-conserved two-component system in Streptococcus pneumoniae

Pathogens of the Streptococcus genus inhabit many different environmental niches during the course of an infection in a human host and the bacteria must adjust their metabolism according to available nutrients. Despite their lack of the citric-acid cycle, some streptococci proliferate in niches devoid of a readily available carbohydrate source. Instead they rely on carbohydrate scavenging for energy acquisition, which are obtained from the host. Here we discover a two-component system (TCS07) of Streptococcus pneumoniae that responds to glycoconjugated structures on proteins present on the host cells. Using next-generation RNA sequencing we find that the uncharacterized TCS07 regulon encodes proteins important for host-glycan processing and transporters of the released glycans, as well as intracellular carbohydrate catabolizing enzymes. We find that a functional TCS07 allele is required for growth on the glycoconjugated model protein fetuin. Consistently, we see a TCS07-dependent activation of the glycan degradation pathway. Thus, we pinpoint the molecular constituents responsible for sensing host derived glycans and link this to the induction of the proteins necessary for glycan degradation. Furthermore, we connect the TCS07 regulon to virulence in a mouse model, thereby establishing that host-derived glycan-metabolism is important for infection in vivo. Finally, a comparative phylogenomic analysis of strains from the Streptococcus genus reveal that TCS07 and most of its regulon is specifically conserved in species that utilize host-glycans for growth.

Worldwide, Streptococcus pneumoniae is the most common cause of community acquired pneumonia with high mortality rates. Interestingly, S. pneumoniae strictly relies on carbohydrate scavenging for energy acquisition, which are obtained from the host. This is a critical step in pathogenesis and a common mechanism among Streptococcal species. In this study, we discover an uncharacterized two-component system that responds to the carbohydrate structures present on the host cells. These are important findings as we describe the molecular mechanism responsible for sensing these host derived glycans, and how this mechanism is linked to virulence, thus highlighting that glycan metabolism is important for infection in vivo, thereby posing a novel target for intervention. Our phylogenetic analysis reveals that the two-component system and the genetic regulon co-occur and are specifically conserved among Streptococcal species capable of degrading host-glycans.

Carbon source regulates polysaccharide capsule biosynthesis in Streptococcus pneumoniae

The exopolysaccharide capsule of Streptococcus pneumoniae is an important virulence factor, but the mechanisms that regulate capsule thickness are not fully understood. Here, we investigated the effects of various exogenously supplied carbohydrates on capsule production and gene expression in several pneumococcal serotypes. Microscopy analyses indicated a near absence of the capsular polysaccharide (CPS) when S. pneumoniae was grown on fructose. Moreover, serotype 7F pneumococci produced much less CPS than strains of other serotypes (6B, 6C, 9V, 15, and 23F) when grown on glucose or sucrose. RNA-sequencing revealed carbon source-dependent regulation of distinct genes of WT strains and capsule-switch mutants of serotypes 6B and 7F, but could not explain the mechanism of capsule thickness regulation. In contrast, ³¹P NMR of whole-cell extract from capsule-knockout strains (Δcps) clearly revealed the accumulation or absence of capsule precursor metabolites when cells were grown on glucose or fructose, respectively. This finding suggests that fructose uptake mainly results in intracellular fructose 1-phosphate, which is not converted to CPS precursors. In addition, serotype 7F strains accumulated more precursors than did 6B strains, indicating less efficient conversion of precursor metabolites into the CPS in 7F, in line with its thinner capsule. Finally, isotopologue sucrose labeling and NMR analyses revealed that the uptake of the labeled fructose subunit into the capsule is <10% that of glucose. Our findings on the effects of carbon sources on CPS production in different S. pneumoniae serotypes may contribute to a better understanding of pneumococcal diseases and could inform future therapeutic approaches.

iDS372, a Phenotypically Reconciled Model for the Metabolism of Streptococcus pneumoniae Strain R6

A high-quality GSM model for Streptococcus pneumoniae R6 model strain (iDS372), comprising 372 genes and 529 reactions, was developed. The construction of this model involved performing a genome-wide reannotation to identify the metabolic capacity of the bacterium. A reaction representing the abstraction of the biomass composition was reconciled from several studies reported in the literature and previous models, and included in the model. The final model comprises two compartments and manifold automatically generated gene rules. The validation was performed with experimental data from recent studies, regarding the usability of carbon sources, the effect of the presence of oxygen, and the requirement of amino acids for growth. This model can be used to better understand the metabolism of this major pathogen, provide clues regarding new drug targets, and eventually design strategies for fighting infections by these bacteria.

