Cross-talk between bacterial pathogens and their host cells

A ParDE toxin-antitoxin system is responsible for the maintenance of the Yersinia virulence plasmid but not for type III secretion-associated growth inhibition

Many Gram-negative pathogens utilize the type III secretion system (T3SS) to translocate virulence-promoting effector proteins into eukaryotic host cells. The activity of this system results in a severe reduction of bacterial growth and division, summarized as secretion-associated growth inhibition (SAGI). In Yersinia enterocolitica, the T3SS and related proteins are encoded on a virulence plasmid. We identified a ParDE-like toxin-antitoxin system on this virulence plasmid in genetic proximity to yopE, encoding a T3SS effector. Effectors are strongly upregulated upon activation of the T3SS, indicating a potential role of the ParDE system in the SAGI or maintenance of the virulence plasmid. Expression of the toxin ParE in trans resulted in reduced growth and elongated bacteria, highly reminiscent of the SAGI. Nevertheless, the activity of ParDE is not causal for the SAGI. T3SS activation did not influence ParDE activity; conversely, ParDE had no impact on T3SS assembly or activity itself. However, we found that ParDE ensures the presence of the T3SS across bacterial populations by reducing the loss of the virulence plasmid, especially under conditions relevant to infection. Despite this effect, a subset of bacteria lost the virulence plasmid and regained the ability to divide under secreting conditions, facilitating the possible emergence of T3SS-negative bacteria in late acute and persistent infections.

Scar/WAVE has Rac GTPase-independent functions during cell wound repair

Rho family GTPases regulate both linear and branched actin dynamics by activating downstream effectors to facilitate the assembly and function of complex cellular structures such as lamellipodia and contractile actomyosin rings. Wiskott-Aldrich Syndrome (WAS) family proteins are downstream effectors of Rho family GTPases that usually function in a one-to-one correspondence to regulate branched actin nucleation. In particular, the WAS protein Scar/WAVE has been shown to exhibit one-to-one correspondence with Rac GTPase. Here we show that Rac and SCAR are recruited to cell wounds in the Drosophila repair model and are required for the proper formation and maintenance of the dynamic actomyosin ring formed at the wound periphery. Interestingly, we find that SCAR is recruited to wounds earlier than Rac and is still recruited to the wound periphery in the presence of a potent Rac inhibitor. We also show that while Rac is important for actin recruitment to the actomyosin ring, SCAR serves to organize the actomyosin ring and facilitate its anchoring to the overlying plasma membrane. These differing spatiotemporal recruitment patterns and wound repair phenotypes highlight the Rac-independent functions of SCAR and provide an exciting new context in which to investigate these newly uncovered SCAR functions.

Coordinated efforts of different actin filament populations are needed for optimal cell wound repair

Cells are subjected to a barrage of daily insults that often lead to their cortices being ripped open and requiring immediate repair. An important component of the cell's repair response is the formation of an actomyosin ring at the wound periphery to mediate its closure. Here we show that inhibition of myosin or the linear actin nucleation factors Diaphanous and/or dishevelled associated activator of morphogenesis results in a disrupted contractile apparatus and delayed wound closure. We also show that the branched actin nucleators WASp and SCAR function nonredundantly as scaffolds to assemble and maintain this contractile actomyosin cable. Removing branched actin leads to the formation of smaller circular actin-myosin structures at the cell cortex and to slow wound closure. Removing linear and branched actin simultaneously results in failed wound closure. Surprisingly, removal of branched actin and myosin results in the formation of parallel linear F-actin filaments that undergo a chiral swirling movement to close the wound, uncovering a new mechanism of cell wound closure. Taken together, we demonstrate the roles of different actin substructures that are required for optimal actomyosin ring formation and the extraordinary resilience of the cell to undergo wound repair when it is unable to form different subsets of these substructures.

Bacterial Protein Tyrosine Phosphatases as Possible Targets for Antimicrobial Therapies in Response to Antibiotic Resistance

The review is focused on the bacterial protein tyrosine phosphatases (PTPs) utilized by bacteria as virulence factors necessary for pathogenicity. The inhibition of bacterial PTPs could contribute to the arrest of the bacterial infection process. This mechanism could be utilized in the design of antimicrobial therapy as adjuvants to antibiotics. The review summaries knowledge on pathogenic bacterial protein tyrosine phosphatases (PTPs) involved in infection process, such as: PTPA and PTPB from Staphylococcus aureus and Mycobacterium tuberculosis; SptP from Salmonella typhimurium; YopH from Yersinia sp. and TbpA from Pseudomonas aeruginosa. The review focuses also on the potential inhibitory compounds of bacterial virulence factors and inhibitory mechanisms such as the reversible oxidation of tyrosine phosphatases.

