Capsule type of Streptococcus pneumoniae determines growth phenotype

Bacterial capsules: Occurrence, mechanism, and function

In environments characterized by extended multi-stress conditions, pathogens develop a variety of immune escape mechanisms to enhance their ability to infect the host. The capsules, polymers that bacteria secrete near their cell wall, participates in numerous bacterial life processes and plays a crucial role in resisting host immune attacks and adapting to their niche. Here, we discuss the relationship between capsules and bacterial virulence, summarizing the molecular mechanisms of capsular regulation and pathogenesis to provide new insights into the research on the pathogenesis of pathogenic bacteria.

Capsules and their traits shape phage susceptibility and plasmid conjugation efficiency

Bacterial evolution is affected by mobile genetic elements like phages and conjugative plasmids, offering new adaptive traits while incurring fitness costs. Their infection is affected by the bacterial capsule. Yet, its importance has been difficult to quantify because of the high diversity of confounding mechanisms in bacterial genomes such as anti-viral systems and surface receptor modifications. Swapping capsule loci between Klebsiella pneumoniae strains allowed us to quantify their impact on plasmid and phage infection independently of genetic background. Capsule swaps systematically invert phage susceptibility, revealing serotypes as key determinants of phage infection. Capsule types also influence conjugation efficiency in both donor and recipient cells, a mechanism shaped by capsule volume and conjugative pilus structure. Comparative genomics confirmed that more permissive serotypes in the lab correspond to the strains acquiring more conjugative plasmids in nature. The least capsule-sensitive pili (F-like) are the most frequent in the species' plasmids, and are the only ones associated with both antibiotic resistance and virulence factors, driving the convergence between virulence and antibiotics resistance in the population. These results show how traits of cellular envelopes define slow and fast lanes of infection by mobile genetic elements, with implications for population dynamics and horizontal gene transfer.

Impact of Difluoromethylornithine and AMXT 1501 on Gene Expression and Capsule Regulation in Streptococcus pneumoniae

Streptococcus pneumoniae (Spn), a Gram-positive bacterium, poses a significant threat to human health, causing mild respiratory infections to severe invasive conditions. Despite the availability of vaccines, challenges persist due to serotype replacement and antibiotic resistance, emphasizing the need for alternative therapeutic strategies. This study explores the intriguing role of polyamines, ubiquitous, small organic cations, in modulating virulence factors, especially the capsule, a crucial determinant of Spn's pathogenicity. Using chemical inhibitors, difluoromethylornithine (DFMO) and AMXT 1501, this research unveils distinct regulatory effects on the gene expression of the Spn D39 serotype in response to altered polyamine homeostasis. DFMO inhibits polyamine biosynthesis, disrupting pathways associated with glucose import and the interconversion of sugars. In contrast, AMXT 1501, targeting polyamine transport, enhances the expression of polyamine and glucose biosynthesis genes, presenting a novel avenue for regulating the capsule independent of glucose availability. Despite ample glucose availability, AMXT 1501 treatment downregulates the glycolytic pathway, fatty acid synthesis, and ATP synthase, crucial for energy production, while upregulating two-component systems responsible for stress management. This suggests a potential shutdown of energy production and capsule biosynthesis, redirecting resources towards stress management. Following DFMO and AMXT 1501 treatments, countermeasures, such as upregulation of stress response genes and ribosomal protein, were observed but appear to be insufficient to overcome the deleterious effects on capsule production. This study highlights the complexity of polyamine-mediated regulation in S. pneumoniae, particularly capsule biosynthesis. Our findings offer valuable insights into potential therapeutic targets for modulating capsules in a polyamine-dependent manner, a promising avenue for intervention against S. pneumoniae infections.

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.

NAD+ metabolism is a key modulator of bacterial respiratory epithelial infections

Lower respiratory tract infections caused by Streptococcus pneumoniae (Spn) are a leading cause of death globally. Here we investigate the bronchial epithelial cellular response to Spn infection on a transcriptomic, proteomic and metabolic level. We found the NAD+ salvage pathway to be dysregulated upon infection in a cell line model, primary human lung tissue and in vivo in rodents, leading to a reduced production of NAD+. Knockdown of NAD+ salvage enzymes (NAMPT, NMNAT1) increased bacterial replication. NAD+ treatment of Spn inhibited its growth while growth of other respiratory pathogens improved. Boosting NAD+ production increased NAD+ levels in immortalized and primary cells and decreased bacterial replication upon infection. NAD+ treatment of Spn dysregulated the bacterial metabolism and reduced intrabacterial ATP. Enhancing the bacterial ATP metabolism abolished the antibacterial effect of NAD+. Thus, we identified the NAD+ salvage pathway as an antibacterial pathway in Spn infections, predicting an antibacterial mechanism of NAD+.

Establishment of a modified opsonophagocytic killing assay for anti-pneumococcal surface protein A antibody

Streptococcus pneumoniae (pneumococcus) is a pathogenic gram-positive bacterium that causes pneumonia, meningitis, and sepsis. Pneumococcal surface protein A (PspA) induces antibodies that protect against lethal infections by pneumococci. PspA is a choline-binding protein present on the cell surface of almost all pneumococcal strains and is a non-capsular polysaccharide vaccine candidate. For research and development of PspA-based vaccines, an in-vitro test system to measure the activity of functional antibodies capable of killing pneumococci is essential. The opsonophagocytic killing (OPK) assay is used to evaluate the opsonic activity of functional antibodies induced by capsular polysaccharide (CPS)-based vaccines (standard OPK assay). Despite the potential of anti-PspA antibodies to protect against lethal infections in mice, the standard OPK assay fails to evaluate anti-PspA antibodies. Using a pneumococcal surface protein C-deficient strain and extending the incubation time of opsonized bacteria, complement, and HL-60 cells reportedly results in enhanced bactericidal activity (modified OPK assay). We aimed to measure the bactericidal activity of anti-PspA antibodies in intact pneumococcal strains. We optimized the pneumococcal culture method used in the OPK assay to increase the efficiency of anti-PspA antibody-mediated phagocytosis of HL-60 cells. As thick capsules hinder phagocytosis, we attempted to obtain pneumococci with thin capsules through an improved culture method. As pneumococci attached to cells exhibit thin capsules, pneumococci cultured in Todd Hewitt yeast extract (THY) broth were spread on blood agar plates and incubated for 4 h. cpsA mRNA transcript levels in pneumococci cultured on blood agar were lower than those in pneumococci cultured in THY broth. OPK activity against pneumococci expressing PspA of clades 1-5 was reasonably well detected using pneumococci cultured on blood agar in the modified OPK assay. The modified OPK assay for anti-PspA antibody using pneumococci cultured on blood agar represents a useful assay to determine the killing activity of functional anti-PspA antibodies against pneumococci.

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.

Influence of glycan structure on the colonization of Streptococcus pneumoniae on human respiratory epithelial cells

Virtually all living cells are encased in glycans. They perform key cellular functions such as immunomodulation and cell-cell recognition. Yet, how their composition and configuration affect their functions remains enigmatic. Here, we constructed isogenic capsule-switch mutants harboring 84 types of capsular polysaccharides (CPSs) in Streptococcus pneumoniae. This collection enables us to systematically measure the affinity of structurally related CPSs to primary human nasal and bronchial epithelial cells. Contrary to the paradigm, the surface charge does not appreciably affect epithelial cell binding. Factors that affect adhesion to respiratory cells include the number of rhamnose residues and the presence of human-like glycomotifs in CPS. Besides, pneumococcal colonization stimulated the production of interleukin 6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), and monocyte chemoattractantprotein-1 (MCP-1) in nasal epithelial cells, which also appears to be dependent on the serotype. Together, our results reveal glycomotifs of surface polysaccharides that are likely to be important for colonization and survival in the human airway.