Assessing the function of pneumococcal neuraminidases NanA, NanB and NanC in in vitro and in vivo lung infection models using monoclonal antibodies

Streptococcus pneumoniae isolates express up to three neuraminidases (sialidases), NanA, NanB and NanC, all of which cleave the terminal sialic acid of glycan-structures that decorate host cell surfaces. Most research has focused on the role of NanA with limited investigations evaluating the roles of all three neuraminidases in host-pathogen interactions. We generated two highly potent monoclonal antibodies (mAbs), one that blocks the enzymatic activity of NanA and one cross-neutralizing NanB and NanC. Total neuraminidase activity of clinical S. pneumoniae isolates could be inhibited by this mAb combination in enzymatic assays. To detect desialylation of cell surfaces by pneumococcal neuraminidases, primary human tracheal/bronchial mucocilial epithelial tissues were infected with S. pneumoniae and stained with peanut lectin. Simultaneous targeting of the neuraminidases was required to prevent desialylation, suggesting that inhibition of NanA alone is not sufficient to preserve terminal lung glycans. Importantly, we also found that all three neuraminidases increased the interaction of S. pneumoniae with human airway epithelial cells. Lectin-staining of lung tissues of mice pre-treated with mAbs before intranasal challenge with S. pneumoniae confirmed that both anti-NanA and anti-NanBC mAbs were required to effectively block desialylation of the respiratory epithelium in vivo. Despite this, no effect on survival, reduction in pulmonary bacterial load, or significant changes in cytokine responses were observed. This suggests that neuraminidases have no pivotal role in this murine pneumonia model that is induced by high bacterial challenge inocula and does not progress from colonization as it happens in the human host.

An Uncharacterized Member of the Gls24 Protein Superfamily Is a Putative Sensor of Essential Amino Acid Availability in Streptococcus pneumoniae

Proteins belonging to the Gls24 superfamily are involved in survival of pathogenic Gram-positive cocci under oligotrophic conditions and other types of stress, by a still unknown molecular mechanism. In Firmicutes, this superfamily includes three different valine-rich orthologal families (Gls24A, B, C) with different potential interactive partners. Whereas the Streptococcus pneumoniae Δgls24A deletion mutant experienced a general long growth delay, the Δgls24B mutant grew as the parental strain in the semisynthetic AGCH medium but failed to grow in the complex Todd-Hewitt medium. Bovine seroalbumin (BSA) was the component responsible for this phenotype. The effect of BSA on growth was concentration-dependent and was maintained when the protein was proteolyzed but not when heat-denatured, suggesting that BSA dependence was related to oligopeptide supplementation. Global transcriptional analyses of the knockout mutant revealed catabolic derepression and induction of chaperone and oligopeptide transport genes. This mutant also showed increased sensibility to cadmium and high temperature. The Δgls24B mutant behaved as a poor colonizer in the nasopharynx of mice and showed 20-fold competence impairment. Experimental data suggest that Gls24B plays a central role as a sensor of amino acid availability and its connection to sugar catabolism. This metabolic rewiring can be compensated in vitro, at the expenses of external oligopeptide supplementation, but reduce important bacteria skills prior to efficiently address systemic virulence traits. This is an example of how metabolic factors conserved in enterococci, streptococci, and staphylococci can be essential for survival in poor oligopeptide environments prior to infection progression.

Capacity To Utilize Raffinose Dictates Pneumococcal Disease Phenotype

S. pneumoniae is a component of the commensal nasopharyngeal microflora of humans, but from this reservoir, it can progress to localized or invasive disease with a frequency that translates into massive global morbidity and mortality. However, the factors that govern the switch from commensal to pathogen, as well as those that determine disease tropism, are poorly understood. Here we show that capacity to utilize raffinose can determine the nature of the disease caused by a given pneumococcal strain. Moreover, our findings provide an interesting example of convergent evolution, whereby pneumococci belonging to two unrelated serotypes/lineages exhibit SNPs in separate genes affecting raffinose uptake and utilization that correlate with distinct pathogenic profiles in vivo. This further underscores the critical role of differential carbohydrate metabolism in the pathogenesis of localized versus invasive pneumococcal disease.

