Role of Yops and adhesins in resistance of Yersinia enterocolitica to phagocytosis

The yad and yeh fimbrial loci influence gene expression and virulence in enterohemorrhagic Escherichia coli O157:H7

Fimbriae are essential virulence factors for many bacterial pathogens. Fimbriae are extracellular structures that attach bacteria to surfaces. Thus, fimbriae mediate a critical step required for any pathogen to establish infection by anchoring a bacterium to host tissue. The human pathogen enterohemorrhagic Escherichia coli (EHEC) O157:H7encodes 16 fimbriae that may be important for EHEC to initiate infection and allow for productive expression of virulence traits important in later stages of infection, including a type III secretion system (T3SS) and Shiga toxin; however, the roles of most EHEC fimbriae are largely uncharacterized. Here, we provide evidence that two EHEC fimbriae, Yad and Yeh, modulate expression of diverse genes including genes encoding T3SS and Shiga toxin and that these fimbriae are required for robust colonization of the gastrointestinal tract. These findings reveal a significant and previously unappreciated role for fimbriae in bacterial pathogenesis as important determinants of virulence gene expression.IMPORTANCEFimbriae are extracellular proteinaceous structures whose defining role is to anchor bacteria to surfaces. This is a fundamental step for bacterial pathogens to establish infection in a host. Here, we show that the contributions of fimbriae to pathogenesis are more complex. Specifically, we demonstrate that fimbriae influence expression of virulence traits essential for disease progression in the intestinal pathogen enterohemorrhagic Escherichia coli. Gram-positive and Gram-negative bacteria express multiple fimbriae; therefore, these findings may have broad implications for understanding how pathogens use fimbriae, beyond adhesion, to initiate infection and coordinate gene expression, which ultimately results in disease.

Yersinia pestis and pneumonic plague: Insight into how a lethal pathogen interfaces with innate immune populations in the lung to cause severe disease

Yersinia pestis is the causative agent of bubonic, septicemic and pneumonic plague. The historical importance and potential of plague to re-emerge as a threat worldwide are indisputable. The most severe manifestion of plague is pneumonic plague, which results in disease that is 100% lethal without treatment. Y. pestis suppresses host immune responses early in the lung to establish infection. The later stages of infection see the rapid onset of hyperinflammatory responses that prove lethal. The study of Y. pestis host/pathogen interactions have largely been investigated during bubonic plague and with attenuated strains in cell culture models. There remains a somewhat limited understanding of the interactions between virulent Y. pestis and immune populations in the lung that drive severe disease. In this review we give a broad overview of the progression of pneumonic plague and highlighting how Y. pestis interfaces with host innate immune populations in the lung to cause lethal disease.

A TNF-IL-1 circuit controls Yersinia within intestinal pyogranulomas

Tumor necrosis factor (TNF) is a pleiotropic inflammatory cytokine that mediates antimicrobial defense and granuloma formation in response to infection by numerous pathogens. We previously reported that Yersinia pseudotuberculosis colonizes the intestinal mucosa and induces the recruitment of neutrophils and inflammatory monocytes into organized immune structures termed pyogranulomas (PG) that control Yersinia infection. Inflammatory monocytes are essential for the control and clearance of Yersinia within intestinal PG, but how monocytes mediate Yersinia restriction is poorly understood. Here, we demonstrate that TNF signaling in monocytes is required for bacterial containment following enteric Yersinia infection. We further show that monocyte-intrinsic TNFR1 signaling drives the production of monocyte-derived interleukin-1 (IL-1), which signals through IL-1 receptors on non-hematopoietic cells to enable PG-mediated control of intestinal Yersinia infection. Altogether, our work reveals a monocyte-intrinsic TNF-IL-1 collaborative inflammatory circuit that restricts intestinal Yersinia infection.

The polymer and materials science of the bacterial fimbriae Caf1

Fimbriae are long filamentous polymeric protein structures located upon the surface of bacteria. Often implicated in pathogenicity, the biosynthesis and function of fimbriae has been a productive topic of study for many decades. Evolutionary pressures have ensured that fimbriae possess unique structural and mechanical properties which are advantageous to bacteria. These properties are also difficult to engineer with well-known synthetic and natural fibres, and this has raised an intriguing question: can we exploit the unique properties of bacterial fimbriae in useful ways? Initial work has set out to explore this question by using Capsular antigen fragment 1 (Caf1), a fimbriae expressed naturally by Yersina pestis. These fibres have evolved to 'shield' the bacterium from the immune system of an infected host, and thus are rather bioinert in nature. Caf1 is, however, very amenable to structural mutagenesis which allows the incorporation of useful bioactive functions and the modulation of the fibre's mechanical properties. Its high-yielding recombinant synthesis also ensures plentiful quantities of polymer are available to drive development. These advantageous features make Caf1 an archetype for the development of new polymers and materials based upon bacterial fimbriae. Here, we cover recent advances in this new field, and look to future possibilities of this promising biopolymer.

A TNF-IL-1 circuit controls Yersinia within intestinal granulomas

Tumor necrosis factor (TNF) is a pleiotropic inflammatory cytokine that mediates antimicrobial defense and granuloma formation in response to infection by numerous pathogens. Yersinia pseudotuberculosis colonizes the intestinal mucosa and induces recruitment of neutrophils and inflammatory monocytes into organized immune structures termed pyogranulomas that control the bacterial infection. Inflammatory monocytes are essential for control and clearance of Yersinia within intestinal pyogranulomas, but how monocytes mediate Yersinia restriction is poorly understood. Here, we demonstrate that TNF signaling in monocytes is required for bacterial containment following enteric Yersinia infection. We further show that monocyte-intrinsic TNFR1 signaling drives production of monocyte-derived interleukin-1 (IL-1), which signals through IL-1 receptor on non-hematopoietic cells to enable pyogranuloma-mediated control of Yersinia infection. Altogether, our work reveals a monocyte-intrinsic TNF-IL-1 collaborative circuit as a crucial driver of intestinal granuloma function, and defines the cellular target of TNF signaling that restricts intestinal Yersinia infection.

Bacterial effector kinases and strategies to identify their target host substrates

Post-translational modifications (PTMs) are critical in regulating protein function by altering chemical characteristics of proteins. Phosphorylation is an integral PTM, catalyzed by kinases and reversibly removed by phosphatases, that modulates many cellular processes in response to stimuli in all living organisms. Consequently, bacterial pathogens have evolved to secrete effectors capable of manipulating host phosphorylation pathways as a common infection strategy. Given the importance of protein phosphorylation in infection, recent advances in sequence and structural homology search have significantly expanded the discovery of a multitude of bacterial effectors with kinase activity in pathogenic bacteria. Although challenges exist due to complexity of phosphorylation networks in host cells and transient interactions between kinases and substrates, approaches are continuously being developed and applied to identify bacterial effector kinases and their host substrates. In this review, we illustrate the importance of exploiting phosphorylation in host cells by bacterial pathogens via the action of effector kinases and how these effector kinases contribute to virulence through the manipulation of diverse host signaling pathways. We also highlight recent developments in the identification of bacterial effector kinases and a variety of techniques to characterize kinase-substrate interactions in host cells. Identification of host substrates provides new insights for regulation of host signaling during microbial infection and may serve as foundation for developing interventions to treat infection by blocking the activity of secreted effector kinases.