Magnetic bead-based separation of pneumococcal serotypes

The separation of pneumococcal serotypes from a complex polymicrobial mixture may be required for different applications. For instance, a minority strain could be present at a low frequency in a clinical sample, making it difficult to identify and isolate by traditional culture-based methods. We therefore developed an assay to separate mixed pneumococcal samples using serotype-specific antiserum and a magnetic bead-based separation method. Using qPCR and colony counting methods, we first show that serotypes (12F, 23F, 3, 14, 19A, and 15A) present at ∼0.1% of a dual serotype mixture can be enriched to between 10% and 90% of the final sample. We demonstrate two applications for this method: extraction of known pneumococcal serotypes from saliva samples and efficient purification of capsule switch variants from experimental transformation experiments. This method may have further laboratory or clinical applications when the selection of specific serotypes is required.

Influence of Streptococcus pneumoniae Within-Strain Population Diversity on Virulence and Pathogenesis

The short generation time of many bacterial pathogens allows the accumulation of de novo mutations during routine culture procedures used for the preparation and propagation of bacterial stocks. Taking the major human pathogen Streptococcus pneumoniae as an example, we sought to determine the influence of standard laboratory handling of microbes on within-strain genetic diversity and explore how these changes influence virulence characteristics and experimental outcomes. A single culture of S. pneumoniae D39 grown overnight resulted in the enrichment of previously rare genotypes present in bacterial freezer stocks and the introduction of new variation to the bacterial population through the acquisition of mutations. A comparison of D39 stocks from different laboratories demonstrated how changes in bacterial population structure taking place during individual culture events can cumulatively lead to fixed, divergent change that profoundly alters virulence characteristics. The passage of D39 through mouse models of infection, a process used to standardize virulence, resulted in the enrichment of high-fitness genotypes that were originally rare (<2% frequency) in D39 culture collection stocks and the loss of previously dominant genotypes. In the most striking example, the selection of a <2%-frequency genotype carrying a mutation in sdhB, a gene thought to be essential for the establishment of lung infection, was associated with enhanced systemic virulence. Three separately passaged D39 cultures originating from the same frozen stocks showed considerable genetic divergence despite comparable virulence. IMPORTANCE Laboratory bacteriology involves the use of high-density cultures that we often assume to be clonal but that in reality are populations consisting of multiple genotypes at various abundances. We have demonstrated that the genetic structure of a single population of a widely used Streptococcus pneumoniae strain can be substantially altered by even short-term laboratory handling and culture and that, over time, this can lead to changes in virulence characteristics. Our findings suggest that caution should be applied when comparing data generated in different laboratories using the same strain but also when comparing data within laboratories over time. Given the dramatic reductions in the cost of next-generation sequencing technology in recent years, we advocate for the frequent sampling and sequencing of bacterial isolate collections.

Meningitis-associated pneumococcal serotype 8, ST 53, strain is hypervirulent in a rat model and has non-haemolytic pneumolysin which can be attenuated by liposomes

Introduction

Streptococcus pneumoniae bacteria cause life-threatening invasive pneumococcal disease (IPD), including meningitis. Pneumococci are classified into serotypes, determined by differences in capsular polysaccharide and both serotype and pneumolysin toxin are associated with disease severity. Strains of serotype 8, ST 53, are increasing in prevalence in IPD in several countries.

Methods

Here we tested the virulence of such an isolate in a rat model of meningitis in comparison with a serotype 15B and a serotype 14 isolate. All three were isolated from meningitis patients in South Africa in 2019, where serotype 8 is currently the most common serotype in IPD.

Results and Discussion

Only the serotype 8 isolate was hypervirulent causing brain injury and a high mortality rate. It induced a greater inflammatory cytokine response than either the serotype 15B or 14 strain in the rat model and from primary mixed-glia cells isolated from mouse brains. It had the thickest capsule of the three strains and produced non-haemolytic pneumolysin. Pneumolysin-sequestering liposomes reduced the neuroinflammatory cytokine response in vitro indicating that liposomes have the potential to be an effective adjuvant therapy even for hypervirulent pneumococcal strains with non-haemolytic pneumolysin.

Bacterial growth comparison of vaccine and non-vaccine type Streptococcus pneumoniae in different enrichment broths

Broth enrichment is used to enhance pneumococcal carriage detection. Identifying serotypical growth difference during enrichment can prevent detection biases. We discovered, using supplemented Todd-Hewitt broth (0.5% yeast extract) at 4-h incubation and supplemented Brain Heart Infusion broth at 5-h incubation, provide no significant growth difference between vaccine and non-vaccine types.

Differential Pneumococcal Growth Features in Severe Invasive Disease Manifestations

The nasopharyngeal commensal Streptococcus pneumoniae can become invasive and cause metastatic infection. This requires the pneumococcus to have the ability to adapt, grow, and reside in diverse host environments. Therefore, we studied whether the likelihood of severe disease manifestations was related to pneumococcal growth kinetics. For 383 S. pneumoniae blood isolates and 25 experimental mutants, we observed highly reproducible growth curves in nutrient-rich medium. The derived growth features were lag time, maximum growth rate, maximum density, and stationary-phase time before lysis. First, the pathogenicity of each growth feature was probed by comparing isolates from patients with and without marked preexisting comorbidity. Then, growth features were related to the propensity of causing severe manifestations of invasive pneumococcal disease (IPD). A high maximum bacterial density was the most pronounced pathogenic growth feature, which was also an independent predictor of 30-day mortality (P = 0.03). Serotypes with an epidemiologically higher propensity for causing meningitis displayed a relatively high maximum density (P < 0.005) and a short stationary phase (P < 0.005). Correspondingly, isolates from patients diagnosed with meningitis showed an especially high maximum density and short stationary phase compared to isolates from the same serotype that had caused uncomplicated bacteremic pneumonia. In contrast, empyema-associated strains were characterized by a relatively long lag phase (P < 0.0005), and slower growth (P < 0.005). The course and dissemination of IPD may partly be attributable to the pneumococcal growth features involved. If confirmed, we should tailor the prevention and treatment strategies for the different infection sites that can complicate IPD.

IMPORTANCEStreptococcus pneumoniae is a leading infectious cause of deaths worldwide. To understand the course and outcome of pneumococcal infection, most research has focused on the host and its response to contain bacterial growth. However, bacterial epidemiology suggest that certain pneumococcal serotypes are particularly prone to causing complicated infections. Therefore, we took the bacterial point of view, simply examining in vitro growth features for hundreds of pneumococcal blood isolates. Their growth curves were very reproducible. Certain poles of pneumococcal growth features were indeed associated with specific clinical manifestations like meningitis or pleural empyema. This indicates that bacterial growth style potentially affects the progression of infection. Further research on bacterial growth and adaptation to different host environments may therefore provide key insight into pathogenesis of complicated invasive disease. Such knowledge could lead to more tailored vaccine targets or therapeutic approaches to reduce the million deaths that are caused by pneumococcal disease every year.