Protective function of surface layer protein from Lactobacillus casei fb05 against intestinal pathogens in vitro

Escherichia coli and Salmonella are common pathogenic bacteria in human intestine, which can infect epithelial cells and cause diseases. Adhesion to intestinal tissue is the first step of pathogen infection. This work was to investigate the protective function of surface layer protein (SLP) from Lactobacillus casei fb05 against the harmful effects of E. coli and Salmonella on intestinal tissue (collagen and HT-29 cells). The SLP of L. casei fb05 was identified by transmission electron microscopy and SDS-PAGE. The purified SLP could reduce the adhesion of E. coli and Salmonella to collagen and HT-29 cells as observed by light microscope. The flow cytometry results showed that the L. casei fb05 SLP decreased the two pathogens-induced apoptosis of HT-29 cells by about 45%-49%. In addition, the activation of caspase-9 and caspase-3 caused by the two pathogens was significantly declined by the interference of the L. casei fb05 SLP. All the findings demonstrated that the L. casei fb05 SLP could decrease the deleterious effects of E. coli and Salmonella on intestinal tract in two ways: reducing pathogen adhesion and inhibiting pathogen-induced apoptosis. The potential of L. casei fb05 SLP in the treatment of intestinal diseases might be explored in this work.

Autocrine insulin pathway signaling regulates actin dynamics in cell wound repair

Cells are exposed to frequent mechanical and/or chemical stressors that can compromise the integrity of the plasma membrane and underlying cortical cytoskeleton. The molecular mechanisms driving the immediate repair response launched to restore the cell cortex and circumvent cell death are largely unknown. Using microarrays and drug-inhibition studies to assess gene expression, we find that initiation of cell wound repair in the Drosophila model is dependent on translation, whereas transcription is required for subsequent steps. We identified 253 genes whose expression is up-regulated (80) or down-regulated (173) in response to laser wounding. A subset of these genes were validated using RNAi knockdowns and exhibit aberrant actomyosin ring assembly and/or actin remodeling defects. Strikingly, we find that the canonical insulin signaling pathway controls actin dynamics through the actin regulators Girdin and Chickadee (profilin), and its disruption leads to abnormal wound repair. Our results provide new insight for understanding how cell wound repair proceeds in healthy individuals and those with diseases involving wound healing deficiencies.

Listeria monocytogens - Amended understanding of its pathogenesis with a complete picture of its membrane vesicles, quorum sensing, biofilm and invasion

Listeria monocytogenes is a ubiquitous, intracellular foodborne pathogen that causes listeriosis in animals and humans. Pathogenic Listeria monocytogenes easily adapted to the conditions of human gastrointestinal tract and tolerate the counter changes such as acidity, bile, osmolarity, and antimicrobial peptides. They secrete specialized biologically active extra organ called membrane vesicles which comprises proteins, lipids, and lipopolysaccharides. Listerial vesicles possess functional versatility and play a significant role in pathogenesis by cell-free intercellular communication and toxin packaging. L. monocytogenes can attach promptly and decisively to inert substratum including intestinal mucosa, and forms biofilms and causes detrimental effects. Further, they invade the host cells through quorum sensing (QS) controlled virulence determinants and biofilms. The precise degree to which the bacterium retains the intracellular ambiance of host cells remains unknown. The machinery associated with intracellular survival, and the role of membrane vesicles, quorum sensing, and the Agr system in Listeria monocytogenes largely remains unclear. The current review focused to understand the role of membrane vesicles mediated pathogenesis biofilms, and delivers auxiliary impetus to understanding the potentials of virulence mediated invasion in Listeria monocytogenes.

