Carbon catabolite repression by seryl phosphorylated HPr is essential to Streptococcus pneumoniae in carbohydrate-rich environments

Systematic analysis of the glucose-PTS in Streptococcus sanguinis highlighted its importance in central metabolism and bacterial fitness

Previous work reported that deletion of the Enzyme IIAB subunits (EIIABMan and manL) of the glucose phosphotransferase system (PTS) (glucose-PTS, manLMNO) in Streptococcus sanguinis impacted carbon catabolite repression and bacterial fitness. Here, a single-nucleotide polymorphism in ManN, ManNA91E, produced the unusual phenotype of increased excretion of organic acids and H2O2 yet elevated PTS activities. To characterize the contributions of each component of the glucose-PTS to bacterial fitness, we performed genetic analyses by deleting from S. sanguinis SK36 the entire operon and each EIIMan subunit individually; and genes encoding the catabolite control protein A (ΔccpA) and the redox regulator Rex (Δrex) for comparison. Deletion of each subunit incurred a growth defect on glucose partly due to elevated excretion of H2O2; when supplemented with catalase, this defect was rescued, instead resulting in a significantly higher yield than the parent. All glucose-PTS deletion mutants presented an increased antagonism against the oral pathobiont Streptococcus mutans, a phenotype absent in ΔccpA despite increased H2O2 output. A shift in the pyruvate node toward mixed acid fermentation and increased arginine deiminase activity enhanced pH homeostasis in glucose-PTS mutants but not ΔccpA. Despite the purported ability of Rex to regulate central carbon metabolism, deletion of rex had no significant impact on most of the phenotypes discussed here. These findings place glucose-PTS in the pivotal position of controlling central carbon flux in streptococci, with critical outcomes impacting acidogenicity, aciduricity, pH homeostasis, and antagonism, highlighting its potential as a therapeutic target for treating diseases with a dysbiotic microbiome.

IMPORTANCE

Management of carbohydrate metabolism and environmental stress is key to the survival of oral commensal species such as S. sanguinis. Antagonism of oral pathobionts and modulation of the environmental pH and oxidative potential by commensals are crucial to the maintenance of microbial homeostasis and prevention of oral diseases including dental caries. It is therefore vital to understand how these species regulate sugar fermentation, production of acids and ammonia, and stress management in an environment known for a feast-and-famine cycle of carbohydrates and similar fluctuations in pH and oxygen tension. Here, we detail that genetic alterations of the glucose-PTS transporter in S. sanguinis can significantly affect the regulation of factors required for bacterial fitness and homeostatic ability independent of known catabolic regulators. It is then discussed how these changes may impact the survival of streptococcal species and affect caries onset.

Glycerol metabolism contributes to competition by oral streptococci through production of hydrogen peroxide

As a biological byproduct from both humans and microbes, glycerol's contribution to microbial homeostasis in the oral cavity remains understudied. In this study, we examined glycerol metabolism by Streptococcus sanguinis, a commensal associated with oral health. Genetic mutants of glucose-PTS enzyme II (manL), glycerol metabolism (glp and dha pathways), and transcriptional regulators were characterized with regard to glycerol catabolism, growth, production of hydrogen peroxide (H2O2), transcription, and competition with Streptococcus mutans. Biochemical assays identified the glp pathway as a novel source for H2O2 production by S. sanguinis that is independent of pyruvate oxidase (SpxB). Genetic analysis indicated that the glp pathway requires glycerol and a transcriptional regulator, GlpR, for expression and is negatively regulated by PTS, but not the catabolite control protein, CcpA. Conversely, deletion of either manL or ccpA increased the expression of spxB and a second, H2O2-non-producing glycerol metabolic pathway (dha), indicative of a mode of regulation consistent with conventional carbon catabolite repression (CCR). In a plate-based antagonism assay and competition assays performed with planktonic and biofilm-grown cells, glycerol greatly benefited the competitive fitness of S. sanguinis against S. mutans. The glp pathway appears to be conserved in several commensal streptococci and actively expressed in caries-free plaque samples. Our study suggests that glycerol metabolism plays a more significant role in the ecology of the oral cavity than previously understood. Commensal streptococci, though not able to use glycerol as a sole carbohydrate source for growth, benefit from the catabolism of glycerol through production of both ATP and H2O2.