Streptococcus pneumoniae Strains Isolated From a Single Pediatric Patient Display Distinct Phenotypes

Streptococcus pneumoniae is the leading cause of bacterial paediatric meningitis after the neonatal period worldwide, but the bacterial factors and pathophysiology that drive pneumococcal meningitis are not fully understood. In this work, we have identified differences in raffinose utilization by S. pneumoniae isolates of identical serotype and sequence type from the blood and cerebrospinal fluid (CSF) of a single pediatric patient with meningitis. The blood isolate displayed defective raffinose metabolism, reduced transcription of the raffinose utilization pathway genes, and an inability to grow in vitro when raffinose was the sole carbon source. The fitness of these strains was then assessed using a murine intranasal infection model. Compared with the CSF isolate, mice infected with the blood isolate displayed higher bacterial numbers in the nose, but this strain was unable to invade the ears of infected mice. A premature stop codon was identified in the aga gene in the raffinose locus, suggesting that this protein likely displays impaired alpha-galactosidase activity. These closely related strains were assessed by Illumina sequencing, which did not identify any single nucleotide polymorphisms (SNPs) between the two strains. However, these wider genomic analyses identified the presence of an alternative alpha-galactosidase gene that appeared to display altered sequence coverage between the strains, which may account for the observed differences in raffinose metabolic capacity. Together, these studies support previous findings that raffinose utilization capacity contributes to disease progression, and provide insight into a possible alternative means by which perturbation of this pathway may influence the behavior of pneumococci in the host environment, particularly in meningitis.

Host Stress Signals Stimulate Pneumococcal Transition from Colonization to Dissemination into the Lungs

Streptococcus pneumoniae is an asymptomatic colonizer of the nasopharynx, but it is also one of the most important bacterial pathogens of humans, causing a wide range of mild to life-threatening diseases. The basis of the pneumococcal transition from a commensal to a parasitic lifestyle is not fully understood. We hypothesize that exposure to host catecholamine stress hormones is important for this transition. In this study, we demonstrated that pneumococci preexposed to a hormone released during stress, norepinephrine (NE), have an increased capacity to translocate from the nasopharynx into the lungs compared to untreated pneumococci. Examination of NE-treated pneumococci revealed major alterations in metabolic profiles, cell associations, capsule synthesis, and cell size. By systemically mutating all 12 two-component and 1 orphan regulatory systems, we also identified a unique genetic regulatory circuit involved in pneumococcal recognition and responsiveness to human stress hormones. IMPORTANCE Microbes acquire unique lifestyles under different environmental conditions. Although this is a widespread occurrence, our knowledge of the importance of various host signals and their impact on microbial behavior is not clear despite the therapeutic value of this knowledge. We discovered that catecholamine stress hormones are the host signals that trigger the passage of Streptococcus pneumoniae from a commensal to a parasitic state. We identify that stress hormone treatment of this microbe leads to reductions in cell size and capsule synthesis and renders it more able to migrate from the nasopharynx into the lungs in a mouse model of infection. The microbe requires the TCS09 protein for the recognition and processing of stress hormone signals. Our work has particular clinical significance as catecholamines are abundant in upper respiratory fluids as well as being administered therapeutically to reduce inflammation in ventilated patients, which may explain why intubation in the critically ill is a recognized risk factor for the development of pneumococcal pneumonia.

Cytokine response in cerebrospinal fluid of meningitis patients and outcome associated with pneumococcal serotype

Streptococcus pneumoniae causes life-threatening meningitis. Its capsular polysaccharide determines the serotype and influences disease severity but the mechanism is largely unknown. Due to evidence of elevated cytokines levels in the meningeal inflammatory response, we measured 41 cytokines/chemokines and growth factors in cerebrospinal fluid (CSF) samples from 57 South African meningitis patients (collected in the period 2018–2019), with confirmed S. pneumoniae serotypes, using a multiplexed bead-based immunoassay. Based on multivariable Bayesian regression, using serotype 10A as a reference and after adjusting for HIV and age, we found IL-6 concentrations significantly lower in patients infected with serotypes 6D (undetectable) and 23A (1601 pg/ml), IL-8 concentrations significantly higher in those infected with 22A (40,459 pg/ml), 7F (32,400 pg/ml) and 15B/C (6845 pg/ml), and TNFα concentration significantly higher in those infected with serotype 18A (33,097 pg/ml). Although a relatively small number of clinical samples were available for this study and 28% of samples could not be assigned to a definitive serotype, our data suggests 15B/C worthy of monitoring during surveillance as it is associated with in-hospital case fatality and not included in the 13-valent polysaccharide conjugate vaccine, PCV13. Our data provides average CSF concentrations of a range of cytokines and growth factors for 18 different serotypes (14, 19F, 3, 6A, 7F, 19A, 8, 9N, 10A, 12F, 15B/C, 22F, 16F, 23A, 31, 18A, 6D, 22A) to serve as a basis for future studies investigating host–pathogen interaction during pneumococcal meningitis. We note that differences in induction of IL-8 between serotypes may be particularly worthy of future study.

Streptolysin O concentration and activity is central to in vivo phenotype and disease outcome in Group A Streptococcus infection

Group A Streptoccocus (GAS) is among the most diverse of all human pathogens, responsible for a range of clinical manifestations, from mild superficial infections such as pharyngitis to serious invasive infections such as necrotising fasciitis and sepsis. The drivers of these different disease phenotypes are not known. The GAS cholesterol-dependent cytolysin, Streptolysin O (SLO), has well established cell and tissue destructive activity. We investigated the role of SLO in determining disease outcome in vivo, by using two different clinical lineages; the recently emerged hypervirulent outbreak emm type 32.2 strains, which result in sepsis, and the emm type 1.0 strains which cause septic arthritis. Using clinically relevant in vivo mouse models of sepsis and a novel septic arthritis model, we found that the amount and activity of SLO was vital in determining the course of infection. The emm type 32.2 strain produced large quantities of highly haemolytic SLO that resulted in rapid development of sepsis. By contrast, the reduced concentration and lower haemolytic activity of emm type 1.0 SLO led to translocation of bacteria from blood to joints. Importantly, sepsis associated strains that were attenuated by deletion or inhibition of SLO, then also translocated to the joint, confirming the key role of SLO in determining infection niche. Our findings demonstrate that SLO is key to in vivo phenotype and disease outcome. Careful consideration should be given to novel therapy or vaccination strategies that target SLO. Whilst neutralising SLO activity may reduce severe invasive disease, it has the potential to promote chronic inflammatory conditions such as septic arthritis.

Capsule Independent Antimicrobial Activity Induced by Nanochitosan against Streptococcus pneumoniae

Background: Streptococcus pneumoniae remains a major cause of community-acquired pneumonia, meningitis, and other diseases, contributing significantly to high morbidity and mortality worldwide. Although it responds to antibiotics, their use is becoming limited due to the rise in antibiotic resistance, which necessitates the development of new therapeutics. Nanotechnology is used to counteract antimicrobial resistance. In this regard, polymeric nanoparticles (NPs) made of natural, biodegradable, biocompatible, and cationic polymers such as Chitosan (CNPs) exhibit wide-spectrum antimicrobial activity. Therefore, this study aimed to prepare CNPs, characterize their physiochemical characteristics: particle size (PZ), polydispersity index (PDI), and zeta potential (ZP), and investigate their antimicrobial activity against Streptococcus pneumoniae TIGR4 (virulent serotype 4) and its capsular mutant (∆cps). Methods: CNPs were prepared at 1, 2.5, and 5 mg/mL concentrations using the ion gelation method. Then, PZ, PDI, and ZP were characterized using a Zetasizer. Transmission electron microscopy (TEM) was used to visualize the CNP’s morphology. Broth and agar dilution methods were used to assess their antimicrobial activity. Cytotoxicity of prepared NPs on A549 cells and their effect on pneumococcal hemolysis were also investigated. Results: Spherical CNPs were produced with PZ ranging from 133.3 nm ± 0.57 to 423 nm ± 12.93 PDI < 0.35, and ZP from 19 ± 0.115 to 27 ± 0.819. The prepared CNPs exhibited antibacterial activity against TIGR4 and its capsule mutant with a minimum inhibitory concentration (MIC90) of 0.5 to 2.5 mg/mL in a non-acidic environment. The hemolysis assay results revealed that CNPs reduced bacterial hemolysis in a concentration-dependent manner. Their mammalian cytotoxicity results indicated that CNPs formed from low concentrations of Chitosan (Cs) were cytocompatible. Conclusion: Nanochitosan particles showed anti-pneumococcal activity regardless of the presence of capsules. They resulted in a concentration-dependent reduction in bacterial hemolysis and were cytocompatible at a lower concentration of Cs. These findings highlight the potential of CNPs in the treatment of pneumococcal diseases.