AAA+ Molecular Chaperone ClpB in Leptospira interrogans: Its Role and Significance in Leptospiral Virulence and Pathogenesis of Leptospirosis

Bacterial ClpB is an ATP-dependent disaggregase that belongs to the Hsp100/Clp subfamily of the AAA+ ATPases and cooperates with the DnaK chaperone system in the reactivation of aggregated proteins, as well as promotes bacterial survival under adverse environmental conditions, including thermal and oxidative stresses. In addition, extensive evidence indicates that ClpB supports the virulence of numerous bacteria, including pathogenic spirochaete Leptospira interrogans responsible for leptospirosis in animals and humans. However, the specific function of ClpB in leptospiral virulence still remains to be fully elucidated. Interestingly, ClpB was predicted as one of the L. interrogans hub proteins interacting with human proteins, and pathogen-host protein interactions are fundamental for successful invasion of the host immune system by bacteria. The aim of this review is to discuss the most important aspects of ClpB's function in L. interrogans, including contribution of ClpB to leptospiral virulence and pathogenesis of leptospirosis, a zoonotic disease with a significant impact on public health worldwide.

HyperQuant-A Computational Pipeline for Higher Order Multiplexed Quantitative Proteomics

Quantitative proteomics has evolved considerably over the last decade with the advent of higher order multiplexing (HOM) techniques. With the development of methods such as-multitagging, cPILOT, hyperplexing, BONPlex, and MITNCAT, the HOM technique is rapidly taking the center stage in multiplexed quantitative proteomics. These studies combined MS¹ and MS² labels in a single experiment enabling higher sample throughput. While HOM is highly promising, the computational analysis is still a big challenge, as the available tools cannot harness its power completely. We have developed a new quantitative pipeline, HyperQuant to aid in accurately quantitating complex HOM data. The pipeline uses identification results from either MaxQuant or any other search engine and quantitation results from QuantWiz^IQ. The Mapper and Combiner modules of HyperQuant allow facile integration of the labeled data, along with peptide spectrum match (PSM) intensity/ratio integration for proteins, respectively, for each PSM label combination. This also includes appropriate combination of replicates/fractions before summarizing the protein intensity/ratio, leading to robust quantitation. To the best of our knowledge, this is the first tool for the quantitation of HOM data with flexibility for any combination of MS¹ and MS² labels. We demonstrate its utility in analyzing two 18-plex data sets from the hyperplexing and the BONplex studies. The tool is open source and freely available for noncommercial use. HyperQuant is a highly valuable tool that will help in advancing the field of multiplexed quantitative proteomics.

Cinnamaldehyde inhibits type three secretion system in Salmonella enterica serovar Typhimurium by affecting the expression of key effector proteins

The increasing understanding of bacterial pathogenesis has revealed many new targets for the development of non-traditional antibacterial drugs. Interference with bacterial virulence has become a new strategy to treat bacteria-mediated diseases. As an important food-borne pathogen, Salmonella enterica serovar Typhimurium uses type III secretion system (T3SS) to facilitate invasion of host cells. In this study, we identified cinnamaldehyde as a Salmonella pathogenicity island 1 (SPI-1) inhibitor which blocks the secretion of several SPI-1 associated effector proteins and consequently exhibits a strong inhibitory effect on SPI-1-mediated invasion of HeLa cells. Further study revealed that cinnamaldehyde significantly reduced the transcription of some SPI-1 genes, such as sipA and sipB, in S. Typhimurium by affecting multiple SPI-1 regulator genes. In an animal infection model, cinnamaldehyde effectively protected infected mice against S. Typhimurium-induced mortality and pathological damages. In summary, this study presented an effective SPI-1 inhibitor, cinnamaldehyde, which reduces the expression of SPI-1 effector proteins by regulating the transcription of main regulator genes.

Pseudomonas aeruginosa cytotoxicity is attenuated at high cell density and associated with the accumulation of phenylacetic acid

Background

P. aeruginosa is known to cause acute cytotoxicity against various human and animal cells and tissues.

Methodology/Findings

Intriguingly, however, in this study we noticed that while a low cell density inoculum of P. aeruginosa caused severe cytotoxicity against human lung tissue cell line A549, increasing the cell density of bacterial inoculum led to decreased cytotoxicity. Addition of the supernatants from high density bacterial culture to low cell density inoculum protected the human cells from bacterial cytotoxic damage, suggesting that P. aeruginosa may produce and accumulate an inhibitory molecule(s) counteracting its pathogenic infection. The inhibitor was purified from the stationary-phase culture supernatants of P. aeruginosa strain PAO1 using bioassay-guided high performance liquid chromatography (HPLC), and characterized to be phenylacetic acid (PAA) by mass spectrometry and nuclear magnetic resonance spectroscopy. Microarray analysis revealed that treatment of P. aeruginosa with PAA down-regulated the transcriptional expression of Type III secretion system (T3SS) genes and related regulatory genes including rsmA and vfr, which were confirmed by transcriptional and translational analysis.