IMPORTANCE

Glycerol is an abundant carbohydrate in the oral cavity. However, little is understood regarding the metabolism of glycerol by commensal streptococci, some of the most abundant oral bacteria. This was in part because most streptococci cannot grow on glycerol as the sole carbon source. In this study, we show that Streptococcus sanguinis, a commensal associated with dental health, can degrade glycerol for persistence and competition through two pathways, one of which generates hydrogen peroxide at levels capable of inhibiting Streptococcus mutans. Preliminary studies suggest that several additional commensal streptococci are also able to catabolize glycerol, and glycerol-related genes are actively expressed in human dental plaque samples. Our findings reveal the potential of glycerol to significantly impact microbial homeostasis, which warrants further exploration.

Glycerol Metabolism Contributes to Competition by Oral Streptococci through Production of Hydrogen Peroxide

As a biological byproduct from both humans and microbes, glycerol's contribution to microbial homeostasis in the oral cavity remains understudied. Here we examined glycerol metabolism by Streptococcus sanguinis, a commensal associated with oral health. Genetic mutants of glucose-PTS enzyme II ( manL ), glycerol metabolism ( glp and dha pathways), and transcriptional regulators were characterized with regard to glycerol catabolism, growth, production of hydrogen peroxide (H 2 O 2 ), transcription, and competition with Streptococcus mutans . Biochemical assays identified the glp pathway as a novel source of H 2 O 2 production by S. sanguinis that is independent of pyruvate oxidase (SpxB). Genetic analysis indicated that the glp pathway requires glycerol and a transcriptional regulator, GlpR, for expression and is negatively regulated by PTS, but not the catabolite control protein, CcpA. Conversely, deletion of either manL or ccpA increased expression of spxB and a second, H 2 O 2 -non-producing glycerol metabolic pathway ( dha ), indicative of a mode of regulation consistent with conventional carbon catabolite repression (CCR). In a plate-based antagonism assay and competition assays performed with planktonic and biofilm-grown cells, glycerol greatly benefited the competitive fitness of S. sanguinis against S. mutans. The glp pathway appears to be conserved in several commensal streptococci and actively expressed in caries-free plaque samples. Our study suggests that glycerol metabolism plays a more significant role in the ecology of the oral cavity than previously understood. Commensal streptococci, though not able to use glycerol as a sole carbohydrate for growth, benefit from catabolism of glycerol through production of both ATP and H 2 O 2 .

Importance

Glycerol is an abundant carbohydrate found in oral cavity, both due to biological activities of humans and microbes, and as a common ingredient of foods and health care products. However, very little is understood regarding the metabolism of glycerol by some of the most abundant oral bacteria, commensal streptococci. This was in part because most streptococci cannot grow on glycerol as the sole carbon source. Here we show that Streptococcus sanguinis , an oral commensal associated with dental health, can degrade glycerol for persistence and competition through two independent pathways, one of which generates hydrogen peroxide at levels capable of inhibiting a dental pathobiont, Streptococcus mutans . Preliminary studies suggest that several other commensal streptococci are also able to catabolize glycerol, and glycerol-related genes are being actively expressed in human dental plaque samples. Our findings reveal the potential of glycerol to significantly impact microbial homeostasis which warrants further exploration.

comCDE (Competence) Operon Is Regulated by CcpA in Streptococcus pneumoniae D39

Natural transformation plays an important role in the formation of drug-resistant bacteria. Exploring the regulatory mechanism of natural transformation can aid the discovery of new antibacterial targets and reduce the emergence of drug-resistant bacteria. Competence is a prerequisite of natural transformation in Streptococcus pneumoniae, in which comCDE operon is the core regulator of competence. To date, only ComE has been shown to directly regulate comCDE transcription. In this study, a transcriptional regulator, the catabolite control protein A (CcpA), was identified that directly regulated comCDE transcription. We confirmed that CcpA binds to the cis-acting catabolite response elements (cre) in the comCDE promoter region to regulate comCDE transcription and transformation. Moreover, CcpA can coregulate comCDE transcription with phosphorylated and dephosphorylated ComE. Regulation of comCDE transcription and transformation by CcpA was also affected by carbon source signals. Together, these insights demonstrate the versatility of CcpA and provide a theoretical basis for reducing the emergence of drug-resistant bacteria. IMPORTANCE Streptococcus pneumoniae is a major cause of bacterial infections in humans, such as pneumonia, bacteremia, meningitis, otitis media, and sinusitis. Like most streptococci, S. pneumoniae is naturally competent and employs this ability to augment its adaptive evolution. The current study illustrates CcpA, a carbon catabolite regulator, can participate in the competence process by regulating comCDE transcription, and this process is regulated by different carbon source signals. These hidden abilities are likely critical for adaptation and colonization in the environment.