Unraveling the molecular determinants of the anti-phagocytic protein cloak of plague bacteria

The pathogenic bacterium Yersina pestis is protected from macrophage engulfment by a capsule like antigen, F1, formed of long polymers of the monomer protein, Caf1. However, despite the importance of this pathogen, the mechanism of protection was not understood. Here we demonstrate how F1 protects the bacteria from phagocytosis. First, we show that Escherichia coli expressing F1 showed greatly reduced adherence to macrophages. Furthermore, the few cells that did adhere remained on the macrophage surface and were not engulfed. We then inserted, by mutation, an “RGDS” integrin binding motif into Caf1. This did not change the number of cells adhering to macrophages but increased the fraction of adherent cells that were engulfed. Therefore, F1 protects in two separate ways, reducing cell adhesion, possibly by acting as a polymer brush, and hiding innate receptor binding sites needed for engulfment. F1 is very robust and we show that E. coli expressing weakened mutant polymers are engulfed like the RGDS mutant. This suggests that innate attachment sites on the native cell surface are exposed if F1 is weakened. Single-molecule force spectroscopy (SMFS) experiments revealed that wild-type F1 displays a very high mechanical stability of 400 pN. However, the mechanical resistance of the destabilised mutants, that were fully engulfed, was only 20% weaker. By only marginally exceeding the mechanical force applied to the Caf1 polymer during phagocytosis it may be that the exceptional tensile strength evolved to resist the forces applied at this stage of engulfment.

Macrophages, a type of white blood cell, form an important element of our immune defence. They interrogate other cells’ surfaces for molecular clues and ingest those presenting a threat in a process known as phagocytosis. Not surprisingly, pathogenic bacteria have developed ways to evade this fate. The plague bacterium, Yersinia pestis, produces the long polymeric F1 coat protein which enables it to avoid ingestion, but the mechanism was unclear. We show that equipping Escherichia coli cells with an F1 coat protected them from phagocytosis by two separate mechanisms, reducing contact with the macrophage surface and hiding the signals that tell the macrophages they are targets. F1 is also a very stable protein polymer and using single molecule force spectroscopy we showed it also has a very high resistance to pulling forces. Surprisingly, mutations which reduced this by only 20% caused adherent bacteria to be fully ingested, indicating that cells are subject to significant forces prior to recognition and ingestion. Thus, F1 has evolved three notable properties (i) physical; creation of a hydrated polymer brush to inhibit surface interactions, (ii) chemical; absence of molecular recognition clues needed for engulfment and (iii) mechanical; strength that maintains the camouflage layer during surface stretching.

Yersinia pseudotuberculosis YopE prevents uptake by M cells and instigates M cell extrusion in human ileal enteroid-derived monolayers

Many pathogens use M cells to access the underlying Peyer's patches and spread to systemic sites via the lymph as demonstrated by ligated loop murine intestinal models. However, the study of interactions between M cells and microbial pathogens has stalled due to the lack of cell culture systems. To overcome this obstacle, we use human ileal enteroid-derived monolayers containing five intestinal cell types including M cells to study the interactions between the enteric pathogen, Yersinia pseudotuberculosis (Yptb), and M cells. The Yptb type three secretion system (T3SS) effector Yops inhibit host defenses including phagocytosis and are critical for colonization of the intestine and Peyer's patches. Therefore, it is not understood how Yptb traverses through M cells to breach the epithelium. By growing Yptb under two physiological conditions that mimic the early infectious stage (low T3SS-expression) or host-adapted stage (high T3SS-expression), we found that large numbers of Yptb specifically associated with M cells, recapitulating murine studies. Transcytosis through M cells was significantly higher by Yptb expressing low levels of T3SS, because YopE and YopH prevented Yptb uptake. YopE also caused M cells to extrude from the epithelium without inducing cell-death or disrupting monolayer integrity. Sequential infection with early infectious stage Yptb reduced host-adapted Yptb association with M cells. These data underscore the strength of enteroids as a model by discovering that Yops impede M cell function, indicating that early infectious stage Yptb more effectively penetrates M cells while the host may defend against M cell penetration of host-adapted Yptb.

Galectin-1 Cooperates with Yersinia Outer Protein (Yop) P to Thwart Protective Immunity by Repressing Nitric Oxide Production

Yersinia enterocolitica (Ye) inserts outer proteins (Yops) into cytoplasm to infect host cells. However, in spite of considerable progress, the mechanisms implicated in this process, including the association of Yops with host proteins, remain unclear. Here, we evaluated the functional role of Galectin-1 (Gal1), an endogenous β-galactoside-binding protein, in modulating Yop interactions with host cells. Our results showed that Gal1 binds to Yops in a carbohydrate-dependent manner. Interestingly, Gal1 binding to Yops protects these virulence factors from trypsin digestion. Given that early control of Ye infection involves activation of macrophages, we evaluated the role of Gal1 and YopP in the modulation of macrophage function. Although Gal1 and YopP did not influence production of superoxide anion and/or TNF by Ye-infected macrophages, they coordinately inhibited nitric oxide (NO) production. Notably, recombinant Gal1 (rGal1) did not rescue NO increase observed in Lgals1-/- macrophages infected with the YopP mutant Ye ∆yopP. Whereas NO induced apoptosis in macrophages, no significant differences in cell death were detected between Gal1-deficient macrophages infected with Ye ∆yopP, and WT macrophages infected with Ye wt. Strikingly, increased NO production was found in WT macrophages treated with MAPK inhibitors and infected with Ye wt. Finally, rGal1 administration did not reverse the protective effect in Peyer Patches (PPs) of Lgals1-/- mice infected with Ye ∆yopP. Our study reveals a cooperative role of YopP and endogenous Gal1 during Ye infection.

Plague Prevention and Therapy: Perspectives on Current and Future Strategies

Plague, caused by the bacterial pathogen Yersinia pestis, is a vector-borne disease that has caused millions of human deaths over several centuries. Presently, human plague infections continue throughout the world. Transmission from one host to another relies mainly on infected flea bites, which can cause enlarged lymph nodes called buboes, followed by septicemic dissemination of the pathogen. Additionally, droplet inhalation after close contact with infected mammals can result in primary pneumonic plague. Here, we review research advances in the areas of vaccines and therapeutics for plague in context of Y. pestis virulence factors and disease pathogenesis. Plague continues to be both a public health threat and a biodefense concern and we highlight research that is important for infection mitigation and disease treatment.

Bacterial proteomics and its application for pathogenesis studies

Bacteria build their structures by implementing several macromolecules such as proteins, polysaccharides, phospholipids, and nucleic acids, which leads to preserve their lives and play an essential role in their pathogenesis. There are two genomic and proteomic methods to study various macromolecules of bacteria, which are complementary methods and provide comprehensive information. Proteomic approaches are used to identify proteins and their cell applications. Furthermore, to study bacterial proteins, macromolecules are involved in the bacteria's structures and functions. These protein-based methods provide comprehensive information about the cells, such as the external structures, internal compositions, post-translational modifications, and mechanisms of particular actions such as biofilm formation, antibiotic resistance, and adaptation to the environment, which are helpful in promoting bacterial pathogenesis. These methods use various devices such as MALDI-TOF MS, LC-MS, and two-dimensional electrophoresis, which are valuable tools for studying different structural and functional proteins of the bacteria and their mechanisms of pathogenesis that causes rapid, easy, and accurate diagnosis of the infections.

Host manipulation by bacterial type III and type IV secretion system effector proteases

Proteases are powerful enzymes, which cleave peptide bonds, leading most of the time to irreversible fragmentation or degradation of their substrates. Therefore they control many critical cell fate decisions in eukaryotes. Bacterial pathogens exploit this power and deliver protease effectors through specialised secretion systems into host cells. Research over the past years revealed that the functions of protease effectors during infection are diverse, reflecting the lifestyles and adaptations to specific hosts; however, only a small number of peptidase families seem to have given rise to most of these protease virulence factors by the evolution of different substrate-binding specificities, intracellular activation and subcellular targeting mechanisms. Here, we review our current knowledge about the enzymology and function of protease effectors, which Gram-negative bacterial pathogens translocate via type III and IV secretion systems to irreversibly manipulate host processes. We highlight emerging concepts such as signalling by protease cleavage products and effector-triggered immunity, which host cells employ to detect and defend themselves against a protease attack. TAKE AWAY: Proteases irreversibly cleave proteins to control critical cell fate decisions. Gram-negative bacteria use type III and IV secretion systems to inject effectors. Protease effectors are integral weapons for the manipulation of host processes. Effectors evolved from few peptidase families to target diverse substrates. Effector-triggered immunity upon proteolytic attack emerges as host defence.