Conclusions

Identification of bacterial metabolite PAA as a T3SS-specific inhibitor explains this intriguing inverse cell-density-dependent-cytotoxicity phenomenon as T3SS is known to be a key virulence factor associated with cytotoxicity and acute infection. The findings may provide useful clues for design and development of new strategies to combat this formidable bacterial pathogen.

Extracellular amastigotes of Trypanosoma cruzi are potent inducers of phagocytosis in mammalian cells

The protozoan parasite Trypanosoma cruzi, the aetiological agent of Chagas' disease, has two infective life cycle stages, trypomastigotes and amastigotes. While trypomastigotes actively enter mammalian cells, highly infective extracellular amastigotes (type I T. cruzi) rely on actin-mediated uptake, which is generally inefficient in non-professional phagocytes. We found that extracellular amastigotes (EAs) of T. cruzi G strain (type I), but not Y strain (type II), were taken up 100-fold more efficiently than inert particles. Mammalian cell lines showed levels of parasite uptake comparable to macrophages, and extensive actin recruitment and polymerization was observed at the site of entry. EA uptake was not dependent on parasite-secreted molecules and required the same molecular machinery utilized by professional phagocytes during large particle phagocytosis. Transcriptional silencing of synaptotagmin VII and CD63 significantly inhibited EA internalization, demonstrating that delivery of supplemental lysosomal membrane to form the phagosome is involved in parasite uptake. Importantly, time-lapse live imaging using fluorescent reporters revealed phagosome-associated modulation of phosphoinositide metabolism during EA uptake that closely resembles what occurs during phagocytosis by macrophages. Collectively, our results demonstrate that T. cruzi EAs are potent inducers of phagocytosis in non-professional phagocytes, a process that may facilitate parasite persistence in infected hosts.

HLA-B27 modulates intracellular growth of Salmonella pathogenicity island 2 mutants and production of cytokines in infected monocytic U937 cells

BACKGROUND

Salmonella enterica serovar Enteritidis PT4 KS8822/88 replicates rapidly in HLA-B27-transfected human monocytic U937 cells. In this process, Salmonella pathogenicity island 2 (SPI-2) genes play a crucial role. Our previous study indicated that 118 Salmonella genes, including 8 SPI-2 genes were affected by HLA-B27 antigen during Salmonella infection of U937 cells.

METHODS/PRINCIPAL FINDINGS

To further investigate Salmonella replication in HLA-B27-positive U937 monocytic cells, two SPI-2 genes, ssaS and sscA up-regulated most during Salmonella infection of HLA-B27-transfected U937 cells, were mutated by using one-step gene disruption method. Intracellular survival and replication of the mutants in the U937 cells was compared to that of the wild type strain. Surprisingly, the two mutated strains replicated significantly more than the wild type bacteria in HLA-B27-transfected cells. Secretion of tumor necrosis factor alpha (TNF-α) and interleukin 10 (IL-10) was significantly induced during the infection of HLA-B27-transfected U937 cells with the mutants. The results indicated that the certain SPI-2 genes in wild type bacteria suppress Salmonella intracellular growth and production of cytokines in infected HLA-B27-transfected cells. HLA-B27-associated modulation of Salmonella SPI-2 genes and cytokine production may have importance in the persistent infection of the bacteria and the pathogenesis of reactive arthritis.

CONCLUSIONS

The study provides evidence that certain virulence factors of pathogens can reduce the intracellular growth in the host cells. We suggest that the limiting intracellular growth might be a strategy for persistence of bacteria in host cells, keeping a balance between pathogenic growth and pathogenesis.