Pn-AqpC-Mediated Fermentation Pattern Coordination with the Two-Component System 07 Regulates Host N-Glycan Degradation of Streptococcus pneumoniae

The opportunistic pathogen Streptococcus pneumoniae (pneumococcus) is a human nasopharyngeal commensal, and host N-glycan metabolism promotes its colonization and invasion. It has been reported that glucose represses, while fetuin, a glycoconjugated model protein, induces, the genes involved in N-glycan degradation through the two-component system TCS07. However, the mechanisms of glucose repression and TCS07 induction remain unknown. Previously, we found that the pneumococcal aquaglyceroporin Pn-AqpC facilitates oxygen uptake, thereby contributing to the antioxidant potential and virulence. In this study, through Tandem Mass Tag (TMT) quantitative proteomics, we found that the deletion of Pn-aqpC caused a marked upregulation of 11 proteins involved in N-glycan degradation in glucose-grown pneumococcus R6. Both quantitative RT-PCR and GFP fluorescence reporters revealed that the upregulation of N-glycan genes was completely dependent on response regulator (RR) 07, but not on the histidine kinase HK07 of TCS07 or the phosphoryl-receiving aspartate residue of RR07 in ΔPn-aqpC, indicating that RR07 was activated in an HK07-independent manner when Pn-AqpC was absent. The deletion of Pn-aqpC also enhanced the expression of pyruvate formate lyase and increased formate production, probably due to reduced cellular oxygen content, indicating that a shunt of glucose catabolism to mixed acid fermentation occurs. Notably, formate induced the N-glycan degradation genes in glucose-grown R6, but the deletion of rr07 abolished this induction, indicating that formate activates RR07. However, the induction of N-glycan degradation proteins reduced the intraspecies competition of R6 in glucose. Therefore, although N-glycan degradation promotes pneumococcal pathogenesis, the glucose metabolites-based RR07 regulation reported here is of importance for balancing growth fitness and the pathogenicity of pneumococcus. IMPORTANCE Pneumococcus, a human opportunistic pathogen, is capable of metabolizing host complex N-glycans. N-glycan degradation promotes pneumococcus colonization in the nasopharynx as well as invasion into deeper tissues, thus significantly contributing to pathogenesis. It is known that the two-component system 07 induces the N-glycan metabolizing genes; however, how TCS07 is activated remains unknown. This study reveals that formate, the anaerobic fermentation metabolite of pneumococcus, is a novel activator of the response regulator (RR) 07. Although the high expression of N-glycan degradation genes promotes pneumococcal colonization in the nasopharynx and pathogenesis, this reduces pneumococcal growth fitness in glucose as indicated in this work. Notably, the presence of Pn-AqpC, an oxygen-transporting aquaglyceroporin, enables pneumococcus to maintain glucose homolactic acid fermentation, thus reducing formate production, maintaining RR07 inactivation, and controlling N-glycan degrading genes at a non-induced status. Thus, this study highlights a novel fermentation metabolism pattern linking TCS-regulated carbohydrate utilization strategies as a trade-off between the fitness and the pathogenicity of pneumococcus.

Spontaneous Mutants of Streptococcus sanguinis with Defects in the Glucose-Phosphotransferase System Show Enhanced Post-Exponential-Phase Fitness