Inactivation of Transcriptional Regulator FabT Influences Colony Phase Variation of Streptococcus pneumoniae

Streptococcus pneumoniae is an opportunistic pathogen that can alter its cell surface phenotype in response to the host environment. We demonstrated that the transcriptional regulator FabT is an indirect regulator of capsular polysaccharide, an important virulence factor of Streptococcus pneumoniae. Transcriptome analysis between the wild-type D39s and D39ΔfabT mutant strains unexpectedly identified a differentially expressed gene encoding a site-specific recombinase, PsrA. PsrA catalyzes the inversion of 3 homologous hsdS genes in a type I restriction-modification (RM) system SpnD39III locus and is responsible for the reversible switch of phase variation. Our study demonstrated that upregulation of PsrA in a D39ΔfabT mutant correlated with an increased ratio of transparent (T) phase variants. Inactivation of the invertase PsrA led to uniform opaque (O) variants. Direct quantification of allelic variants of hsdS derivatives and inversions of inverted repeats indicated that the recombinase PsrA fully catalyzes the inversion mediated by IR1 and IR3, and FabT mediated the recombination of the hsdS alleles in PsrA-dependent and PsrA-independent manners. In addition, compared to D39s, the ΔfabT mutant exhibited reduced nasopharyngeal colonization and was more resistant to phagocytosis and less adhesive to epithelial cells. These results indicated that phase variation in the ΔfabT mutant also affects other cell surface components involved in host interactions.

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).

Media Matters, Examining Historical and Modern Streptococcus pneumoniae Growth Media and the Experiments They Affect

While some bacteria can thrive for generations in minerals and salts, many require lavish nutrition and specific chemicals to survive to the point where they can be observed and researched. Although researchers once boiled and rendered animal flesh and bones to obtain a media that facilitated bacterial growth, we now have a plethora of formulations and manufacturers to provide dehydrated flavors of historical, modified, and modern media. The purpose of media has evolved from simple isolation to more measured study. However, in some instances, media formulated to aid the metabolic, nutritional, or physical properties of microbes may not be best suited for studying pathogen behavior or resilience as a function of host interactions. While there have been comparative studies on handfuls of these media in Streptococcus pneumoniae, this review focuses on describing both the historical and modern composition of common complex (Todd Hewitt and M17), semi-defined (Adams and Roe), and defined pneumococcal media (RPMI and Van de Rijn and Kessler), key components discovered/needed for cultivation/growth enhancement, and effects these different media have on bacterial phenotypes and experimental outcomes. While many researchers find the best conditions to grow and experiment on their bacteria of choice, the reasons for some researchers to use a specific medium is at best, not discussed, and at worst, arbitrary. As such, the goal of this review is to highlight the differences in pneumococcal media to encourage investigators to challenge their decisions on why they use a given medium, discuss the recipe, and explain their reasoning.

Capsular polysaccharide switching in Streptococcus suis modulates host cell interactions and virulence

The capsular polysaccharide (CPS) of Streptococcus suis defines various serotypes based on its composition and structure. Though serotype switching has been suggested to occur between S. suis strains, its impact on pathogenicity and virulence remains unknown. Herein, we experimentally generated S. suis serotype-switched mutants from a serotype 2 strain that express the serotype 3, 4, 7, 8, 9, or 14 CPS. The effects of serotype switching were then investigated with regards to classical properties conferred by presence of the serotype 2 CPS, including adhesion to/invasion of epithelial cells, resistance to phagocytosis by macrophages, killing by whole blood, dendritic cell-derived pro-inflammatory mediator production and virulence using mouse and porcine infection models. Results demonstrated that these properties on host cell interactions were differentially modulated depending on the switched serotypes, although some different mutations other than loci of CPS-related genes were found in each the serotype-switched mutant. Among the serotype-switched mutants, the mutant expressing the serotype 8 CPS was hyper-virulent, whereas mutants expressing the serotype 3 or 4 CPSs had reduced virulence. By contrast, switching to serotype 7, 9, or 14 CPSs had little to no effect. These findings suggest that serotype switching can drastically alter S. suis virulence and host cell interactions.

The Two-Component System 09 Regulates Pneumococcal Carbohydrate Metabolism and Capsule Expression

Streptococcus pneumoniae two-component regulatory systems (TCSs) are important systems that perceive and respond to various host environmental stimuli. In this study, we have explored the role of TCS09 on gene expression and phenotypic alterations in S. pneumoniae D39. Our comparative transcriptomic analyses identified 67 differently expressed genes in total. Among those, agaR and the aga operon involved in galactose metabolism showed the highest changes. Intriguingly, the encapsulated and nonencapsulated hk09-mutants showed significant growth defects under nutrient-defined conditions, in particular with galactose as a carbon source. Phenotypic analyses revealed alterations in the morphology of the nonencapsulated hk09- and tcs09-mutants, whereas the encapsulated hk09- and tcs09-mutants produced higher amounts of capsule. Interestingly, the encapsulated D39∆hk09 showed only the opaque colony morphology, while the D39∆rr09- and D39∆tcs09-mutants had a higher proportion of transparent variants. The phenotypic variations of D39ΔcpsΔhk09 and D39ΔcpsΔtcs09 are in accordance with their higher numbers of outer membrane vesicles, higher sensitivity against Triton X-100 induced autolysis, and lower resistance against oxidative stress. In conclusion, these results indicate the importance of TCS09 for pneumococcal metabolic fitness and resistance against oxidative stress by regulating the carbohydrate metabolism and thereby, most likely indirectly, the cell wall integrity and amount of capsular polysaccharide.

Zinc limitation in Klebsiella pneumoniae profiled by quantitative proteomics influences transcriptional regulation and cation transporter-associated capsule production

Background

Microbial organisms encounter a variety of environmental conditions, including changes to metal ion availability. Metal ions play an important role in many biological processes for growth and survival. As such, microbes alter their cellular protein levels and secretion patterns in adaptation to a changing environment. This study focuses on Klebsiella pneumoniae, an opportunistic bacterium responsible for nosocomial infections. By using K. pneumoniae, we aim to determine how a nutrient-limited environment (e.g., zinc depletion) modulates the cellular proteome and secretome of the bacterium. By testing virulence in vitro, we provide novel insight into bacterial responses to limited environments in the presence of the host.

Results

Analysis of intra- and extracellular changes identified 2380 proteins from the total cellular proteome (cell pellet) and 246 secreted proteins (supernatant). Specifically, HutC, a repressor of the histidine utilization operon, showed significantly increased abundance under zinc-replete conditions, which coincided with an expected reduction in expression of genes within the hut operon from our validating qRT-PCR analysis. Additionally, we characterized a putative cation transport regulator, ChaB that showed significantly higher abundance under zinc-replete vs. -limited conditions, suggesting a role in metal ion homeostasis. Phenotypic analysis of a chaB deletion strain demonstrated a reduction in capsule production, zinc-dependent growth and ion utilization, and reduced virulence when compared to the wild-type strain.

Conclusions

This is first study to comprehensively profile the impact of zinc availability on the proteome and secretome of K. pneumoniae and uncover a novel connection between zinc transport and capsule production in the bacterial system.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02091-8.

