The choline-binding proteins PspA, PspC, and LytA of Streptococcus pneumoniae and their interaction with human endothelial and red blood cells.

Streptococcus pneumoniae is a Gram-positive opportunistic pathogen that can colonize the upper respiratory tract. It is a leading cause of a wide range of infectious diseases, including community-acquired pneumonia and meningitis. Pneumococcal infections cause 1-2 million deaths per year, most of which occur in developing countries. Here, we focused on three choline-binding proteins (CBPs), i.e., PspC, PspA, and LytA. These pneumococcal proteins have different surface-exposed regions but share related choline-binding anchors. These surface-exposed pneumococcal proteins are in direct contact with host cells and have diverse functions. We explored the role of the three CBPs on adhesion and pathogenicity in a human host by performing relevant imaging and functional analyses, such as electron microscopy, confocal laser scanning microscopy, and functional quantitative assays, targeting biofilm formation and the hemolytic capacity of S. pneumoniae. In vitro biofilm formation assays and electron microscopy experiments were used to examine the ability of knockout mutant strains lacking the lytA, pspC, or pspA genes to adhere to surfaces. We found that LytA plays an important role in robust synthesis of the biofilm matrix. PspA and PspC appeared crucial for the hemolytic effects of S. pneumoniae on human red blood cells. Furthermore, all knockout mutants caused less damage to endothelial cells than wild-type bacteria, highlighting the significance of each CPB for the overall pathogenicity of S. pneumoniae. Hence, in addition to their structural function within the cell wall of S. pneumoniae, each of these three surface-exposed CBPs controls or mediates multiple steps during bacterial pathogenesis.

Soluble Enolase 1 of Candida albicans and Aspergillus fumigatus Stimulates Human and Mouse B Cells and Monocytes.

Because of the growing numbers of immunocompromised patients, the incidence of life-threatening fungal infections caused by Candida albicans and Aspergillus fumigatus is increasing. We have recently identified enolase 1 (Eno1) from A. fumigatus as an immune evasion protein. Eno1 is a fungal moonlighting protein that mediates adhesion and invasion of human cells and also immune evasion through complement inactivation. We now show that soluble Eno1 has immunostimulatory activity. We observed that Eno1 from both C. albicans and A. fumigatus directly binds to the surface of lymphocytes, preferentially human and mouse B cells. Functionally, Eno1 upregulated CD86 expression on B cells and induced proliferation. Although the receptor for fungal Eno1 on B lymphocytes is still unknown, the comparison of B cells from wild-type and MyD88-deficient mice showed that B cell activation by Eno1 required MyD88 signaling. With respect to infection biology, we noted that mouse B cells stimulated by Eno1 secreted IgM and IgG2b. These Igs bound C. albicans hyphae in vitro, suggesting that Eno1-induced Ab secretion might contribute to protection from invasive fungal disease in vivo. Eno1 also triggered the release of proinflammatory cytokines from monocytes, particularly IL-6, which is a potent activator of B cells. Together, our data shed new light on the role of secreted Eno1 in infections with C. albicans and A. fumigatus. Eno1 secretion by these pathogenic microbes appears to be a double-edged sword by supporting fungal pathogenicity while triggering (antifungal) immunity.

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

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

The Protease SplB of Staphylococcus aureus Targets Host Complement Components and Inhibits Complement-Mediated Bacterial Opsonophagocytosis.