Effects of mesoporous SiO2-CaO nanospheres on the murine peritoneal macrophages/Candidaalbicans interface

The use of nanoparticles for intracellular drug delivery could reduce the toxicity and side effects of the drug but, the uptake of these nanocarriers could induce adverse effects on cells and tissues after their incorporation. Macrophages play a central role in host defense and are responsible for in vivo nanoparticle trafficking. Assessment of their defense capacity against pathogenic micro-organisms after nanoparticle uptake, is necessary to prevent infections associated with nanoparticle therapies. In this study, the effects of hollow mesoporous SiO2-CaO nanospheres labeled with fluorescein isothiocyanate (FITC-NanoMBGs) on the function of peritoneal macrophages was assessed by measuring their ability to phagocytize Candidaalbicans expressing a red fluorescent protein. Two macrophage/fungus ratios (MOI1 and MOI5) were used and two experimental strategies were carried out: a) pretreatment of macrophages with FITC-NanoMBGs and subsequent fungal infection; b) competition assays after simultaneous addition of fungus and nanospheres. Macrophage pro-inflammatory phenotype markers (CD80 expression and interleukin 6 secretion) were also evaluated. Significant decreases of CD80+ macrophage percentage and interleukin 6 secretion were observed after 30 min, indicating that the simultaneous incorporation of NanoMBG and fungus favors the macrophage non-inflammatory phenotype. The present study evidences that the uptake of these nanospheres in all the studied conditions does not alter the macrophage function. Moreover, intracellular FITC-NanoMBGs induce a transitory increase of the fungal phagocytosis by macrophages at MOI 1 and after a short time of interaction. In the competition assays, as the intracellular fungus quantity increased, the intracellular FITC-NanoMBG content decreased in a MOI- and time-dependent manner. These results have confirmed that macrophages clearly distinguish between inert material and the live yeast in a dynamic intracellular incorporation. Furthermore, macrophage phagocytosis is a critical determinant to know their functional state and a valuable parameter to study the nanomaterial / macrophages / Candida albicans interface.

Yersinia pestis escapes entrapment in thrombi by targeting platelet function

BACKGROUND

Platelets are classically recognized for their role in hemostasis and thrombosis. Recent work has demonstrated that platelets can also execute a variety of immune functions. The dual prothrombotic and immunological roles of platelets suggest that they may pose a barrier to the replication or dissemination of extracellular bacteria. However, some bloodborne pathogens, such as the plague bacterium Yersinia pestis, routinely achieve high vascular titers that are necessary for pathogen transmission.

OBJECTIVES

It is not currently known how or if pathogens circumvent platelet barriers to bacterial dissemination and replication. We sought to determine whether extracellular bloodborne bacterial pathogens actively interfere with platelet function, using Y  pestis as a model system.

METHODS

The interactions and morphological changes of human platelets with various genetically modified Y pestis strains were examined using aggregation assays, immunofluorescence, and scanning electron microscopy.

RESULTS

Yersinia pestis directly destabilized platelet thrombi, preventing bacterial entrapment in fibrin/platelet clots. This activity was dependent on two well-characterized bacterial virulence factors: the Y pestis plasminogen activator Pla, which stimulates host-mediated fibrinolysis, and the bacterial type III secretion system (T3SS), which delivers bacterial proteins into the cytoplasm of targeted host cells to reduce or prevent effective immunological responses. Platelets intoxicated by the Y pestis T3SS were unable to respond to prothrombotic stimuli, and T3SS expression decreased the formation of neutrophil extracellular traps in platelet thrombi.

CONCLUSIONS

These findings are the first demonstration of a bacterial pathogen using its T3SS and an endogenous protease to manipulate platelet function and to escape entrapment in platelet thrombi.

Molecular evaluation of quorum quenching potential of vanillic acid against Yersinia enterocolitica through transcriptomic and in silico analysis

Introduction. Yersinia enterocolitica is one of the leading food-borne entero-pathogens causing various illnesses ranging from gastroenteritis to systemic infections. Quorum sensing (QS) is one of the prime mechanisms that control the virulence in Y. enterocolitica .

Hypothesis/Gap Statement. Vanillic acid inhibits the quorum sensing and other virulence factors related to Y. enterocolitica . It has been evaluated by transcriptomic and Insilico analysis. Therefore, it can be a prospective agent to develop a therapeutic combination against Y. enterocolitica .

Aim. The present study is focused on screening natural anti-quorum-sensing agents against Y. enterocolitica . The effect of selected active principle on various virulence factors was evaluated.

Methodology. In total, 12 phytochemicals were screened by swarming assay. MATH assay, EPS and surfactant production assay, SEM analysis, antibiotic and blood sensitivity assay were performed to demonstrate the anti-virulence activity. Further, RNA sequencing and molecular docking studies were carried out to substantiate the anti-QS activity.

Results. Vanillic acid (VA) has exhibited significant motility inhibition, thus indicating the anti-QS activity with MQIC of 400 µg ml−1 without altering the cell viability. It has also inhibited the violacein production in Chromobacterium violaceum ATCC 12472, which further confirms the anti-QS activity. VA has inhibited 16 % of cell-surface hydrophobicity (CSH), 52 % of EPS production and 60 % of surfactant production. Moreover, it has increased the sensitivity of Y. enterocolitica towards antibiotics. It has also made the cells upto 91 % more vulnerable towards human immune cells. The transcriptomic analysis by RNA sequencing revealed the down regulation of genes related to motility, virulence, chemotaxis, siderophores and drug resistance. VA treatment has also positively regulated the expression of several stress response genes. In furtherance, the anti-QS potential of VA has been validated with QS regulatory protein YenR by in silico molecular simulation and docking study.

Conclusion. The present study is possibly the first attempt to demonstrate the anti-QS and anti-pathogenic potential of VA against Y. enterocolitica by transcriptomic and in silico analysis. It also deciphers that VA can be a promising lead to develop biopreservative and therapeutic regimens to treat Y. enterocolitica infections.

Host Delipidation Mediated by Bacterial Effectors

Protein lipidation, the covalent attachment of a lipid moiety to a target protein, plays a critical role in many cellular processes in eukaryotic cells. Bacterial pathogens secrete various effectors to subvert the host signaling pathway as a mechanism of microbial pathogenesis. An increasing number of effectors from diverse bacterial pathogens function as cysteine proteases to cause irreversible delipidation of host lipidated proteins. This in turn results in disruption of crucial lipidation-mediated host signal transduction, thereby enabling pathogen survival and replication. In this review, we discuss the role of the bacterial effectors in interactions with the host and highlight our knowledge of irreversible host delipidation, with a focus on the common concerted biochemical mechanisms of the bacterial effectors.