Lower Density and Shorter Duration of Nasopharyngeal Carriage by Pneumococcal Serotype 1 (ST217) May Explain Its Increased Invasiveness over Other Serotypes

Streptococcus pneumoniae is a frequent colonizer of the human nasopharynx and a major cause of life-threating invasive infections such as pneumonia, meningitis and sepsis. Over 1 million people die every year due to invasive pneumococcal disease (IPD), mainly in developing countries. Serotype 1 is a common cause of IPD; however, unlike other serotypes, it is rarely found in the carrier state in the nasopharynx, which is often considered a prerequisite for disease. The aim of this study was to understand this dichotomy. We used murine models of carriage and IPD to characterize the pathogenesis of African serotype 1 (sequence type 217) pneumococcal strains obtained from the Queen Elizabeth Central Hospital in Blantyre, Malawi. We found that ST217 pneumococcal strains were highly virulent in a mouse model of invasive pneumonia, but in contrast to the generally accepted assumption, can also successfully establish nasopharyngeal carriage. Interestingly, we found that cocolonizing serotypes may proliferate in the presence of serotype 1, suggesting that acquisition of serotype 1 carriage could increase the risk of developing IPD by other serotypes. RNA sequencing analysis confirmed that key virulence genes associated with inflammation and tissue invasiveness were upregulated in serotype 1. These data reveal important new insights into serotype 1 pathogenesis, with implications for carriage potential and risk of invasive disease through interactions with other cocolonizing serotypes, an often-overlooked factor in transmission and disease progression.IMPORTANCE The pneumococcus causes serious diseases such as pneumonia, sepsis, and meningitis and is a major cause of morbidity and mortality worldwide. Serotype 1 accounts for the majority of invasive pneumococcal disease cases in sub-Saharan Africa but is rarely found during nasopharyngeal carriage. Understanding the mechanisms leading to nasopharyngeal carriage and invasive disease by this serotype can help reduce its burden on health care systems worldwide. In this study, we also uncovered the potential impact of serotype 1 on disease progression of other coinfecting serotypes, which can have important implications for vaccine efficacy. Understanding the interactions between different serotypes during nasopharyngeal carriage may lead to improved intervention methods and therapies to reduce pneumococcal invasive disease levels.

Chronic cigarette smoke exposure and pneumococcal infection induce oropharyngeal microbiota dysbiosis and contribute to long-lasting lung damage in mice

Environmental factors, such as cigarette smoking or lung infections, may influence chronic obstructive pulmonary disease (COPD) progression by modifying the respiratory tract microbiome. However, whether the disease itself induces or maintains dysbiosis remains undefined. In this longitudinal study, we investigated the oropharyngeal microbiota composition and disease progression of mice (in cages of 5-10 mice per cage) before, during and up to 3 months after chronic cigarette smoke exposure or exposure to room air for 6 months. Cigarette smoke exposure induced pulmonary emphysema measurable at the end of exposure for 6 months, as well as 3 months following smoke exposure cessation. Using both classical culture methods and 16S rRNA sequencing, we observed that cigarette smoke exposure altered the relative composition of the oropharyngeal microbiota and reduced its diversity (P <0.001). More than 60 taxa were substantially reduced after 6 months of smoke exposure (P <0.001) However, oropharyngeal microbiota disordering was reversed 3 months after smoke exposure cessation and no significant difference was observed compared to age-matched control mice. The effects of lung infection with Streptococcus pneumoniae on established smoke-induced emphysema and on the oropharyngeal microbiota were also evaluated. Inoculation with S. pneumoniae induced lung damage and altered the microbiota composition for a longer time compared to control groups infected but not previously exposed to smoke (P=0.01). Our data demonstrate effects of cigarette smoke and pneumococcus infection leading to altered microbiota and emphysema development. The reversal of the disordering of the microbiota composition, but not lung damage, following smoke exposure cessation and after clearance of infection suggest that changes in lung structure are not sufficient to sustain a disordered microbiota in mice. Whether changes in the airway microbiota contribute to inducing emphysema requires further investigation.

Streptococci and the complement system: interplay during infection, inflammation and autoimmunity

Streptococci are a broad group of Gram-positive bacteria. This genus includes various human pathogens causing significant morbidity and mortality. Two of the most important human pathogens are Streptococcus pneumoniae (pneumococcus) and Streptococcus pyogenes (group A streptococcus or GAS). Streptococcal pathogens have evolved to express virulence factors that enable them to evade complement-mediated attack. These include factor H-binding M (S. pyogenes) and pneumococcal surface protein C (PspC) (S. pneumoniae) proteins. In addition, S. pyogenes and S. pneumoniae express cytolysins (streptolysin and pneumolysin), which are able to destroy host cells. Sometimes, the interplay between streptococci, the complement, and antistreptococcal immunity may lead to an excessive inflammatory response or autoimmune disease. Understanding the fundamental role of the complement system in microbial clearance and the bacterial escape mechanisms is of paramount importance for understanding microbial virulence, in general, and, the conversion of commensals to pathogens, more specifically. Such insights may help to identify novel antibiotic and vaccine targets in bacterial pathogens to counter their growing resistance to commonly used antibiotics.

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.

Deletion of the Zinc Transporter Lipoprotein AdcAII Causes Hyperencapsulation of Streptococcus pneumoniae Associated with Distinct Alleles of the Type I Restriction-Modification System

The capsule is the dominant Streptococcus pneumoniae virulence factor, yet how variation in capsule thickness is regulated is poorly understood. Here, we describe an unexpected relationship between mutation of adcAII, which encodes a zinc uptake lipoprotein, and capsule thickness. Partial deletion of adcAII in three of five capsular serotypes frequently resulted in a mucoid phenotype that biochemical analysis and electron microscopy of the D39 adcAII mutants confirmed was caused by markedly increased capsule thickness. Compared to D39, the hyperencapsulated ΔadcAII mutant strain was more resistant to complement-mediated neutrophil killing and was hypervirulent in mouse models of invasive infection. Transcriptome analysis of D39 and the ΔadcAII mutant identified major differences in transcription of the Sp_0505-0508 locus, which encodes an SpnD39III (ST5556II) type I restriction-modification system and allelic variation of which correlates with capsule thickness. A PCR assay demonstrated close linkage of the SpnD39IIIC and F alleles with the hyperencapsulated ΔadcAII strains. However, transformation of ΔadcAII with fixed SpnD39III alleles associated with normal capsule thickness did not revert the hyperencapsulated phenotype. Half of hyperencapsulated ΔadcAII strains contained the same single nucleotide polymorphism in the capsule locus gene cps2E, which is required for the initiation of capsule synthesis. These results provide further evidence for the importance of the SpnD39III (ST5556II) type I restriction-modification system for modulating capsule thickness and identified an unexpected linkage between capsule thickness and mutation of ΔadcAII Further investigation will be needed to characterize how mutation of adcAII affects SpnD39III (ST5556II) allele dominance and results in the hyperencapsulated phenotype.IMPORTANCE The Streptococcus pneumoniae capsule affects multiple interactions with the host including contributing to colonization and immune evasion. During infection, the capsule thickness varies, but the mechanisms regulating this are poorly understood. We have identified an unsuspected relationship between mutation of adcAII, a gene that encodes a zinc uptake lipoprotein, and capsule thickness. Mutation of adcAII resulted in a striking hyperencapsulated phenotype, increased resistance to complement-mediated neutrophil killing, and increased S. pneumoniae virulence in mouse models of infection. Transcriptome and PCR analysis linked the hyperencapsulated phenotype of the ΔadcAII strain to specific alleles of the SpnD39III (ST5556II) type I restriction-modification system, a system which has previously been shown to affect capsule thickness. Our data provide further evidence for the importance of the SpnD39III (ST5556II) type I restriction-modification system for modulating capsule thickness and identify an unexpected link between capsule thickness and ΔadcAII, further investigation of which could further characterize mechanisms of capsule regulation.