Lactobacillus S-layer protein inhibition of Salmonella-induced reorganization of the cytoskeleton and activation of MAPK signalling pathways in Caco-2 cells

Surface layer (S-layer) proteins are crystalline arrays of proteinaceous subunits that are present as the outermost component of the cell wall in several Lactobacillus species. The S-layer proteins have been shown to play a role in the antimicrobial activity of certain lactobacilli. However, it is not fully understood how the S-layer proteins exert this biological function. The aim of this study was to test the hypothesis that Lactobacillus acidophilus S-layer proteins antagonize Salmonella Typhimurium (S. Typhimurium) infection by protecting against F-actin cytoskeleton rearrangements and the activation of mitogen-activated protein kinase (MAPK) signalling pathways. Monolayer transepithelial electrical resistance (TER) was measured after S. Typhimurium infection in Caco-2 cultured human intestinal cells with L. acidophilus S-layer proteins. F-actin rearrangement and MAPK activation were also assessed by immunofluorescence staining or Western blotting. The results showed that when S. Typhimurium was co-incubated with S-layer proteins, the S. Typhimurium-induced Caco-2 cell F-actin rearrangement was reduced, and the S. Typhimurium-induced TER decrease and interleukin 8 (IL-8) secretion were attenuated. Additionally, L. acidophilus S-layer proteins could inhibit S. Typhimurium-induced phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinase (JNK) and p38. This study indicates that L. acidophilus S-layer proteins are able to inhibit S. Typhimurium infection through blocking S. Typhimurium-induced F-actin rearrangements and S. Typhimurium-induced ERK1/2, JNK and p38 activation in Caco-2 cells. These data provide a rationale for the use of lactobacillus S-layer proteins as therapeutic and preventative agents, at least in infectious diarrhoea.

Oral treatment with Saccharomyces cerevisiae strain UFMG 905 modulates immune responses and interferes with signal pathways involved in the activation of inflammation in a murine model of typhoid fever

Salmonella spp. are Gram-negative, facultative, intracellular pathogens that cause several diarrheal diseases ranging from self-limiting gastroenteritis to typhoid fever. Previous results from our laboratory showed that Saccharomyces cerevisiae strain UFMG 905 isolated from 'cachaça' production presented probiotic properties due to its ability to protect against experimental infection with Salmonella enterica serovar Typhimurium. In this study, the effects of oral treatment with S. cerevisiae 905 were evaluated at the immunological level in a murine model of typhoid fever. Treatment with S. cerevisiae 905 inhibited weight loss and increased survival rate after Salmonella challenge. Immunological data demonstrated that S. cerevisiae 905 decreased levels of proinflammatory cytokines and modulated the activation of mitogen-activated protein kinases (p38 and JNK, but not ERK1/2), NF-κB and AP-1, signaling pathways which are involved in the transcriptional activation of proinflammatory mediators. Experiments in germ-free mice revealed that probiotic effects were due, at least in part, to the binding of Salmonella to the yeast. In conclusion, S. cerevisiae 905 acts as a potential new biotherapy against S. Typhimurium infection due to its ability to bind bacteria and modulate signaling pathways involved in the activation of inflammation in a murine model of typhoid fever.

The draft genome sequence of Arsenophonus nasoniae, son-killer bacterium of Nasonia vitripennis, reveals genes associated with virulence and symbiosis

Four percent of female Nasonia vitripennis carry the son-killer bacterium Arsenophonus nasoniae, a microbe with notably different biology from other inherited parasites and symbionts. In this paper, we examine a draft genome sequence of the bacterium for open reading frames (ORFs), structures and pathways involved in interactions with its insect host. The genome data suggest that A. nasoniae carries multiple type III secretion systems, and an array of toxin and virulence genes found in Photorhabdus, Yersinia and other gammaproteobacteria. Of particular note are ORFs similar to those known to affect host innate immune functioning in other bacteria, and four ORFs related to pro-apoptotic exotoxins. The genome sequences for both A. nasoniae and its Nasonia host are useful tools for examining functional genomic interactions of microbial survival in hostile immune environments, and mechanisms of passage through gut epithelia, in a whole organism context.

Interaction of Saccharomyces boulardii with Salmonella enterica serovar Typhimurium protects mice and modifies T84 cell response to the infection

Background

Salmonella pathogenesis engages host cells in two-way biochemical interactions: phagocytosis of bacteria by recruitment of cellular small GTP-binding proteins induced by the bacteria, and by triggering a pro-inflammatory response through activation of MAPKs and nuclear translocation of NF-κB. Worldwide interest in the use of functional foods containing probiotic bacteria for health promotion and disease prevention has increased significantly. Saccharomyces boulardii is a non-pathogenic yeast used as a probiotic in infectious diarrhea.