Early evolutionary loss of the lipid A modifying enzyme PagP resulting in innate immune evasion in Yersinia pestis

Immune evasion through membrane remodeling is a hallmark of Yersinia pestis pathogenesis. Yersinia remodels its membrane during its life cycle as it alternates between mammalian hosts (37 °C) and ambient (21 °C to 26 °C) temperatures of the arthropod transmission vector or external environment. This shift in growth temperature induces changes in number and length of acyl groups on the lipid A portion of lipopolysaccharide (LPS) for the enteric pathogens Yersinia pseudotuberculosis (Ypt) and Yersinia enterocolitica (Ye), as well as the causative agent of plague, Yersinia pestis (Yp). Addition of a C16 fatty acid (palmitate) to lipid A by the outer membrane acyltransferase enzyme PagP occurs in immunostimulatory Ypt and Ye strains, but not in immune-evasive Yp Analysis of Yp pagP gene sequences identified a single-nucleotide polymorphism that results in a premature stop in translation, yielding a truncated, nonfunctional enzyme. Upon repair of this polymorphism to the sequence present in Ypt and Ye, lipid A isolated from a Yp pagP+ strain synthesized two structures with the C16 fatty acids located in acyloxyacyl linkage at the 2' and 3' positions of the diglucosamine backbone. Structural modifications were confirmed by mass spectrometry and gas chromatography. With the genotypic restoration of PagP enzymatic activity in Yp, a significant increase in lipid A endotoxicity mediated through the MyD88 and TRIF/TRAM arms of the TLR4-signaling pathway was observed. Discovery and repair of an evolutionarily lost lipid A modifying enzyme provides evidence of lipid A as a crucial determinant in Yp infectivity, pathogenesis, and host innate immune evasion.

Yersinia pseudotuberculosis YopH targets SKAP2-dependent and independent signaling pathways to block neutrophil antimicrobial mechanisms during infection

Yersinia suppress neutrophil responses by using a type 3 secretion system (T3SS) to inject 6–7 Yersinia effector proteins (Yops) effectors into their cytoplasm. YopH is a tyrosine phosphatase that causes dephosphorylation of the adaptor protein SKAP2, among other targets in neutrophils. SKAP2 functions in reactive oxygen species (ROS) production, phagocytosis, and integrin-mediated migration by neutrophils. Here we identify essential neutrophil functions targeted by YopH, and investigate how the interaction between YopH and SKAP2 influence Yersinia pseudotuberculosis (Yptb) survival in tissues. The growth defect of a ΔyopH mutant was restored in mice defective in the NADPH oxidase complex, demonstrating that YopH is critical for protecting Yptb from ROS during infection. The growth of a ΔyopH mutant was partially restored in Skap2-deficient (Skap2KO) mice compared to wild-type (WT) mice, while induction of neutropenia further enhanced the growth of the ΔyopH mutant in both WT and Skap2KO mice. YopH inhibited both ROS production and degranulation triggered via integrin receptor, G-protein coupled receptor (GPCR), and Fcγ receptor (FcγR) stimulation. SKAP2 was required for integrin receptor and GPCR-mediated ROS production, but dispensable for degranulation under all conditions tested. YopH blocked SKAP2-independent FcγR-stimulated phosphorylation of the proximal signaling proteins Syk, SLP-76, and PLCγ2, and the more distal signaling protein ERK1/2, while only ERK1/2 phosphorylation was dependent on SKAP2 following integrin receptor activation. These findings reveal that YopH prevents activation of both SKAP2-dependent and -independent neutrophilic defenses, uncouple integrin- and GPCR-dependent ROS production from FcγR responses based on their SKAP2 dependency, and show that SKAP2 is not required for degranulation.

Pathogenic Yersinia species carry a virulence plasmid encoding a type 3 secretion system that translocates 6–7 effector Yops into host cells. We demonstrate that YopH protects Yersinia pseudotuberculosis from neutrophil-produced reactive oxygen species (ROS) and degranulation by interfering with signaling pathways downstream of three major receptor classes in neutrophils. We show that a previously identified target of YopH, SKAP2, controls some of the pathways essential for YopH to inactivate during infection. SKAP2 is essential in mediating ROS production downstream of two major receptors; however, it is dispensable for degranulation from the three major receptors tested. Our study illustrates that YopH protects Y. pseudotuberculosis by blocking both SKAP2-dependent and independent signaling pathways that regulate several neutrophil functions.

Programmed Cell Death in the Evolutionary Race against Bacterial Virulence Factors

Innate immune sensors can recognize when host cells are irrevocably compromised by pathogens, and in response can trigger programmed cell death (pyroptosis, apoptosis, and necroptosis). Innate sensors can directly bind microbial ligands; for example, NAIP/NLRC4 detects flagellin/rod/needle, whereas caspase-11 detects lipopolysaccharide. Other sensors are guards that monitor normal function of cellular proteins; for instance, pyrin monitors Rho GTPases, whereas caspase-8 and receptor-interacting protein kinase (RIPK)3 guards RIPK1 transcriptional signaling. Some proteins that need to be guarded can be duplicated as decoy domains, as seen in the integrated decoy domains within NLRP1 that watch for microbial attack. Here, we discuss the evolutionary battle between pathogens and host innate immune sensors/guards, illustrated by the Red Queen hypothesis. We discuss in depth four pathogens, and how they either fail in this evolutionary race (Chromobacterium violaceum, Burkholderia thailandensis), or how the evolutionary race generates increasingly complex virulence factors and host innate immune signaling pathways (Yersinia species, and enteropathogenic Escherichia coli [EPEC]).

Invasiveness of the Yersinia pestis ail protein contributes to host dissemination in pneumonic and oral plague

Yersinia pestis, a Gram-negative bacterium, is the etiologic agent of plague. A hallmark of Y. pestis infection is the organism's ability to rapidly disseminate through an animal host. Y. pestis expresses the outer membrane protein, Ail (Attachment invasion locus), which is associated with host invasion and serum resistance. However, whether Ail plays a role in host dissemination remains unclear. In this study, C57BL/6J mice were challenged with a defined Y. pestis strain, KimD27, or an isogenic ail-deleted mutant derived from KimD27 via metacarpal paw pad inoculation, nasal drops, orogastric infection, or tail vein injection to mimic bubonic, pneumonic, oral, or septicemic plague, respectively. Our results showed that ail-deleted Y. pestis KimD27 lost the ability to invade host cells, leading to failed host dissemination in the pneumonic and oral plague models but not in the bubonic or septicemic plague models, which do not require invasiveness. Therefore, this study demonstrated that whether Ail plays a role in Y. pestis pathogenesis depends on the infection route.

Inhibition of Neutrophil Primary Granule Release during Yersinia pestis Pulmonary Infection

Inhalation of Yersinia pestis causes primary pneumonic plague, the most severe manifestation of plague that is characterized by a dramatic neutrophil influx to the lungs. Neutrophils are ineffective during primary pneumonic plague, failing to control Y. pestis growth in the airways. However, the mechanisms by which Y. pestis resists neutrophil killing are incompletely understood. Here, we show that Y. pestis inhibits neutrophil degranulation, an important line of host innate immune defense. We observed that neutrophils from the lungs of mice infected intranasally with Y. pestis fail to release primary granules throughout the course of disease. Using a type III secretion system (T3SS) injection reporter strain, we determined that Y. pestis directly inhibits neutrophil granule release by a T3SS-dependent mechanism. Combinatorial mutant analysis revealed that a Y. pestis strain lacking both effectors YopE and YopH did not inhibit primary granule release and is killed by neutrophils both in vivo and in vitro Similarly, Y. pestis strains injecting only YopE or YopH are able to inhibit the majority of primary granule release from human neutrophils. We determined that YopE and YopH block Rac2 activation and calcium flux, respectively, to inhibit neutrophil primary granule release in isolated human neutrophils. These results demonstrate that Y. pestis coordinates the inhibition of neutrophil primary granule release through the activities of two distinct effectors, and this inhibition promotes Y. pestis survival during primary pneumonic plague.IMPORTANCEYersinia pestis is the causative agent of plague and is one of the deadliest human pathogens. The pneumonic form of Y. pestis infection has played a critical role in the severity of both historical and modern plague outbreaks, yet the host-pathogen interactions that govern the lethality of Yersinia pestis pulmonary infections are incompletely understood. Here, we report that Yersinia pestis inhibits neutrophil degranulation during infection, rendering neutrophils ineffective and allowing unrestricted growth of Y. pestis in the lungs. This coordinated inhibition of granule release not only demonstrates the pathogenic benefit of "silencing" lung neutrophils but also reveals specific host processes and pathways that could be manipulated to reduce the severity of primary pneumonic plague.