Genetic truncations in a gene encoding a putative glucose-phosphotransferase system (PTS) protein (manL, EIIAB^Man) were identified in subpopulations of two separate laboratory stocks of Streptococcus sanguinis SK36; the mutants had reduced PTS activities on glucose and other monosaccharides. To understand the emergence of these mutants, we engineered deletion mutants of manL and showed that the ManL-deficient strain had improved bacterial viability in the stationary phase and was better able to inhibit the growth of the dental caries pathogen Streptococcus mutans. Transcriptional analysis and biochemical assays suggested that the manL mutant underwent reprograming of central carbon metabolism that directed pyruvate away from production of lactate, increasing production of hydrogen peroxide (H₂O₂) and excretion of pyruvate. Addition of pyruvate to the medium enhanced the survival of SK36 in overnight cultures. Meanwhile, elevated pyruvate levels were detected in the cultures of a small but significant percentage (∼10%) of clinical isolates of oral commensal bacteria. Furthermore, the manL mutant showed higher expression of the arginine deiminase system than the wild type, which enhanced the ability of the mutant to raise environmental pH when arginine was present. To our surprise, significant discrepancies in genome sequence were identified between strain SK36 obtained from ATCC and the sequence deposited in GenBank. As the conditions that are likely associated with the emergence of spontaneous manL mutations, i.e., excess carbohydrates and low pH, are those associated with caries development, we propose that glucose-PTS strongly influences commensal-pathogen interactions by altering the production of ammonia, pyruvate, and H₂O₂. IMPORTANCE A health-associated dental microbiome provides a potent defense against pathogens and diseases. Streptococcus sanguinis is an abundant member of a health-associated oral flora that antagonizes pathogens by producing hydrogen peroxide. There is a need for a better understanding of the mechanisms that allow bacteria to survive carbohydrate-rich and acidic environments associated with the development of dental caries. We report the isolation and characterization of spontaneous mutants of S. sanguinis with impairment in glucose transport. The resultant reprograming of the central metabolism in these mutants reduced the production of lactic acid and increased pyruvate accumulation; the latter enables these bacteria to better cope with hydrogen peroxide and low pH. The implications of these discoveries in the development of dental caries are discussed.

Genome-wide analysis of in vivo CcpA binding with and without its key co-factor HPr in the major human pathogen group A Streptococcus

Catabolite control protein A (CcpA) is a master regulator of carbon source utilization and contributes to the virulence of numerous medically important Gram‐positive bacteria. Most functional assessments of CcpA, including interaction with its key co‐factor HPr, have been performed in nonpathogenic bacteria. In this study we aimed to identify the in vivo DNA binding profile of CcpA and assess the extent to which HPr is required for CcpA‐mediated regulation and DNA binding in the major human pathogen group A Streptococcus (GAS). Using a combination RNAseq/ChIP‐seq approach, we found that CcpA affects transcript levels of 514 of 1667 GAS genes (31%) whereas direct DNA binding was identified for 105 GAS genes. Three of the directly regulated genes encode the key GAS virulence factors Streptolysin S, PrtS (IL‐8 degrading proteinase), and SpeB (cysteine protease). Mutating CcpA Val301 to Ala (strain 2221‐CcpA‐V301A) abolished interaction between CcpA and HPr and impacted the transcript levels of 205 genes (40%) in the total CcpA regulon. By ChIP‐seq analysis, CcpAV301A bound to DNA from 74% of genes bound by wild‐type CcpA, but generally with lower affinity. These data delineate the direct CcpA regulon and clarify the HPr‐dependent and independent activities of CcpA in a key pathogenic bacterium.

Site-Specific Mutations of GalR Affect Galactose Metabolism in Streptococcus pneumoniae

Streptococcus pneumoniae (the pneumococcus) is a formidable human pathogen that is capable of asymptomatically colonizing the nasopharynx. Progression from colonization to invasive disease involves adaptation to distinct host niches, which vary markedly in the availability of key nutrients such as sugars. We previously reported that cell-cell signaling via the autoinducer 2 (AI-2)/LuxS quorum-sensing system boosts the capacity of S. pneumoniae to utilize galactose as a carbon source by upregulation of the Leloir pathway. This resulted in increased capsular polysaccharide production and a hypervirulent phenotype. We hypothesized that this effect was mediated by phosphorylation of GalR, the transcriptional activator of the Leloir pathway. GalR is known to possess three putative phosphorylation sites, S317, T319, and T323. In the present study, derivatives of S. pneumoniae D39 with putative phosphorylation-blocking alanine substitution mutations at each of these GalR sites (singly or in combination) were constructed. Growth assays and transcriptional analyses revealed complex phenotypes for these GalR mutants, with impacts on the regulation of both the Leloir and tagatose 6-phosphate pathways. The alanine substitution mutations significantly reduced the capacity of pneumococci to colonize the nasopharynx, middle ear, and lungs in a murine intranasal challenge model.IMPORTANCE Pneumococcal survival in the host and capacity to transition from a commensal to a pathogenic lifestyle are closely linked to the organism's ability to utilize specific nutrients in distinct niches. Galactose is a major carbon source for pneumococci in the upper respiratory tract. We have shown that both the Leloir and tagatose 6-phosphate pathways are necessary for pneumococcal growth in galactose and demonstrated GalR-mediated interplay between the two pathways. Moreover, the three putative phosphorylation sites in the transcriptional regulator GalR play a critical role in galactose metabolism and are important for pneumococcal colonization of the nasopharynx, middle ear, and lungs.