Pneumococcal serotype determines growth and capsule size in human cerebrospinal fluid

Background

The polysaccharide capsule is a major virulence factor of S. pneumoniae in diseases such as meningitis. While some capsular serotypes are more often found in invasive disease, high case fatality rates are associated with those serotypes more commonly found in asymptomatic colonization. We tested whether growth patterns and capsule size in human cerebrospinal fluid depends on serotype using a clinical isolate of S. pneumoniae and its capsule switch mutants.

Results

We found that the growth pattern differed markedly from that in culture medium by lacking the exponential and lysis phases. Growth in human cerebrospinal fluid was reduced when strains lost their capsules. When a capsule was present, growth was serotype-specific: high carriage serotypes (6B, 9 V, 19F and 23F) grew better than low carriage serotypes (7F, 14, 15B/C and 18C). Growth correlated with the case-fatality rates of serotypes reported in the literature. Capsule size in human cerebrospinal fluid also depended on serotype.

Conclusions

We propose that serotype-specific differences in disease severity observed in meningitis patients may, at least in part, be explained by differences in growth and capsule size in human cerebrospinal fluid. This information could be useful to guide future vaccine design.

Variation of growth characteristics of pneumococcus with environmental conditions

BACKGROUND

Pneumococcus is exposed to a variety of temperature and oxygen levels in the upper respiratory tract and as it invades the lung, tissues, and blood. We sought to determine the effect of environmental variability on growth in vitro and to assess variability between strains. We evaluated the effect of temperature and oxygen on the growth of 256 isolates representing 53 serotypes, recovered from healthy carriers and disease patients. Strains were grown at a range of temperatures, anaerobically or in ambient air with catalase, and were monitored by reading the optical density. Regression models evaluated variation in the characteristics of the growth curves.

RESULTS

Most isolates grew to the maximal density at low temperatures (~33C) and under aerobic conditions. There was considerable variability between strains, and some of this variability was linked to serotype. However, capsule-switch experiments suggest that the production of different capsules might not be sufficient to explain this variation, suggesting there could be interactions between the capsule and genetic background.

CONCLUSIONS

Pneumococcal strains vary in how they respond to environmental variations, some of this variation can be explained by the capsule type being produced, but capsule production itself is not sufficient to explain the variability. This variability could help to explain why different lineages of pneumococcus are more common in carriage or disease.

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.

Peptide Occurring in Enterobacteriaceae Triggers Streptococcus pneumoniae Cell Death

Non-encapsulated Streptococcus pneumoniae often possess two genes, aliB-like ORF 1 and aliB-like ORF 2, in place of capsule genes. AliB-like ORF 1 is thought to encode a substrate binding protein of an ABC transporter which binds peptide SETTFGRDFN, found in 50S ribosomal subunit protein L4 of Enterobacteriaceae. Here, we investigated the effect of binding of AliB-like ORF 1 peptide on the transcriptome and proteome of non-encapsulated pneumococci. We found upregulation of gene expression of a metacaspase and a gene encoding N-acetylmuramoyl-L-alanine amidase, both of which are proposed to be involved in programmed cell death in prokaryotic cells. Proteome profiling indicated upregulation of transcriptional regulators and downregulation of metabolism-associated genes. Exposure to the peptide specifically triggered death in pneumococci which express AliB-like ORF 1, with the bacteria having an apoptotic appearance by electron microscopy. We propose that binding of the AliB-like ORF 1 peptide ligand by the pneumococcus signals a challenging environment with hostile bacterial species leading to death of a proportion of the pneumococcal population.

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.

Pneumolysin and the bacterial capsule of Streptococcus pneumoniae cooperatively inhibit taxis and motility of microglia

Background

Streptococcus pneumoniae is the cause of a highly lethal form of meningitis in humans. Microglial cells in the brain represent the first line of defense against pathogens, and they participate in the inflammatory response. The cholesterol-dependent cytolysin pneumolysin and the bacterial capsule are key pathogenic factors, known to exacerbate the course of pneumococcal meningitis.

Methods

We utilized live imaging and immunostaining of glial cells in dissociated and acute brain slice cultures to study the effect of pneumococcal factors, including the cholesterol-dependent cytolysin pneumolysin and the pneumococcal capsule, on microglial motility and taxis.

Results

In brain tissue, primary microglia cells showed an enhanced response towards lysates from bacteria lacking capsules and pneumolysin as they moved rapidly to areas with an abundance of bacterial factors. The presence of bacterial capsules and pneumolysin cumulatively inhibited microglial taxis. In mixed cultures of astrocytes and microglia, the motility of microglia was inhibited by capsular components within minutes after exposure. The reduced motility was partially reversed by mannan, a mannose receptor inhibitor. The effects on microglia were not mediated by astrocytes because pure microglial cells responded to various pneumococcal lysates similarly with distinct cell shape changes as seen in mixed cultures.

Conclusions

Our data indicate that microglia possess the capacity for a very agile response towards bacterial pathogens, but key pathogenic factors, such as pneumococcal capsules and pneumolysin, inhibited this response shortly after a bacterial challenge. Furthermore, we demonstrate for the first time that the bacterial capsule affects cellular behaviors such as motility and taxis.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1491-7) contains supplementary material, which is available to authorized users.

Characterization of a Signaling System in Streptococcus mitis That Mediates Interspecies Communication with Streptococcus pneumoniae

Streptococcus mitis is found in the oral cavity and nasopharynx and forms a significant portion of the human microbiome. In this study, in silico analyses indicated the presence of an Rgg regulator and short hydrophobic peptide (Rgg/SHP) cell-to-cell communication system in S. mitis Although Rgg presented greater similarity to a repressor in Streptococcus pyogenes, autoinducing assays and genetic mutation analysis revealed that in S. mitis Rgg acts as an activator. Transcriptome analysis showed that in addition to shp, the system regulates two other downstream genes, comprising a segment of a putative lantibiotic gene cluster that is in a conjugative element locus in different members of the mitis group. Close comparison to a similar lantibiotic gene cluster in Streptococcus pneumoniae indicated that S. mitis lacked the full set of genes. Despite the potential of SHP to trigger a futile cycle of autoinduction, growth was not significantly affected for the rgg mutant under normal or antibiotic stress conditions. The S. mitis SHP was, however, fully functional in promoting cross-species communication and increasing S. pneumoniae surface polysaccharide production, which in this species is regulated by Rgg/SHP. The activity of SHPs produced by both species was detected in cocultures using a S. mitis reporter strain. In competitive assays, a slight advantage was observed for the rgg mutants. We conclude that the Rgg/SHP system in S. mitis regulates the expression of its own shp and activates an Rgg/SHP system in S. pneumoniae that regulates surface polysaccharide synthesis. Fundamentally, cross-communication of such systems may have a role during multispecies interactions.IMPORTANCE Bacteria secrete signal molecules into the environment which are sensed by other cells when the density reaches a certain threshold. In this study, we describe a communication system in Streptococcus mitis, a commensal species from the oral cavity, which we also found in several species and strains of streptococci from the mitis group. Further, we show that this system can promote cross-communication with S. pneumoniae, a closely related major human pathogen. Importantly, we show that this cross-communication can take place during coculture. While the genes regulated in S. mitis are likely part of a futile cycle of activation, the target genes in S. pneumoniae are potentially involved in virulence. The understanding of such complex communication networks can provide important insights into the dynamics of bacterial communities.