Life After Secretion-Yersinia enterocolitica Rapidly Toggles Effector Secretion and Can Resume Cell Division in Response to Changing External Conditions

Many pathogenic bacteria use the type III secretion system (T3SS) injectisome to manipulate host cells by injecting virulence-promoting effector proteins into the host cytosol. The T3SS is activated upon host cell contact, and its activation is accompanied by an arrest of cell division; hence, many species maintain a T3SS-inactive sibling population to propagate efficiently within the host. The enteric pathogen Yersinia enterocolitica utilizes the T3SS to prevent phagocytosis and inhibit inflammatory responses. Unlike other species, almost all Y. enterocolitica are T3SS-positive at 37°C, which raises the question, how these bacteria are able to propagate within the host, that is, when and how they stop secretion and restart cell division after a burst of secretion. Using a fast and quantitative in vitro secretion assay, we have examined the initiation and termination of type III secretion. We found that effector secretion begins immediately once the activating signal is present, and instantly stops when this signal is removed. Following effector secretion, the bacteria resume division within minutes after being introduced to a non-secreting environment, and the same bacteria are able to re-initiate effector secretion at later time points. Our results indicate that Y. enterocolitica use their type III secretion system to promote their individual survival when necessary, and are able to quickly switch their behavior toward replication afterwards, possibly gaining an advantage during infection.

Unraveling neutrophil- Yersinia interactions during tissue infection

The human and animal pathogens Yersinia pestis, which causes bubonic and pneumonic plague, and Yersinia pseudotuberculosis and Yersinia enterocolitica, which cause gastroenteritis, share a type 3 secretion system which injects effector proteins, Yops, into host cells. This system is critical for virulence of all three pathogens in tissue infection. Neutrophils are rapidly recruited to infected sites and all three pathogens frequently interact with and inject Yops into these cells during tissue infection. Host receptors, serum factors, and bacterial adhesins appear to collaborate to promote neutrophil- Yersinia interactions in tissues. The ability of neutrophils to control infection is mixed depending on the stage of infection and points to the efficiency of Yops and other bacterial factors to mitigate bactericidal effects of neutrophils. Yersinia in close proximity to neutrophils has higher levels of expression from yop promoters, and neutrophils in close proximity to Yersinia express higher levels of pro-survival genes than migrating neutrophils. In infected tissues, YopM increases neutrophil survival and YopH targets a SKAP2/SLP-76 signal transduction pathway. Yet the full impact of these and other Yops and other Yersinia factors on neutrophils in infected tissues has yet to be understood.

Visualization of the type III secretion mediated Salmonella-host cell interface using cryo-electron tomography

Many important gram-negative bacterial pathogens use highly sophisticated type III protein secretion systems (T3SSs) to establish complex host-pathogen interactions. Bacterial-host cell contact triggers the activation of the T3SS and the subsequent insertion of a translocon pore into the target cell membrane, which serves as a conduit for the passage of effector proteins. Therefore the initial interaction between T3SS-bearing bacteria and host cells is the critical step in the deployment of the protein secretion machine, yet this process remains poorly understood. Here, we use high-throughput cryo-electron tomography (cryo-ET) to visualize the T3SS-mediated Salmonella-host cell interface. Our analysis reveals the intact translocon at an unprecedented level of resolution, its deployment in the host cell membrane, and the establishment of an intimate association between the bacteria and the target cells, which is essential for effector translocation. Our studies provide critical data supporting the long postulated direct injection model for effector translocation.

Gain-of-Function Analysis Reveals Important Virulence Roles for the Yersinia pestis Type III Secretion System Effectors YopJ, YopT, and YpkA

Virulence of Yersinia pestis in mammals requires the type III secretion system, which delivers seven effector proteins into the cytoplasm of host cells to undermine immune responses. All seven of these effectors are conserved across Y. pestis strains, but three, YopJ, YopT, and YpkA, are apparently dispensable for virulence. Some degree of functional redundancy between effector proteins would explain both observations. Here, we use a combinatorial genetic approach to define the minimal subset of effectors required for full virulence in mice following subcutaneous infection. We found that a Y. pestis strain lacking YopJ, YopT, and YpkA is attenuated for virulence in mice and that addition of any one of these effectors to this strain increases lethality significantly. YopJ, YopT, and YpkA likely contribute to virulence via distinct mechanisms. YopJ is uniquely able to cause macrophage cell death in vitro and to suppress accumulation of inflammatory cells to foci of bacterial growth in deep tissue, whereas YopT and YpkA cannot. The synthetic phenotypes that emerge when YopJ, YopT, and YpkA are removed in combination provide evidence that each effector enhances Y. pestis virulence and that YopT and YpkA act through a mechanism distinct from that of YopJ.

Differential impact of lipopolysaccharide defects caused by loss of RfaH in Yersinia pseudotuberculosis and Yersinia pestis

RfaH enhances transcription of a select group of operons controlling bacterial surface features such as lipopolysaccharide (LPS). Previous studies have suggested that rfaH may be required for Yersinia pseudotuberculosis resistance to antimicrobial chemokines and survival during mouse infections. In order to further investigate the role of RfaH in LPS synthesis, resistance to host defense peptides, and virulence of Yersinia, we constructed ΔrfaH mutants of Y. pseudotuberculosis IP32953 and Y. pestis KIM6+. Loss of rfaH affected LPS synthesis in both species, resulting in a shorter core oligosaccharide. Susceptibility to polymyxin and the antimicrobial chemokine CCL28 was increased by loss of rfaH in Y. pseudotuberculosis but not in Y. pestis. Transcription of genes in the ddhD-wzz O-antigen gene cluster, but not core oligosaccharide genes, was reduced in ΔrfaH mutants. In addition, mutants with disruptions in specific ddhD-wzz O-antigen cluster genes produced LPS that was indistinguishable from the ΔrfaH mutant. This suggests that both Y. pseudotuberculosis and Y. pestis produce an oligosaccharide core with a single O-antigen unit attached in an RfaH-dependent fashion. Despite enhanced sensitivity to host defense peptides, the Y. pseudotuberculosis ΔrfaH strain was not attenuated in mice, suggesting that rfaH is not required for acute infection.

Yersinia enterocolitica YopH-Deficient Strain Activates Neutrophil Recruitment to Peyer's Patches and Promotes Clearance of the Virulent Strain

Yersinia enterocolitica evades the immune response by injecting Yersinia outer proteins (Yops) into the cytosol of host cells. YopH is a tyrosine phosphatase critical for Yersinia virulence. However, the mucosal immune mechanisms subverted by YopH during in vivo orogastric infection with Y. enterocolitica remain elusive. The results of this study revealed neutrophil recruitment to Peyer's patches (PP) after infection with a YopH-deficient mutant strain (Y. enterocolitica ΔyopH). While the Y. enterocolitica wild-type (WT) strain in PP induced the major neutrophil chemoattractant CXCL1 mRNA and protein levels, infection with the Y. enterocolitica ΔyopH mutant strain exhibited a higher expression of the CXCL1 receptor, CXCR2, in blood neutrophils, leading to efficient neutrophil recruitment to the PP. In contrast, migration of neutrophils into PP was impaired upon infection with Y. enterocolitica WT strain. In vitro infection of blood neutrophils revealed the involvement of YopH in CXCR2 expression. Depletion of neutrophils during Y. enterocolitica ΔyopH infection raised the bacterial load in PP. Moreover, the clearance of WT Y. enterocolitica was improved when an equal mixture of Y. enterocolitica WT and Y. enterocolitica ΔyopH strains was used in infecting the mice. This study indicates that Y. enterocolitica prevents early neutrophil recruitment in the intestine and that the effector protein YopH plays an important role in the immune evasion mechanism. The findings highlight the potential use of the Y. enterocolitica YopH-deficient strain as an oral vaccine carrier.