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.

Phosphotransferase System Uptake and Metabolism of the β-Glucoside Salicin Impact Group A Streptococcal Bloodstream Survival and Soft Tissue Infection

Streptococcus pyogenes (group A Streptococcus [GAS]), a major human-specific pathogen, relies on efficient nutrient acquisition for successful infection within its host. The phosphotransferase system (PTS) couples the import of carbohydrates with their phosphorylation prior to metabolism and has been linked to GAS pathogenesis. In a screen of an insertional mutant library of all 14 annotated PTS permease (EIIC) genes in MGAS5005, the annotated β-glucoside PTS transporter (bglP) was found to be crucial for GAS growth and survival in human blood and was validated in another M1T1 GAS strain, 5448. In 5448, bglP was shown to be in an operon with a putative phospho-β-glucosidase (bglB) downstream and a predicted antiterminator (licT) upstream. Using defined nonpolar mutants of the β-glucoside permease (bglP) and β-glucosidase enzyme (bglB) in 5448, we showed that bglB, not bglP, was important for growth in blood. Furthermore, transcription of the licT-blgPB operon was found to be repressed by glucose and induced by the β-glucoside salicin as the sole carbon source. Investigation of the individual bglP and bglB mutants determined that they influence in vitro growth in the β-glucoside salicin; however, only bglP was necessary for growth in other non-β-glucoside PTS sugars, such as fructose and mannose. Additionally, loss of BglP and BglB suggests that they are important for the regulation of virulence-related genes that control biofilm formation, streptolysin S (SLS)-mediated hemolysis, and localized ulcerative lesion progression during subcutaneous infections in mice. Thus, our results indicate that the β-glucoside PTS transports salicin and its metabolism can differentially influence GAS pathophysiology during soft tissue infection.

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.

Biology of Oral Streptococci

Bacteria belonging to the genus Streptococcus are the first inhabitants of the oral cavity, which can be acquired right after birth and thus play an important role in the assembly of the oral microbiota. In this article, we discuss the different oral environments inhabited by streptococci and the species that occupy each niche. Special attention is given to the taxonomy of Streptococcus, because this genus is now divided into eight distinct groups, and oral species are found in six of them. Oral streptococci produce an arsenal of adhesive molecules that allow them to efficiently colonize different tissues in the mouth. Also, they have a remarkable ability to metabolize carbohydrates via fermentation, thereby generating acids as byproducts. Excessive acidification of the oral environment by aciduric species such as Streptococcus mutans is directly associated with the development of dental caries. However, less acid-tolerant species such as Streptococcus salivarius and Streptococcus gordonii produce large amounts of alkali, displaying an important role in the acid-base physiology of the oral cavity. Another important characteristic of certain oral streptococci is their ability to generate hydrogen peroxide that can inhibit the growth of S. mutans. Thus, oral streptococci can also be beneficial to the host by producing molecules that are inhibitory to pathogenic species. Lastly, commensal and pathogenic streptococci residing in the oral cavity can eventually gain access to the bloodstream and cause systemic infections such as infective endocarditis.

ManLMN is a glucose transporter and central metabolic regulator in Streptococcus pneumoniae

Streptococcus pneumoniae is a common colonizer of the human nasopharynx and a leading cause of bacterial pneumonia and otitis media, among other invasive diseases. During both colonization and invasive disease S. pneumoniae ferments host-derived carbohydrates as its primary means of generating energy. This pathogen is adept at transporting and metabolizing a wide variety of carbohydrates. We found the highly conserved PTS ManLMN contributes to growth on glucose and is also essential for growth on a variety of nonpreferred carbohydrates, suggesting it is a multisubstrate transporter. Exploration of this phenotype revealed ManLMN is required for inducing expression of downstream metabolic genes in response to carbohydrate stimuli. We further demonstrate that ManLMN's role as a constitutively expressed transporter is likely unique and integral to pneumococcus's strategy of carbon catabolite repression (CCR). Using a selection for suppressors, we explored how ManLMN is integrated into the CCR regulatory framework in S. pneumoniae. We identified two hypothetical small proteins and the virulence regulator SmrC as potential mediators of CCR in connection with ManLMN. Characterization of these two hypothetical proteins revealed they influence transcriptional regulation of carbohydrate transporters. We propose a model unifying these observations in which ManLMN is a versatile surveyor of available carbohydrates in S. pneumoniae.