Methodology/Principal Findings

In this study, we reported that S. boulardii (Sb) protected mice from Salmonella enterica serovar Typhimurium (ST)-induced death and prevented bacterial translocation to the liver. At a molecular level, using T84 human colorectal cancer cells, we demonstrate that incubation with Sb before infection totally abolished Salmonella invasion. This correlates with a decrease of activation of Rac1. Sb preserved T84 barrier function and decreased ST-induced IL-8 synthesis. This anti-inflammatory effect was correlated with an inhibitory effect of Sb on ST-induced activation of the MAPKs ERK1/2, p38 and JNK as well as on activation of NF-κB. Electron and confocal microscopy experiments showed an adhesion of bacteria to yeast cells, which could represent one of the mechanisms by which Sb exerts its protective effects.

Conclusions

Sb shows modulating effects on permeability, inflammation, and signal transduction pathway in T84 cells infected by ST and an in vivo protective effect against ST infection. The present results also demonstrate that Sb modifies invasive properties of Salmonella.

Modulation of bacterial Type III secretion system by a spermidine transporter dependent signaling pathway

Background

Many gram-negative bacterial pathogens employ Type III secretion systems (T3SS) to inject effector proteins into host cells in infectious processes.

Methodology/Principal Findings

By screening a transposon mutant library of P. aeruginosa, we found that mutation of spuDEFGH, which encode a major spermidine uptake system, abolished the expression of the exsCEBA operon that codes for key T3SS regulators under inducing conditions (low calcium). Whole genome microarray analysis revealed that inactivation of the spermidine uptake system significantly decreased the transcriptional expression of most, if not all, T3SS genes. Consistently, the spermidine uptake mutants showed decreased expression of the T3SS genes in responding to host cell extract and attenuated cytotoxicity. Furthermore, exogenous addition of spermidine to the wild type strain PAO1 enhanced the expression of exsCEBA and also the effector protein genes.

Conclusion/Significance

Cumulatively, these data have depicted a novel spermidine transporter-dependent signaling pathway, which appears to play an essential role in modulation of T3SS expression in P. aeruginosa.

Bringing order to a complex molecular machine: the assembly of the bacterial flagella

The bacterial flagellum is an example of elegance in molecular engineering. Flagella dependent motility is a widespread and evolutionarily ancient trait. Diverse bacterial species have evolved unique structural adaptations enabling them to migrate in their environmental niche. Variability exists in the number, location and configuration of flagella, and reflects unique adaptations of the microorganism. The most detailed analysis of flagellar morphogenesis and structure has focused on Escherichia coli and Salmonella enterica. The appendage assembles sequentially from the inner to the outer-most structures. Additionally the temporal order of gene expression correlates with the assembly order of encoded proteins into the final structure. The bacterial flagellar apparatus includes an essential basal body complex that comprises the export machinery required for assembly of the hook and flagellar filament. A review outlining the current understanding of the protein interactions that make up this remarkable structure will be presented, and the associated temporal genetic regulation will be briefly discussed.

Immunology of Crohn's disease

The immense microbiological load of the gastrointestinal tract poses a daunting challenge for the mucosal immune system: whereas it should tolerate the vast number of commensal bacteria, it should adequately attack pathogenic organisms. Millions of years of co-evolution have produced an intricate system in which interactions between the endogenous flora and mucosal immune system manage to perform this difficult balancing act. When components of this interaction are defective, for instance by mutation, inflammatory bowel disease may result. In the present review, we comprehensively discuss the mucosal immune system in the context of Crohn's disease (CD) and its genetic risk factors, describe the clinical management of the disease, and discuss how knowledge of the mucosal immune system may yield novel therapeutical avenues for dealing with this debilitating disease.