'Add, stir and reduce': Yersinia spp. as model bacteria for pathogen evolution

Pathogenic species in the Yersinia genus have historically been targets for research aimed at understanding how bacteria evolve into mammalian pathogens. The advent of large-scale population genomic studies has greatly accelerated the progress in this field, and Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica have once again acted as model organisms to help shape our understanding of the evolutionary processes involved in pathogenesis. In this Review, we highlight the gene gain, gene loss and genome rearrangement events that have been identified by genomic studies in pathogenic Yersinia species, and we discuss how these findings are changing our understanding of pathogen evolution. Finally, as these traits are also found in the genomes of other species in the Enterobacteriaceae, we suggest that they provide a blueprint for the evolution of enteropathogenic bacteria.

Yersinia virulence factors - a sophisticated arsenal for combating host defences

The human pathogens Yersinia pseudotuberculosis and Yersinia enterocolitica cause enterocolitis, while Yersinia pestis is responsible for pneumonic, bubonic, and septicaemic plague. All three share an infection strategy that relies on a virulence factor arsenal to enable them to enter, adhere to, and colonise the host while evading host defences to avoid untimely clearance. Their arsenal includes a number of adhesins that allow the invading pathogens to establish a foothold in the host and to adhere to specific tissues later during infection. When the host innate immune system has been activated, all three pathogens produce a structure analogous to a hypodermic needle. In conjunction with the translocon, which forms a pore in the host membrane, the channel that is formed enables the transfer of six ‘effector’ proteins into the host cell cytoplasm. These proteins mimic host cell proteins but are more efficient than their native counterparts at modifying the host cell cytoskeleton, triggering the host cell suicide response. Such a sophisticated arsenal ensures that yersiniae maintain the upper hand despite the best efforts of the host to counteract the infecting pathogen.

CD8(+) T cells specific to a single Yersinia pseudotuberculosis epitope restrict bacterial replication in the liver but fail to provide sterilizing immunity

CD8(+) T cells use contact-dependent cytolysis of target cells to protect the host against intracellular pathogens. We have previously shown that CD8(+) T cells and perforin are required to protect against the extracellular pathogen Yersinia pseudotuberculosis. Here we establish an experimental system where CD8(+) T cells specific to a single model antigen are the only memory response present at time of challenge. Using mice immunized with a vaccine strain of Listeria monocytogenes that expresses secreted ovalbumin (Lm-OVA), we show that OVA-specific CD8(+) T cells are generated and provide limited protection against challenge with virulent OVA(+)Y. pseudotuberculosis. Perforin expression by OVA-specific CD8(+) T cells was required, as Lm-OVA-immunized perforin-deficient mice showed higher bacterial burden as compared to Lm-OVA-immunized perforin-sufficient mice. Surprisingly, antigen-specific T cell protection waned over time, as Lm-OVA-immune mice eventually succumbed to Yersinia infection. Kinetic analysis of infection in mice with and without OVA-specific CD8(+) T cells revealed that bacterial numbers increased sharply in OVA-naïve mice until death, while OVA-immune mice held bacterial burden to a lower level throughout the duration of illness until death. Clonal analysis of bacterial populations in OVA-naïve and OVA-immune mice at distinct time points revealed equivalent and severe bottle-neck effects for bacteria in both sets of mice immediately after intravenous challenge, demonstrating a dominant role for other aspects of the immune system regardless of CD8(+) T cell status. These studies indicate that CD8(+) T cells against a single antigen can restrict Y. pseudotuberculosis colonization in a perforin-dependent manner, but ultimately are insufficient in their ability to provide sterilizing immunity and protect against death.

A bacterial type III secretion-based protein delivery tool for broad applications in cell biology

Methods enabling the delivery of proteins into eukaryotic cells are essential to address protein functions. Here we propose broad applications to cell biology for a protein delivery tool based on bacterial type III secretion (T3S). We show that bacterial, viral, and human proteins, fused to the N-terminal fragment of the Yersinia enterocolitica T3S substrate YopE, are effectively delivered into target cells in a fast and controllable manner via the injectisome of extracellular bacteria. This method enables functional interaction studies by the simultaneous injection of multiple proteins and allows the targeting of proteins to different subcellular locations by use of nanobody-fusion proteins. After delivery, proteins can be freed from the YopE fragment by a T3S-translocated viral protease or fusion to ubiquitin and cleavage by endogenous ubiquitin proteases. Finally, we show that this delivery tool is suitable to inject proteins in living animals and combine it with phosphoproteomics to characterize the systems-level impact of proapoptotic human truncated BID on the cellular network.

Genomic Insight into the Host-Endosymbiont Relationship of Endozoicomonas montiporae CL-33(T) with its Coral Host

The bacterial genus Endozoicomonas was commonly detected in healthy corals in many coral-associated bacteria studies in the past decade. Although, it is likely to be a core member of coral microbiota, little is known about its ecological roles. To decipher potential interactions between bacteria and their coral hosts, we sequenced and investigated the first culturable endozoicomonal bacterium from coral, the E. montiporae CL-33(T). Its genome had potential sign of ongoing genome erosion and gene exchange with its host. Testosterone degradation and type III secretion system are commonly present in Endozoicomonas and may have roles to recognize and deliver effectors to their hosts. Moreover, genes of eukaryotic ephrin ligand B2 are present in its genome; presumably, this bacterium could move into coral cells via endocytosis after binding to coral's Eph receptors. In addition, 7,8-dihydro-8-oxoguanine triphosphatase and isocitrate lyase are possible type III secretion effectors that might help coral to prevent mitochondrial dysfunction and promote gluconeogenesis, especially under stress conditions. Based on all these findings, we inferred that E. montiporae was a facultative endosymbiont that can recognize, translocate, communicate and modulate its coral host.

Yersinia type III effectors perturb host innate immune responses

The innate immune system is the first line of defense against invading pathogens. Innate immune cells recognize molecular patterns from the pathogen and mount a response to resolve the infection. The production of proinflammatory cytokines and reactive oxygen species, phagocytosis, and induced programmed cell death are processes initiated by innate immune cells in order to combat invading pathogens. However, pathogens have evolved various virulence mechanisms to subvert these responses. One strategy utilized by Gram-negative bacterial pathogens is the deployment of a complex machine termed the type III secretion system (T3SS). The T3SS is composed of a syringe-like needle structure and the effector proteins that are injected directly into a target host cell to disrupt a cellular response. The three human pathogenic Yersinia spp. (Y. pestis, Y. enterocolitica, and Y. pseudotuberculosis) are Gram-negative bacteria that share in common a 70 kb virulence plasmid which encodes the T3SS. Translocation of the Yersinia effector proteins (YopE, YopH, YopT, YopM, YpkA/YopO, and YopP/J) into the target host cell results in disruption of the actin cytoskeleton to inhibit phagocytosis, downregulation of proinflammatory cytokine/chemokine production, and induction of cellular apoptosis of the target cell. Over the past 25 years, studies on the Yersinia effector proteins have unveiled tremendous knowledge of how the effectors enhance Yersinia virulence. Recently, the long awaited crystal structure of YpkA has been solved providing further insights into the activation of the YpkA kinase domain. Multisite autophosphorylation by YpkA to activate its kinase domain was also shown and postulated to serve as a mechanism to bypass regulation by host phosphatases. In addition, novel Yersinia effector protein targets, such as caspase-1, and signaling pathways including activation of the inflammasome were identified. In this review, we summarize the recent discoveries made on Yersinia effector proteins and their contribution to Yersinia pathogenesis.