Staphylococcus aureus is an opportunistic pathogen that can cause life-threatening infections, particularly in immunocompromised individuals. The high-level virulence of S. aureus largely relies on its diverse and variable collection of virulence factors and immune evasion proteins, including the six serine protease-like proteins SplA to SplF. Spl proteins are expressed by most clinical isolates of S. aureus, but little is known about the molecular mechanisms by which these proteins modify the host's immune response for the benefit of the bacteria. Here, we identify SplB as a protease that inactivates central human complement proteins, i.e., C3, C4, and the activation fragments C3b and C4b, by preferentially cleaving their α-chains. SplB maintained its proteolytic activity in human serum, degrading C3 and C4. SplB further cleaved the components of the terminal complement pathway, C5, C6, C7, C8, and C9. In contrast, the important soluble human complement regulators factor H and C4b-binding protein (C4BP), as well as C1q, were left intact. Thereby, SplB reduced C3b-mediated opsonophagocytosis by human neutrophils as well as C5b-9 deposition on the bacterial surface. In conclusion, we identified the first physiological substrates of the S. aureus extracellular protease SplB. This enzyme inhibits all three complement pathways and blocks opsonophagocytosis. Thus, SplB can be considered a novel staphylococcal complement evasion protein. IMPORTANCE The success of bacterial pathogens in immunocompetent humans depends on the control and inactivation of host immunity. S. aureus, like many other pathogens, efficiently blocks host complement attack early in infection. Aiming to understand the role of the S. aureus-encoded orphan proteases of the Spl operon, we asked whether these proteins play a role in immune escape. We found that SplB inhibits all three complement activation pathways as well as the lytic terminal complement pathway. This blocks the opsonophagocytosis of the bacteria by neutrophils. We also clarified the molecular mechanisms: SplB cleaves the human complement proteins C3, C4, C5, C6, C7, C8, and C9 as well as factor B but not the complement inhibitors factor H and C4BP. Thus, we identify the first physiological substrates of the extracellular protease SplB of S. aureus and characterize SplB as a novel staphylococcal complement evasion protein.

Molecular analyses identifies new domains and structural differences among Streptococcus pneumoniae immune evasion proteins PspC and Hic.

The PspC and Hic proteins of Streptococcus pneumoniae are some of the most variable microbial immune evasion proteins identified to date. Due to structural similarities and conserved binding profiles, it was assumed for a long time that these pneumococcal surface proteins represent a protein family comprised of eleven subgroups. Recently, however, the evaluation of more proteins revealed a greater diversity of individual proteins. In contrast to previous assumptions a pattern evaluation of six PspC and five Hic variants, each representing one of the previously defined subgroups, revealed distinct structural and likely functionally regions of the proteins, and identified nine new domains and new domain alternates. Several domains are unique to PspC and Hic variants, while other domains are also present in other virulence factors encoded by pneumococci and other bacterial pathogens. This knowledge improved pattern evaluation at the level of full-length proteins, allowed a sequence comparison at the domain level and identified domains with a modular composition. This novel strategy increased understanding of individual proteins variability and modular domain composition, enabled a structural and functional characterization at the domain level and furthermore revealed substantial structural differences between PspC and Hic proteins. Given the exceptional genomic diversity of the multifunctional PspC and Hic proteins a detailed structural and functional evaluation need to be performed at the strain level. Such knowledge will also be useful for molecular strain typing and characterizing PspC and Hic proteins from new clinical S. pneumoniae strains.

Importance of Pyruvate Sensing and Transport for the Resuscitation of Viable but Nonculturable Escherichia coli K-12.

Escherichia coli and many other bacterial species can enter into a viable but nonculturable (VBNC) state, which is a survival strategy adopted by cells exposed to adverse environmental conditions. Pyruvate is known to be one factor that promotes resuscitation of VBNC cells. Here we studied the role of a pyruvate-sensing network, composed of the histidine kinase-response regulator systems BtsS/BtsR and YpdA/YpdB and the target gene btsT, encoding the high-affinity pyruvate/H+ symporter BtsT, in the resuscitation of VBNC E. coli K-12 cells after exposure to cold for 120 days. Analysis of the proteome of VBNC cells revealed upregulation, relative to exponentially growing cells, of BtsT and other proteins involved in pyruvate metabolism. Provision of pyruvate stimulated protein and DNA biosynthesis, and thus resuscitation, in wild-type but not btsSR ypdAB mutant VBNC cells. This result was corroborated by time-dependent tracking of the resuscitation of individual VBNC E. coli cells observed in a microfluidic system. Finally, transport assays revealed that 14C-labeled pyruvate was rapidly taken up into VBNC cells by BtsT. These results provide the first evidence that pyruvate is taken up as a carbon source for the resuscitation of VBNC E. coli cells.IMPORTANCE Viable but nonculturable (VBNC) bacteria do not form colonies in standard medium but otherwise retain their metabolic activity and can express toxic proteins. Many bacterial genera, including Escherichia, Vibrio, and Listeria, have been shown to enter the VBNC state upon exposure to adverse conditions, such as low temperature, radiation, and starvation. Ultimately, these organisms pose a public health risk with potential implications for the pharmaceutical and food industries, as dormant organisms are especially difficult to selectively eliminate and VBNC bacteria can be resuscitated if placed in an environment with appropriate nutrition and temperature. Here we used a microfluidic system to monitor the resuscitation of single VBNC cells over time. We provide new molecular insights into the initiation of resuscitation by demonstrating that VBNC E. coli cells rapidly take up pyruvate with an inducible high-affinity transporter, whose expression is triggered by the BtsSR-YpdAB sensing network.