Streptococcus pneumoniae is commonly carried asymptomatically in the human nasopharynx, but it also causes serious and invasive diseases such as pneumonia, bacteremia, and meningitis, as well as less serious but highly prevalent infections such as otitis media. We have previously shown that closely related pneumococci (of the same capsular serotype and multilocus sequence type [ST]) can display distinct pathogenic profiles in mice that correlate with clinical isolation site (e.g., blood versus ear), suggesting stable niche adaptation within a clonal lineage. This has provided an opportunity to identify determinants of disease tropism. Genomic analysis identified 17 and 27 single nucleotide polymorphisms (SNPs) or insertions/deletions in protein coding sequences between blood and ear isolates of serotype 14 ST15 and serotype 3 ST180, respectively. SNPs in raffinose uptake and utilization genes (rafR or rafK) were detected in both serotypes/lineages. Ear isolates were consistently defective in growth in media containing raffinose as the sole carbon source, as well as in expression of raffinose pathway genes aga, rafG, and rafK, relative to their serotype/ST-matched blood isolates. Similar differences were also seen between serotype 23F ST81 blood and ear isolates. Analysis of rafR allelic exchange mutants of the serotype 14 ST15 blood and ear isolates demonstrated that the SNP in rafR was entirely responsible for their distinct in vitro phenotypes and was also the determinant of differential tropism for the lungs versus ear and brain in a mouse intranasal challenge model. These data suggest that the ability of pneumococci to utilize raffinose determines the nature of disease.

The Role of Pneumococcal Virulence Factors in Ocular Infectious Diseases

Streptococcus pneumoniae is a gram-positive, facultatively anaerobic pathogen that can cause severe infections such as pneumonia, meningitis, septicemia, and middle ear infections. It is also one of the top pathogens contributing to bacterial keratitis and conjunctivitis. Though two pneumococcal vaccines exist for the prevention of nonocular diseases, they do little to fully prevent ocular infections. This pathogen has several virulence factors that wreak havoc on the conjunctiva, cornea, and intraocular system. Polysaccharide capsule aids in the evasion of host complement system. Pneumolysin (PLY) is a cholesterol-dependent cytolysin that acts as pore-forming toxin. Neuraminidases assist in adherence and colonization by exposing cell surface receptors to the pneumococcus. Zinc metalloproteinases contribute to evasion of the immune system and disease severity. The main purpose of this review is to consolidate the multiple studies that have been conducted on several pneumococcal virulence factors and the role each plays in conjunctivitis, keratitis, and endophthalmitis.

Coordinated Regulation of the EIIMan and fruRKI Operons of Streptococcus mutans by Global and Fructose-Specific Pathways

The glucose/mannose-phosphotransferase system (PTS) permease EII^Man encoded by manLMN in the dental caries pathogen Streptococcus mutans has a dominant influence on sugar-specific, CcpA-independent catabolite repression (CR). Mutations in manL affect energy metabolism and virulence-associated traits, including biofilm formation, acid tolerance, and competence. Using promoter::reporter fusions, expression of the manLMN and the fruRKI operons, encoding a transcriptional regulator, a fructose-1-phosphate kinase and a fructose-PTS permease EII^Fru, respectively, was monitored in response to carbohydrate source and in mutants lacking CcpA, FruR, and components of EII^Man Expression of genes for EII^Man and EII^Fru was directly regulated by CcpA and CR, as evinced by in vivo and in vitro methods. Unexpectedly, not only was the fruRKI operon negatively regulated by FruR, but also so was manLMN Carbohydrate transport by EII^Man had a negative influence on expression of manLMN but not fruRKI In agreement with the proposed role of FruR in regulating these PTS operons, loss of fruR or fruK substantially altered growth on a number of carbohydrates, including fructose. RNA deep sequencing revealed profound changes in gene regulation caused by deletion of fruK or fruR Collectively, these findings demonstrate intimate interconnection of the regulation of two major PTS permeases in S. mutans and reveal novel and important contributions of fructose metabolism to global regulation of gene expression.IMPORTANCE The ability of Streptococcus mutans and other streptococcal pathogens to survive and cause human diseases is directly dependent upon their capacity to metabolize a variety of carbohydrates, including glucose and fructose. Our research reveals that metabolism of fructose has broad influences on the regulation of utilization of glucose and other sugars, and mutants with changes in certain genes involved in fructose metabolism display profoundly different abilities to grow and express virulence-related traits. Mutants lacking the FruR regulator or a particular phosphofructokinase, FruK, display changes in expression of a large number of genes encoding transcriptional regulators, enzymes required for energy metabolism, biofilm development, biosynthetic and degradative processes, and tolerance of a spectrum of environmental stressors. Since fructose is a major component of the modern human diet, the results have substantial significance in the context of oral health and the development of dental caries.