Elucidating Host-Pathogen Interactions Based on Post-Translational Modifications Using Proteomics Approaches

Microbes with the capability to survive in the host tissue and efficiently subvert its innate immune responses can cause various health hazards. There is an inherent need to understand microbial infection patterns and mechanisms in order to develop efficient therapeutics. Microbial pathogens display host specificity through a complex network of molecular interactions that aid their survival and propagation. Co-infection states further lead to complications by increasing the microbial burden and risk factors. Quantitative proteomics based approaches and post-translational modification analysis can be efficiently applied to gain an insight into the molecular mechanisms involved. The measurement of the proteome and post-translationally modified proteome dynamics using mass spectrometry, results in a wide array of information, such as significant changes in protein expression, protein abundance, the modification status, the site occupancy level, interactors, functional significance of key players, potential drug targets, etc. This mini review discusses the potential of proteomics to investigate the involvement of post-translational modifications in bacterial pathogenesis and host-pathogen interactions.

Yersinia pestis uses the Ail outer membrane protein to recruit vitronectin

Yersinia pestis, the agent of plague, requires the Ail (attachment invasion locus) outer membrane protein to survive in the blood and tissues of its mammalian hosts. Ail is important for both attachment to host cells and for resistance to complement-dependent bacteriolysis. Previous studies have shown that Ail interacts with components of the extracellular matrix, including fibronectin, laminin and heparan sulfate proteoglycans, and with the complement inhibitor C4b-binding protein. Here, we demonstrate that Ail-expressing Y. pestis strains bind vitronectin - a host protein with functions in cell attachment, fibrinolysis and inhibition of the complement system. The Ail-dependent recruitment of vitronectin resulted in efficient cleavage of vitronectin by the outer membrane Pla (plasminogen activator protease). Escherichia coli DH5α expressing Y. pestis Ail bound vitronectin, but not heat-treated vitronectin. The ability of Ail to directly bind vitronectin was demonstrated by ELISA using purified refolded Ail in nanodiscs.

Spatially distinct neutrophil responses within the inflammatory lesions of pneumonic plague

During pneumonic plague, the bacterium Yersinia pestis elicits the development of inflammatory lung lesions that continue to expand throughout infection. This lesion development and persistence are poorly understood. Here, we examine spatially distinct regions of lung lesions using laser capture microdissection and transcriptome sequencing (RNA-seq) analysis to identify transcriptional differences between lesion microenvironments. We show that cellular pathways involved in leukocyte migration and apoptosis are downregulated in the center of lung lesions compared to the periphery. Probing for the bacterial factor(s) important for the alteration in neutrophil survival, we show both in vitro and in vivo that Y. pestis increases neutrophil survival in a manner that is dependent on the type III secretion system effector YopM. This research explores the complexity of spatially distinct host-microbe interactions and emphasizes the importance of cell relevance in assays in order to fully understand Y. pestis virulence.

Yersinia pestis is a high-priority pathogen and continues to cause outbreaks worldwide. The ability of Y. pestis to be transmitted via respiratory droplets and its history of weaponization has led to its classification as a select agent most likely to be used as a biological weapon. Unrestricted bacterial growth during the initial preinflammatory phase primes patients to be infectious once disease symptoms begin in the proinflammatory phase, and the rapid disease progression can lead to death before Y. pestis infection can be diagnosed and treated. Using in vivo analyses and focusing on relevant cell types during pneumonic plague infection, we can identify host pathways that may be manipulated to extend the treatment window for pneumonic plague patients.

Exploring Oxidovanadium(IV) Complexes as YopH Inhibitors: Mechanism of Action and Modeling Studies

YopH tyrosine phosphatase, a virulence factor produced by pathogenic species of Yersinia, is an attractive drug target. In this work, three oxidovanadium(IV) complexes were assayed against recombinant YopH and showed strong inhibition of the enzyme in the nanomolar range. Molecular modeling indicated that their binding is reinforced by H-bond, cation−π, and π–π interactions conferring specificity toward YopH. These complexes are thus interesting lead molecules for phosphatase inhibitor drug discovery.

Role of the Yersinia YopJ protein in suppressing interleukin-8 secretion by human polymorphonuclear leukocytes

Polymorphonuclear leukocytes, in addition to their direct bactericidal activities, produce cytokines involved in the activation and regulation of the innate and adaptive immune response to infection. In this study we evaluated the cytokine response of human PMNs following incubation with the pathogenic Yersinia species. Yersinia pestis strains with the pCD1 virulence plasmid, which encodes cytotoxic Yop proteins that are translocated into host cells, stimulated little or no cytokine production compared to pCD1-negative strains. In particular, PMNs incubated with pCD1-negative Y. pestis secreted 1000-fold higher levels of interleukin-8 (IL-8 or CXCL8), a proinflammatory chemokine important for PMN recruitment and activation. Deletion of yopE, -H, -T, -M or ypkA had no effect on pCD1-dependent inhibition, whereas deletion of yopJ resulted in significantly increased IL-8 production. Like Y. pestis, the enteropathogenic Yersinia species inhibited IL-8 secretion by PMNs, and strains lacking the virulence plasmid induced high levels of IL-8. Our results show that virulence plasmid-encoded effector Yops, particularly YopJ, prevent IL-8 secretion by human PMNs. Suppression of the chemotactic IL-8 response by Y. pestis may contribute to the delayed PMN recruitment to the infected lymph node that typifies bubonic plague.

Thermal control of virulence factors in bacteria: a hot topic

Pathogenic bacteria sense environmental cues, including the local temperature, to control the production of key virulence factors. Thermal regulation can be achieved at the level of DNA, RNA or protein and although many virulence factors are subject to thermal regulation, the exact mechanisms of control are yet to be elucidated in many instances. Understanding how virulence factors are regulated by temperature presents a significant challenge, as gene expression and protein production are often influenced by complex regulatory networks involving multiple transcription factors in bacteria. Here we highlight some recent insights into thermal regulation of virulence in pathogenic bacteria. We focus on bacteria which cause disease in mammalian hosts, which are at a significantly higher temperature than the outside environment. We outline the mechanisms of thermal regulation and how understanding this fundamental aspect of the biology of bacteria has implications for pathogenesis and human health.

A Lactobacillus plantarum strain isolated from kefir protects against intestinal infection with Yersinia enterocolitica O9 and modulates immunity in mice

Lactobacillus plantarum C4, previously isolated from kefir and characterized as a potential probiotic strain, was tested for its protective and immunomodulatory capacity in a murine model of yersiniosis. The inoculation of BALB/c mice with a low pathogenicity serotype O9 strain of Yersinia enterocolitica results in a prolonged intestinal infection with colonization of Peyer's patches. Pretreatment with C4 was without effect on fecal excretion of yersiniae, but shortened the colonization of Peyer's patches. This protective effect was associated with pro-inflammatory status in the intestinal mucosa (TNF-α production in infected mice was increased by C4) and an increase in total IgA secretion. At a systemic level, C4 did not promote a pro-inflammatory response, although production of the immunoregulatory cytokine IFN-γ was enhanced. These findings suggest that L. plantarum C4 can increase resistance to intestinal infections through its immunomodulatory activity.