High-throughput screening for texturing Lactococcus strains

In the food industry, lactic acid bacteria (LAB) are used in dairy fermentations, extending the shelf life by lowering the pH and also affecting taste and texture of the fermented milk. The texture of fermented milk is an important quality parameter, affecting consumer acceptance. Finding LAB providing desired texture of a product is time consuming and laborious when using standard methods for measuring texture, e.g. rheology measurements. Screening of 986 Lactococcus lactis strains resulted in few strains with the ability to enhance texture, demonstrating the necessity of implementation of high-throughput screening methods. A high-throughput screening assay was developed, combining small-scale 96-well microtiter plates and pressure measurements during liquid handling, e.g. aspiration, to find strains that give good texture in fermented milk. Only about 1% of the strains were found to enhance milk texture. Two of the texturing strains belong to L. lactis subsp. lactis, which are the first texturing strains from this subsp. reported. Mining for eps gene clusters responsible for exocellular polysaccharide production was performed, as polysaccharide production can contribute positively to fermented milk texture. Comparative genomics approach revealed four types of texturing L. lactis strains with diverse eps gene clusters.

Peptide Ligands of AmiA, AliA, and AliB Proteins Determine Pneumococcal Phenotype

The Ami-AliA/AliB oligopeptide permease of Streptococcus pneumoniae has been suggested to play a role in environmental sensing and colonisation of the nasopharynx by this human bacterial pathogen by binding peptides derived from bacterial neighbours of other species in the microbiota. Here, we investigated the effects of the peptide ligands of the permease’s substrate binding proteins AmiA, AliA, and AliB on pneumococcal phenotype. AmiA and AliA ligands reduced pneumococcal growth, increased biofilm production and reduced capsule size. In contrast, AliB ligand increased growth and greatly increased bacterial chain length. A decrease in transformation rate was observed in response to all three peptides. Changes in protein expression were also observed, particularly those associated with metabolism and cell wall synthesis. Understanding interspecies bacterial communication and its effect on development of colonising versus invasive phenotypes has the potential to reveal new targets to tackle and prevent pneumococcal infections.

Adenylate kinase potentiates the capsular polysaccharide by modulating Cps2D in Streptococcus pneumoniae D39

Streptococcus pneumoniae is a polysaccharide-encapsulated bacterium. The capsule thickens during blood invasion compared with the thinner capsules observed in asymptomatic nasopharyngeal colonization. However, the underlying mechanism regulating differential CPS expression remains unclear. CPS synthesis requires energy that is supplied by ATP. Previously, we demonstrated a correlation between ATP levels and adenylate kinase in S. pneumoniae (SpAdK). A dose-dependent induction of SpAdK in serum was also reported. To meet medical needs, this study aimed to elucidate the role of SpAdK in the regulation of CPS production. CPS levels in S. pneumoniae type 2 (D39) increased proportionally with SpAdK levels, but they were not related to pneumococcal autolysis. Moreover, increased SpAdK levels resulted in increased total tyrosine kinase Cps2D levels and phosphorylated Cps2D, which is a regulator of CPS synthesis in the D39 strain. Our results also indicated that the SpAdK and Cps2D proteins interact in the presence of Mg-ATP. In addition, in silico analysis uncovered the mechanism behind this protein–protein interaction, suggesting that SpAdK binds with the Cps2D dimer. This established the importance of the ATP-binding domain of Cps2D. Taken together, the biophysical interaction between SpAdK and Cps2D plays an important role in enhancing Cps2D phosphorylation, which results in increased CPS synthesis.

A physical interaction between two key enzymes explains how the bacterium responsible for causing pneumococcal disease thickens its external capsule during infection of the bloodstream. A team led by Dong-Kwon Rhee from Sungkyunkwan University in Suwon, South Korea, studied strains of Streptococcus pneumoniae expressing varying levels of an enzyme that helps maintain the proper balance of cellular energy. They found that this enzyme stimulated the production of sugar chains that coat the outside of the bacterial capsule by binding and activating an intermediary enzyme involved in the synthesis of these sugar chains. Since the capsule is critical in warding off the human immune response, the findings suggest that drugs designed to disrupt the enzyme-mediated induction of capsule formation could help prevent or treat pneumococcal disease.

The volatile molecular profiles of seven Streptococcus pneumoniae serotypes

In this study, the volatile molecule profile of Streptococcus pneumoniae serotypes was evaluated using solid phase microextraction (SPME) and two dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-TOFMS). Here, seven serotypes (6B, 14, 15, 18C, 19F, 9V, and 23F) were analyzed in an isogenic background. We identified 13 core molecules associated with all seven serotypes, and seven molecules that were differentially produced between serotypes. Serotype 14 was found to have the most distinct volatile profile, and could be discriminated from the other six serotypes in aggregate with an area under the curve (AUC) of 89%. This study suggests that molecules from S. pneumoniae culture headspace show potential for rapid serotype identification.

Evaluation of Two Supplemented Culture Media for Long-Term, Room-Temperature Preservation of Streptococcus pneumoniae Strains

Objective

To produce two supplemented agar types in order to store pneumococci for several months at room temperature.

Methods

Todd-Hewitt/Hemoglobin/Yeast/Charcoal/Agar (TH-HYC) and Todd-Hewitt/Skim-Milk/Yeast/Charcoal/Agar (TH-SYC) were used to prepare two supplemented agar types. Nineteen pneumococci isolated from patients or asymptomatic carriers displaying diverse serotypes and multilocus sequence types (MLST) were subcultured and stored onto supplemented agar types, in four different tests, at room temperature.

Findings

At the end of all tests (4–6 months) all noncontaminated subcultures were viable and maintained all phenotypic characteristics. Survival-time curves revealed a slow decrease of viable CFU over time on agar types, but at the end the number of viable CFU was satisfactory (≥2+ of growth). Decreasing of CFU was significantly higher for clinical versus nasopharyngeal isolates. Subcultures contamination rates were 6.25% and 14.58% after 2 and 6 months of storage, respectively.

Conclusion

TH-HYC and TH-SYC agar types allowed the viability of pneumococci with several serotypes, MLST, and genetic profiles, after 6 months of storage at room temperature. We consider that these agar types are a valid alternative to preserve pneumococci over an extended period, especially when methods as cryopreservation or lyophilization are not available, and are useful for transporting strains between laboratories.

Bacterial Adherence and Dwelling Probability: Two Drivers of Early Alveolar Infection by Streptococcus pneumoniae Identified in Multi-Level Mathematical Modeling

Pneumococcal infection is the most frequent cause of pneumonia, and one of the most prevalent diseases worldwide. The population groups at high risk of death from bacterial pneumonia are infants, elderly and immunosuppressed people. These groups are more vulnerable because they have immature or impaired immune systems, the efficacy of their response to vaccines is lower, and antibiotic treatment often does not take place until the inflammatory response triggered is already overwhelming. The immune response to bacterial lung infections involves dynamic interactions between several types of cells whose activation is driven by intracellular molecular networks. A feasible approach to the integration of knowledge and data linking tissue, cellular and intracellular events and the construction of hypotheses in this area is the use of mathematical modeling. For this paper, we used a multi-level computational model to analyse the role of cellular and molecular interactions during the first 10 h after alveolar invasion of Streptococcus pneumoniae bacteria. By “multi-level” we mean that we simulated the interplay between different temporal and spatial scales in a single computational model. In this instance, we included the intracellular scale of processes driving lung epithelial cell activation together with the scale of cell-to-cell interactions at the alveolar tissue. In our analysis, we combined systematic model simulations with logistic regression analysis and decision trees to find genotypic-phenotypic signatures that explain differences in bacteria strain infectivity. According to our simulations, pneumococci benefit from a high dwelling probability and a high proliferation rate during the first stages of infection. In addition to this, the model predicts that during the very early phases of infection the bacterial capsule could be an impediment to the establishment of the alveolar infection because it impairs bacterial colonization.