BtsT, a Novel and Specific Pyruvate/H+ Symporter in Escherichia coli.

The peptide transporter carbon starvation (CstA) family (transporter classification [TC] 2.A.114) belongs to the second largest superfamily of secondary transporters, the amino acid/polyamine/organocation (APC) superfamily. No representative of the CstA family has previously been characterized either biochemically or structurally, but we have now identified the function of one of its members, the transport protein YjiY of Escherichia coli Expression of the yjiY gene is regulated by the LytS-like histidine kinase BtsS, a sensor of extracellular pyruvate, together with the LytTR-like response regulator BtsR. YjiY consists of 716 amino acids, which form 18 putative transmembrane helices. Transport studies with intact cells provided evidence that YjiY is a specific and high-affinity transporter for pyruvate (Km , 16 µM). Furthermore, reconstitution of the purified YjiY into proteoliposomes revealed that YjiY is a pyruvate/H+ symporter. It has long been assumed that E. coli possesses a transporter(s) for pyruvate, but the present study is the first to definitively identify such a protein. Based on its function, we propose to change the name of the uncharacterized gene yjiY to btsT for Brenztraubensäure (the German word for pyruvate) transporter.IMPORTANCE BtsT (formerly known as YjiY) is found in many commensal and pathogenic representatives of the Enterobacteriaceae This study for the first time characterizes a pyruvate transporter in E. coli, BtsT, as a specific pyruvate/H+ symporter. When nutrients are limiting, BtsT takes up pyruvate from the medium, thus enabling it to be used as a carbon source for the growth and survival of E. coli.

A Single-Cell View of the BtsSR/YpdAB Pyruvate Sensing Network in Escherichia coli and Its Biological Relevance.

Fluctuating environments and individual physiological diversity force bacteria to constantly adapt and optimize the uptake of substrates. We focus here on two very similar two-component systems (TCSs) of Escherichia coli belonging to the LytS/LytTR family: BtsS/BtsR (formerly YehU/YehT) and YpdA/YpdB. Both TCSs respond to extracellular pyruvate, albeit with different affinities, typically during postexponential growth, and each system regulates expression of a single transporter gene, yjiY and yhjX, respectively. To obtain insights into the biological significance of these TCSs, we analyzed the activation of the target promoters at the single-cell level. We found unimodal cell-to-cell variability; however, the degree of variance was strongly influenced by the available nutrients and differed between the two TCSs. We hypothesized that activation of either of the TCSs helps individual cells to replenish carbon resources. To test this hypothesis, we compared wild-type cells with the btsSR ypdAB mutant under two metabolically modulated conditions: protein overproduction and persister formation. Although all wild-type cells were able to overproduce green fluorescent protein (GFP), about half of the btsSR ypdAB population was unable to overexpress GFP. Moreover, the percentage of persister cells, which tolerate antibiotic stress, was significantly lower in the wild-type cells than in the btsSR ypdAB population. Hence, we suggest that the BtsS/BtsR and YpdA/YpdB network contributes to a balancing of the physiological state of all cells within a population.IMPORTANCE Histidine kinase/response regulator (HK/RR) systems enable bacteria to respond to environmental and physiological fluctuations. Escherichia coli and other members of the Enterobacteriaceae possess two similar LytS/LytTR-type HK/RRs, BtsS/BtsR (formerly YehU/YehT) and YpdA/YpdB, which form a functional network. Both systems are activated in response to external pyruvate, typically when cells face overflow metabolism during post-exponential growth. Single-cell analysis of the activation of their respective target genes yjiY and yhjX revealed cell-to-cell variability, and the range of variation was strongly influenced by externally available nutrients. Based on the phenotypic characterization of a btsSR ypdAB mutant compared to the parental strain, we suggest that this TCS network supports an optimization of the physiological state of the individuals within the population.