PtdIns5P activates the host cell PI3-kinase/Akt pathway during Shigella flexneri infection

The virulence factor IpgD, delivered into nonphagocytic cells by the type III secretion system of the pathogen Shigella flexneri, is a phosphoinositide 4-phosphatase generating phosphatidylinositol 5 monophosphate (PtdIns5P). We show that PtdIns5P is rapidly produced and concentrated at the entry foci of the bacteria, where it colocalises with phosphorylated Akt during the first steps of infection. Moreover, S. flexneri-induced phosphorylation of host cell Akt and its targets specifically requires IpgD. Ectopic expression of IpgD in various cell types, but not of its inactive mutant, or addition of short-chain penetrating PtdIns5P is sufficient to induce Akt phosphorylation. Conversely, sequestration of PtdIns5P or reduction of its level strongly decreases Akt phosphorylation in infected cells or in IpgD-expressing cells. Accordingly, IpgD and PtdIns5P production specifically activates a class IA PI 3-kinase via a mechanism involving tyrosine phosphorylations. Thus, S. flexneri parasitism is shedding light onto a new mechanism of PI 3-kinase/Akt activation via PtdIns5P production that plays an important role in host cell responses such as survival.

A Role for the SmpB-SsrA system in Yersinia pseudotuberculosis pathogenesis

Yersinia utilizes a sophisticated type III secretion system to enhance its chances of survival and to overcome the host immune system. SmpB (small protein B) and SsrA (small stable RNA A) are components of a unique bacterial translational control system that help maintain the bacterial translational machinery in a fully operational state. We have found that loss of the SmpB-SsrA function causes acute defects in the ability of Yersinia pseudotuberculosis to survive in hostile environments. Most significantly, we show that mutations in smpB-ssrA genes render the bacterium avirulent and unable to cause mortality in mice. Consistent with these observations, we show that the mutant strain is unable to proliferate in macrophages and exhibits delayed Yop-mediated host cell cytotoxicity. Correspondingly, we demonstrate that the smpB-ssrA mutant suffers severe deficiencies in expression and secretion of Yersinia virulence effector proteins, and that this defect is at the level of transcription. Of further interest is the finding that the SmpB-SsrA system might play a similar role in the related type III secretion system that governs flagella assembly and bacterial motility. These findings highlight the significance of the SmpB-SsrA system in bacterial pathogenesis, survival under adverse environmental conditions, and motility.

Bacteria have evolved sophisticated mechanisms to monitor, adapt, and respond to environmental and host-mediated assaults. Many Gram-negative pathogenic bacteria utilize a needle-like type III secretion system (TTSS) to inject a cocktail of effector proteins into host cells, disabling the host defenses against the pathogen. There is evolutionary, structural, and sequence similarity between this TTSS and the bacterial motility apparatus, the flagellum. Experiments described in this study examine the role played by the SmpB-SsrA system in Yersinia virulence, motility, and adaptation to adverse environments. The authors present evidence to demonstrate that an smpB-ssrA mutant of Yersinia pseudotuberculosis is more sensitive to adverse environmental conditions, lacks motility, exhibits severe defects in Yop secretion, and is avirulent in a mouse infection model. On the basis of these findings, they postulate that the SmpB-SsrA system, through its ribosome rescue, and protein tagging for directed degradation functions, affects the expression of the Ysc-Yop TTSS, and likely the flagellar TTSS, at the level of transcription. Their findings are consistent with a proposed regulatory role for the SmpB-SsrA system in regulation of bacterial gene expression.

Role of Protein Phosphorylation on Serine/Threonine and Tyrosine in the Virulence of Bacterial Pathogens

Bacterial pathogens have developed a diversity of strategies to interact with host cells, manipulate their behaviors, and thus to survive and propagate. During the process of pathogenesis, phosphorylation of proteins on hydroxyl amino acids (serine, threonine, tyrosine) occurs at different stages, including cell-cell interaction and adherence, translocation of bacterial effectors into host cells, and changes in host cellular structure and function induced by infection. The phosphorylation reactions are catalyzed in a reversible fashion by specific protein kinases and phosphatases that belong to either the invading bacterial cells or the infected eukaryotic host cells. Among the various virulence factors involved in bacterial pathogenesis, special attention has been paid recently to the cell wall components, exopolysaccharides. A major breakthrough has been made by showing the existence of a biological link between the activity of certain protein-tyrosine kinases/phosphatases and the production and/or transport of surface polysaccharides. In addition, genetic studies have revealed a key role played by some serine/threonine kinases in pathogenesis. Considering the structural organization and membrane topology of these different kinases, it can be envisaged that they operate as one-component systems in signal transduction pathways, in the form of single proteins containing input and output domains on the same polypeptide chain. From a general standpoint, the demonstration of a direct relationship between protein phosphorylation on serine/threonine/tyrosine and bacterial virulence represents a novel concept of great importance in deciphering the molecular and cellular mechanisms that underlie pathogenesis.