Exogenous Streptococcus pneumoniae Endophthalmitis in Diabetic Rabbits

Diabetics are at increased risk for eye infections including bacterial endophthalmitis. It is unclear whether the severity of endophthalmitis is greater in these patients due to confounding factors such as pre-existing ocular diseases in some but not others. Therefore, we tested the hypothesis that disease severity and/or bacterial loads would be significantly higher in a Type I diabetic rabbit model of Streptococcus pneumoniae endophthalmitis. Rabbits were treated with alloxan to destroy pancreatic islet cells, or mock-treated with vehicle, and maintained for 10 days before intravitreal infection with S. pneumoniae E353. Clinical scoring of the eyes was performed 24 and 48 hours after infection, followed by euthanasia and vitreous harvest to quantitate bacterial loads. There were no significant differences in clinical scores (P ≥ 0.440) or bacterial loads (P = 0.736), however, 4/12 (33%) of the diabetic rabbits became bacteremic. This finding not only indicates a breakdown in the blood-ocular barrier, but also prompts further investigation into the exploitation of the diabetic eye by the streptococci.

Streptococcus pneumoniae Senses a Human-like Sialic Acid Profile via the Response Regulator CiaR

Streptococcus pneumoniae is a human-adapted pathogen that encounters terminally sialylated glycoconjugates and free sialic acid (Sia) in the airways. Upon scavenging by the bacterial sialidase NanA, Sias serve as carbon sources for the bacteria. Unlike most animals in which cytidine-monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) converts Sia N-acetylneuraminic acid (Neu5Ac) into N-glycolylneuraminic acid (Neu5Gc), humans have an inactive CMAH, causing an absence of Neu5Gc and excess Neu5Ac. We find that pneumococcal challenge in Cmah(-/-) mice leads to heightened bacterial loads, virulence, and NanA expression. In vitro, NanA is upregulated in response to Neu5Ac compared with Neu5Gc, a process controlled by the two-component response regulator CiaR and requiring Sia uptake by the transporter SatABC. Additionally, compared with Neu5Gc, Neu5Ac increases pneumococcal resistance to antimicrobial reactive oxygen species in a CiaR-dependent manner. Thus, S. pneumoniae senses and responds to Neu5Ac, leading to CiaR activation and increased virulence and potentially explaining the greater susceptibility in humans.

Amino Sugars Enhance the Competitiveness of Beneficial Commensals with Streptococcus mutans through Multiple Mechanisms

Biochemical and genetic aspects of the metabolism of the amino sugars N-acetylglucosamine (GlcNAc) and glucosamine (GlcN) by commensal oral streptococci and the effects of these sugars on interspecies competition with the dental caries pathogen Streptococcus mutans were explored. Multiple S. mutans wild-type isolates displayed long lag phases when transferred from glucose-containing medium to medium with GlcNAc as the primary carbohydrate source, but commensal streptococci did not. Competition in liquid coculture or dual-species biofilms between S. mutans and Streptococcus gordonii showed that S. gordonii was particularly dominant when the primary carbohydrate was GlcN or GlcNAc. Transcriptional and enzymatic assays showed that the catabolic pathway for GlcNAc was less highly induced in S. mutans than in S. gordonii Exposure to H2O2, which is produced by S. gordonii and antagonizes the growth of S. mutans, led to reduced mRNA levels of nagA and nagB in S. mutans When the gene for the transcriptional regulatory NagR was deleted in S. gordonii, the strain produced constitutively high levels of nagA (GlcNAc-6-P deacetylase), nagB (GlcN-6-P deaminase), and glmS (GlcN-6-P synthase) mRNA. Similar to NagR of S. mutans (NagRSm), the S. gordonii NagR protein (NagRSg) could bind to consensus binding sites (dre) in the nagA, nagB, and glmS promoter regions of S. gordonii Notably, NagRSg binding was inhibited by GlcN-6-P, but G-6-P had no effect, unlike for NagRSm This study expands the understanding of amino sugar metabolism and NagR-dependent gene regulation in streptococci and highlights the potential for therapeutic applications of amino sugars to prevent dental caries.

IMPORTANCE

Amino sugars are abundant in the biosphere, so the relative efficiency of particular bacteria in a given microbiota to metabolize these sources of carbon and nitrogen might have a profound impact on the ecology of the community. Our investigation reveals that several oral commensal bacteria have a much greater capacity to utilize amino sugars than the dental pathogen Streptococcus mutans and that the ability of the model commensal Streptococcus gordonii to compete against S. mutans is substantively enhanced by the presence of amino sugars commonly found in the oral cavity. The mechanisms underlying the greater capacity and competitive enhancements of the commensal are shown to depend on how the genes for the catabolic enzymes are regulated, the role of the allosteric modulators affecting such regulation, and the ability of amino sugars to enhance certain activities of the commensal that are antagonistic to S. mutans.