IL-17 can be protective or deleterious in murine pneumococcal pneumonia

Streptococcus pneumoniae is the major bacterial cause of community-acquired pneumonia, and the leading agent of childhood pneumonia deaths worldwide. Nasal colonization is an essential step prior to infection. The cytokine IL-17 protects against such colonization and vaccines that enhance IL-17 responses to pneumococcal colonization are being developed. The role of IL-17 in host defence against pneumonia is not known. To address this issue, we have utilized a murine model of pneumococcal pneumonia in which the gene for the IL-17 cytokine family receptor, Il17ra, has been inactivated. Using this model, we show that IL-17 produced predominantly from γδ T cells protects mice against death from the invasive TIGR4 strain (serotype 4) which expresses a relatively thin capsule. However, in pneumonia produced by two heavily encapsulated strains with low invasive potential (serotypes 3 and 6B), IL-17 significantly enhanced mortality. Neutrophil uptake and killing of the serotype 3 strain was significantly impaired compared to the serotype 4 strain and depletion of neutrophils with antibody enhanced survival of mice infected with the highly encapsulated SRL1 strain. These data strongly suggest that IL-17 mediated neutrophil recruitment to the lungs clears infection from the invasive TIGR4 strain but that lung neutrophils exacerbate disease caused by the highly encapsulated pneumococcal strains. Thus, whilst augmenting IL-17 immune responses against pneumococci may decrease nasal colonization, this may worsen outcome during pneumonia caused by some strains.

Identification of Antipneumococcal Molecules Effective Against Different Streptococcus pneumoniae Serotypes Using a Resazurin-Based High-Throughput Screen

Streptococcus pneumoniae is a major human pathogen, causing around 1.6 million deaths worldwide each year. By optimizing a resazurin-based assay to detect S. pneumoniae growth in 384-well microplates, we developed a new high-throughput screening (HTS) system for the discovery of antipneumococcal molecules, which was unsuccessful using conventional absorbance measurements. Before applying our protocol to a large-scale screen, we validated the system through a pilot screen targeting about 7,800 bioactive molecules using three different S. pneumoniae serotypes. Primary screenings of a further 27,000 synthetic small molecules facilitated the identification of 3-acyl-2-phenylamino-1,4-dihydropquinolin-4-one (APDQ) derivatives that inhibited growth of S. pneumoniae with MIC₉₀ values <1 μM (0.03-0.81 μM). Five selected APDQ derivatives were also active against Staphylococcus aureus but neither Klebsiella pneumoniae nor Pseudomonas aeruginosa, suggesting that APDQ may act specifically against Gram-positive bacteria. Our results both validated and demonstrated the utility of the resazurin-based HTS system for the identification of new antipneumococcal molecules. Moreover, the identified new antipneumococcal molecules in this study may have potential to be further developed as new antibiotics.

A visual review of the human pathogen Streptococcus pneumoniae

Being the principal causative agent of bacterial pneumonia, otitis media, meningitis and septicemia, the bacterium Streptococcus pneumoniae is a major global health problem. To highlight the molecular basis of this problem, we have portrayed essential biological processes of the pneumococcal life cycle in eight watercolor paintings. The paintings are done to a consistent nanometer scale based on currently available data from structural biology and proteomics. In this review article, the paintings are used to provide a visual review of protein synthesis, carbohydrate metabolism, cell wall synthesis, cell division, teichoic acid synthesis, virulence, transformation and pilus synthesis based on the available scientific literature within the field of pneumococcal biology. Visualization of the molecular details of these processes reveals several scientific questions about how molecular components of the pneumococcal cell are organized to allow biological function to take place. By the presentation of this visual review, we intend to stimulate scientific discussion, aid in the generation of scientific hypotheses and increase public awareness. A narrated video describing the biological processes in the context of a whole-cell illustration accompany this article.

Foreign peptide triggers boost in pneumococcal metabolism and growth

Background

Nonencapsulated Streptococcus pneumoniae bacteria are successful colonizers of the human nasopharynx and often possess genes aliB-like ORF 1 and 2 in place of capsule genes. AliB-like ORF 2 binds peptide FPPQSV, found in Prevotella species, resulting in enhanced colonization. How this response is mediated is so far unknown.

Results

Here we show that the peptide increases expression of genes involved in release of host carbohydrates, carbohydrate uptake and carbohydrate metabolism. In particular, the peptide increased expression of 1,5-anhydro-D-fructose reductase, a metabolic enzyme of an alternative starch and glycogen degrading pathway found in many organisms, in both transcriptomic and proteomic data. The peptide enhanced pneumococcal growth giving a competitive advantage to a strain with aliB-like ORF 2, over its mutant lacking the gene.

Possession of aliB-like ORF 2 did not affect release of inflammatory cytokine CXCL8 from epithelial cells in culture and the nonencapsulated wild type strain was not able to establish disease or inflammation in an infant rat model of meningitis.

Conclusions

We propose that AliB-like ORF 2 confers an advantage in colonization by enhancing carbohydrate metabolism resulting in a boost in growth. This may explain the widespread presence of aliB-like ORF 2 in the nonencapsulated pneumococcal population in the human nasopharynx.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1167-y) contains supplementary material, which is available to authorized users.

Interplay Between Capsule Expression and Uracil Metabolism in Streptococcus pneumoniae D39

Pyrimidine nucleotides play an important role in the biosynthesis of activated nucleotide sugars (NDP-sugars). NDP-sugars are the precursors of structural polysaccharides in bacteria, including capsule, which is a major virulence factor of the human pathogen S. pneumoniae. In this work, we identified a spontaneous non-reversible mutant of strain D39 that displayed a non-producing capsule phenotype. Whole-genome sequencing analysis of this mutant revealed several non-synonymous single base modifications, including in genes of the de novo synthesis of pyrimidines and in the -10 box of capsule operon promoter (Pcps). By directed mutagenesis we showed that the point mutation in Pcps was solely responsible for the drastic decrease in capsule expression. We also demonstrated that D39 subjected to uracil deprivation shows increased biomass and decreased Pcps activity and capsule amounts. Importantly, Pcps expression is further decreased by mutating the first gene of the de novo synthesis of pyrimidines, carA. In contrast, the absence of uracil from the culture medium showed no effect on the spontaneous mutant strain. Co-cultivation of the wild-type and the mutant strain indicated a competitive advantage of the spontaneous mutant (non-producing capsule) in medium devoid of uracil. We propose a model in that uracil may act as a signal for the production of different capsule amounts in S. pneumoniae.

Capsule Prolongs Survival of Streptococcus pneumoniae during Starvation

Person-to-person transmission of Streptococcus pneumoniae (the pneumococcus) may occur via environmental sources in close contact with carriers. Pneumococcal polysaccharide capsules, the determinant of serotype (or type), are heterogeneous in structure and amount, and these differences affect rates of transmission. In this study, we examined the contribution of capsule and its variations to the maintenance of pneumococcal viability under starvation conditions. S. pneumoniae retained its ability to colonize infant mice even after incubation for 24 h in phosphate-buffered saline at 25°C. The expression of capsule by the cps locus prolonged survival under these and other nutrient-poor conditions. Analysis of capsule-switch constructs showed that strain-to-strain differences in survival were due to capsule type rather than genetic background. The addition of glucose was sufficient to rescue the survival defect of the capsule-deficient derivative, demonstrating that in the absence of capsule, survival depends upon nutrient availability. During starvation, there was a decrease in capsule size and amount of capsular polysaccharide that was dependent on bacterial viability and the presence of the cps locus. These observations suggest that pneumococci catabolize their own capsular polysaccharide using the genes involved in its biosynthesis to maintain viability when other carbon sources are unavailable. Our findings describe a new role of the pneumococcal capsule: the prolongation of viability under nutrient-limiting conditions as would be encountered during periods when the organism is between hosts.