Two host clades, two bacterial arsenals: evolution through gene losses in facultative endosymbionts

Bacterial endosymbiosis is an important evolutionary process in insects, which can harbor both obligate and facultative symbionts. The evolution of these symbionts is driven by evolutionary convergence, and they exhibit among the tiniest genomes in prokaryotes. The large host spectrum of facultative symbionts and the high diversity of strategies they use to infect new hosts probably impact the evolution of their genome and explain why they undergo less severe genomic erosion than obligate symbionts. Candidatus Hamiltonella defensa is suitable for the investigation of the genomic evolution of facultative symbionts because the bacteria are engaged in specific relationships in two clades of insects. In aphids, H. defensa is found in several species with an intermediate prevalence and confers protection against parasitoids. In whiteflies, H. defensa is almost fixed in some species of Bemisia tabaci, which suggests an important role of and a transition toward obligate symbiosis. In this study, comparisons of the genome of H. defensa present in two B. tabaci species (Middle East Asia Minor 1 and Mediterranean) and in the aphid Acyrthosiphon pisum revealed that they belong to two distinct clades and underwent specific gene losses. In aphids, it contains highly virulent factors that could allow protection and horizontal transfers. In whiteflies, the genome lost these factors and seems to have a limited ability to acquire genes. However it contains genes that could be involved in the production of essential nutrients, which is consistent with a primordial role for this symbiont. In conclusion, although both lineages of H. defensa have mutualistic interactions with their hosts, their genomes follow distinct evolutionary trajectories that reflect their phenotype and could have important consequences on their evolvability.

Regulation of Yersinia protein kinase A (YpkA) kinase activity by multisite autophosphorylation and identification of an N-terminal substrate-binding domain in YpkA

The serine/threonine protein kinase YpkA is an essential virulence factor produced by pathogenic Yersinia species. YpkA is delivered into host mammalian cells via a type III secretion system and localizes to the inner side of the plasma membrane. We have previously shown that YpkA binds to and phosphorylates the α subunit of the heterotrimeric G protein complex, Gαq, resulting in inhibition of Gαq signaling. To identify residues in YpkA involved in substrate binding activity we generated GFP-YpkA N-terminal deletion mutants and performed coimmunoprecipitation experiments. We located a substrate-binding domain on amino acids 40-49 of YpkA, which lies within the previously identified membrane localization domain on YpkA. Deletion of amino acids 40-49 on YpkA interfered with substrate binding, substrate phosphorylation and substrate inhibition. Autophosphorylation regulates the kinase activity of YpkA. To dissect the mechanism by which YpkA transmits signals, we performed nano liquid chromatography coupled to tandem mass spectrometry to map in vivo phosphorylation sites. Multiple serine phosphorylation sites were identified in the secretion/translocation region, kinase domain, and C-terminal region of YpkA. Using site-directed mutagenesis we generated multiple YpkA constructs harboring specific serine to alanine point mutations. Our results demonstrate that multiple autophosphorylation sites within the N terminus regulate YpkA kinase activation, whereas mutation of serine to alanine within the C terminus of YpkA had no effect on kinase activity. YpkA autophosphorylation on multiple sites may be a strategy used by pathogenic Yersinia to prevent inactivation of this important virulence protein by host proteins.

The Yersinia pestis type III secretion system: expression, assembly and role in the evasion of host defenses

Yersinia pestis, the etiologic agent of plague, utilizes a type III secretion system (T3SS) to subvert the defenses of its mammalian hosts. T3SSs are complex nanomachines that allow bacterial pathogens to directly inject effector proteins into eukaryotic cells. The Y. pestis T3SS is not expressed during transit through the flea vector, but T3SS gene expression is rapidly thermoinduced upon entry into a mammalian host. Assembly of the T3S apparatus is a highly coordinated process that requires the homo- and hetero-oligomerization over 20 Yersinia secretion (Ysc) proteins, several assembly intermediates and the T3S process to complete the assembly of the rod and external needle structures. The activation of effector secretion is controlled by the YopN/TyeA/SycN/YscB complex, YscF and LcrG in response to extracellular calcium and/or contact with a eukaryotic cell. Cell contact triggers the T3S process including the secretion and assembly of a pore-forming translocon complex that facilitates the translocation of effector proteins, termed Yersinia outer proteins (Yops), across the eukaryotic membrane. Within the host cell, the Yop effector proteins function to inhibit bacterial phagocytosis and to suppress the production of pro-inflammatory cytokines.

Identifying Yersinia YopH-targeted signal transduction pathways that impair neutrophil responses during in vivo murine infection

Identifying molecular targets of Yersinia virulence effectors, or Yops, during animal infection is challenging because few cells are targeted by Yops in an infected organ, and isolating these sparse effector-containing cells is difficult. YopH, a tyrosine phosphatase, is essential for full virulence of Yersinia. Investigating the YopH-targeted signal transduction pathway(s) in neutrophils during infection of a murine host, we find that several host proteins, including the essential signaling adaptor SLP-76, are dephosphorylated in the presence of YopH in neutrophils isolated from infected tissues. YopH inactivated PRAM-1/SKAP-HOM and the SLP-76/Vav/PLCγ2 signal transduction axes, leading to an inhibition of calcium response in isolated neutrophils. Consistent with a failure to mount a calcium response, IL-10 production was reduced in neutrophils containing YopH from infected tissues. Finally, a yopH mutant survived better in the absence of neutrophils, indicating that neutrophil inactivation by YopH by targeting PRAM-1/SKAP-HOM and SLP-76/Vav/PLCγ2 signaling hubs may be critical for Yersinia survival.

Modulation of phagosomal pH by Candida albicans promotes hyphal morphogenesis and requires Stp2p, a regulator of amino acid transport

Candida albicans, the most important fungal pathogen of humans, has a unique interaction with macrophages in which phagocytosis induces a switch from the yeast to hyphal form, allowing it to escape by rupturing the immune cell. While a variety of factors induce this switch in vitro, including neutral pH, it is not clear what triggers morphogenesis within the macrophage where the acidic environment should inhibit this transition. In vitro, C. albicans grown in similar conditions in which amino acids are the primary carbon source generate large quantities of ammonia to raise the extracellular pH and induce the hyphal switch. We show here that C. albicans cells neutralize the macrophage phagosome and that neutral pH is a key inducer of germination in phagocytosed cells by using a mutant lacking STP2, a transcription factor that regulates the expression of multiple amino acid permeases, that is completely deficient in alkalinization in vitro. Phagocytosed stp2Δ mutant cells showed significant reduction in hypha formation and escaped from macrophages less readily compared to wild type cells; as a result stp2Δ mutant cells were killed at a higher rate and caused less damage to RAW264.7 macrophages. Stp2p-regulated import leads to alkalinization of the phagosome, since the majority of the wild type cells fail to co-localize with acidophilic dyes, whereas the stp2Δ mutant cells were located in acidic phagosomes. Furthermore, stp2Δ mutant cells were able to form hyphae and escape from neutral phagosomes, indicating that the survival defect in these cells was pH dependent. Finally, these defects are reflected in an attenuation of virulence in a mouse model of disseminated candidiasis. Altogether our results suggest that C. albicans utilizes amino acids to promote neutralization of the phagosomal pH, hyphal morphogenesis, and escape from macrophages.

The innate immune system represents a key barrier that fungal pathogens such as Candida albicans must overcome in order to disseminate through the host. C. albicans cells phagocytosed by macrophages initiate a complex program that involves a large-scale reprogramming of metabolism and transcription and results in the switch to a hyphal form that can penetrate and kill the macrophage. Though a number of signals are known to induce this morphological transition in vitro, what does so following phagocytosis has been unclear. We previously showed that C. albicans rapidly neutralizes acidic, nutrient-poor media that resembles the phagolysosome and that this is deficient in mutants impaired in amino acid import due to a mutation in STP2. In this paper, we investigate whether this happens within the macrophage as well. We show here that, in contrast to wild-type cells, stp2Δ mutants occupy an acidic phagosome and are unable to initiate hyphal differentiation. Because of this, they are more sensitive to killing and do less damage to the macrophages than cells that can neutralize the phagolysosome. We conclude that alteration of phagosomal pH is an important virulence adaptation in this species.