Transcription of Sialic Acid Catabolism Genes in Corynebacterium glutamicum Is Subject to Catabolite Repression and Control by the Transcriptional Repressor NanR

Corynebacterium glutamicum metabolizes sialic acid (Neu5Ac) to fructose-6-phosphate (fructose-6P) via the consecutive activity of the sialic acid importer SiaEFGI, N-acetylneuraminic acid lyase (NanA), N-acetylmannosamine kinase (NanK), N-acetylmannosamine-6P epimerase (NanE), N-acetylglucosamine-6P deacetylase (NagA), and glucosamine-6P deaminase (NagB). Within the cluster of the three operons nagAB, nanAKE, and siaEFGI for Neu5Ac utilization a fourth operon is present, which comprises cg2936, encoding a GntR-type transcriptional regulator, here named NanR. Microarray studies and reporter gene assays showed that nagAB, nanAKE, siaEFGI, and nanR are repressed in wild-type (WT) C. glutamicum but highly induced in a ΔnanR C. glutamicum mutant. Purified NanR was found to specifically bind to the nucleotide motifs A[AC]G[CT][AC]TGATGTC[AT][TG]ATGT[AC]TA located within the nagA-nanA and nanR-sialA intergenic regions. Binding of NanR to promoter regions was abolished in the presence of the Neu5Ac metabolism intermediates GlcNAc-6P and N-acetylmannosamine-6-phosphate (ManNAc-6P). We observed consecutive utilization of glucose and Neu5Ac as well as fructose and Neu5Ac by WT C. glutamicum, whereas the deletion mutant C. glutamicum ΔnanR simultaneously consumed these sugars. Increased reporter gene activities for nagAB, nanAKE, and nanR were observed in cultivations of WT C. glutamicum with Neu5Ac as the sole substrate compared to cultivations when fructose was present. Taken together, our findings show that Neu5Ac metabolism in C. glutamicum is subject to catabolite repression, which involves control by the repressor NanR.

IMPORTANCE

Neu5Ac utilization is currently regarded as a common trait of both pathogenic and commensal bacteria. Interestingly, the nonpathogenic soil bacterium C. glutamicum efficiently utilizes Neu5Ac as a substrate for growth. Expression of genes for Neu5Ac utilization in C. glutamicum is here shown to depend on the transcriptional regulator NanR, which is the first GntR-type regulator of Neu5Ac metabolism not to use Neu5Ac as effector but relies instead on the inducers GlcNAc-6P and ManNAc-6P. The identification of conserved NanR-binding sites in intergenic regions within the operons for Neu5Ac utilization in pathogenic Corynebacterium species indicates that the mechanism for the control of Neu5Ac catabolism in C. glutamicum by NanR as described in this work is probably conserved within this genus.

Transcriptional and metabolic effects of glucose on Streptococcus pneumoniae sugar metabolism

Streptococcus pneumoniae is a strictly fermentative human pathogen that relies on carbohydrate metabolism to generate energy for growth. The nasopharynx colonized by the bacterium is poor in free sugars, but mucosa lining glycans can provide a source of sugar. In blood and inflamed tissues glucose is the prevailing sugar. As a result during progression from colonization to disease S. pneumoniae has to cope with a pronounced shift in carbohydrate nature and availability. Thus, we set out to assess the pneumococcal response to sugars found in glycans and the influence of glucose (Glc) on this response at the transcriptional, physiological, and metabolic levels. Galactose (Gal), N-acetylglucosamine (GlcNAc), and mannose (Man) affected the expression of 8 to 14% of the genes covering cellular functions including central carbon metabolism and virulence. The pattern of end-products as monitored by in vivo¹³C-NMR is in good agreement with the fermentation profiles during growth, while the pools of phosphorylated metabolites are consistent with the type of fermentation observed (homolactic vs. mixed) and regulation at the metabolic level. Furthermore, the accumulation of α-Gal6P and Man6P indicate metabolic bottlenecks in the metabolism of Gal and Man, respectively. Glc added to cells actively metabolizing other sugar(s) was readily consumed and elicited a metabolic shift toward a homolactic profile. The transcriptional response to Glc was large (over 5% of the genome). In central carbon metabolism (most represented category), Glc exerted mostly negative regulation. The smallest response to Glc was observed on a sugar mix, suggesting that exposure to varied sugars improves the fitness of S. pneumoniae. The expression of virulence factors was negatively controlled by Glc in a sugar-dependent manner. Overall, our results shed new light on the link between carbohydrate metabolism, adaptation to host niches and virulence.