Physical and Functional Association of c-Src and Adhesion and Degranulation Promoting Adaptor Protein (ADAP) in Osteoclastogenesis in Vitro

c-Src plays a crucial role in osteoclastogenesis. In this study, we searched for c-Src-binding proteins using a combination of pull-down assays and mass spectrometric analysis, and identified the association of adhesion and degranulation promoting adaptor protein (ADAP) with c-Src in RAW264 cells and osteoclast precursors prepared from bone marrow cells. The kinase activity and the SH2 domain of c-Src were required for this association and Tyr807 in the extreme carboxyl terminus of ADAP was identified as a major recognition site. ADAP was found to be expressed in cells at the prefusion stage and localized mainly in the leading edge of lamellipodia and in pseudopodia. Tyrosine phosphorylation of ADAP was induced in an integrin-dependent manner, and the level was Src kinase-dependent. ADAP-knockdown RAW264 cells showed retarded migration and formed few multinucleated cells. Cas, known to be phosphorylated by c-Src, was identified as a major tyrosine-phosphorylated protein in differentiating RAW264 cells and the phosphorylation appeared to be decreased in ADAP-knockdown cells. ADAP thus may play an important role as a partner of c-Src for cell migration and progression to the multinucleated cell stage in osteoclastogenesis in vitro.

Frequency and polymorphism of sopE in isolates of Salmonella enterica belonging to the ten most prevalent serotypes in England and Wales

Translocated effector protein, SopE, leads to actin cytoskeletal rearrangements and membrane ruffling. Only a subset of Salmonella enterica serotypes possess sopE, with the majority of sopE-carrying S. enterica serotype Typhimurium associated with epidemics. Using real-time PCR and sequencing, sopE was investigated in the ten most prevalent serotypes of S. enterica in England and Wales in 2001. sopE was identified in S. Typhimurium definitive phage types 29, 44, 49, 204b and 204c, all of which either have been involved in major epidemics or are precursors of epidemic strains. The presence of sopE varied in the remaining nine serotypes, but was more common in the top four (Enteritidis, Virchow, Hadar and Newport). Nucleotide changes were detected throughout sopE and may result in altered specificity for certain signal transduction pathways. Since acquisition of sopE may play a key role in emergence of epidemic strains, detection of sopE could aid identification of those Salmonella strains with the potential for epidemic spread.

Cloning, overexpression, and characterization of a serine/threonine protein kinase pknI from Mycobacterium tuberculosis H37Rv

Protein phosphorylation-dephosphorylation is the principal mechanism for translation of external signals into cellular responses. Eukaryotic-like serine/threonine kinases have been reported to play important roles in bacterial development and/or virulence. The PknI protein is one of the 11 eukaryotic-like serine/threonine kinases in Mycobacterium tuberculosis H37Rv. From the bioinformatic studies, PknI protein has been shown to have an N-terminal cytoplasmic domain followed by a transmembrane region and an extracellular C-terminus suggestive of a sensor molecule. In this study, we have cloned, overexpressed, and characterized the entire coding region and the cytoplasmic domain of PknI as a fusion protein with an N-terminal histidine tag, and used immobilized metal affinity chromatography for purification of recombinant proteins. The purified recombinant proteins were found to be functionally active through in vitro phosphorylation assay and phosphoamino acid analysis. In vitro kinase assay of both proteins revealed that PknI is capable of autophosphorylation and showed manganese-dependent activity. Phosphoamino acid analysis indicated phosphorylation at serine and threonine residues. Southern blot analysis with genomic DNA highlighted the conserved nature of pknI among the various mycobacterial species. In silico analysis revealed a close homology of PknI to Stk1 from Streptococcus agalactiae, shown to have a role in virulence and cell segregation of the organism.

Bacterial and host factors involved in the major histocompatibility complex class Ib-restricted presentation of Salmonella Hsp 60: novel pathway

Previously, a peptide epitope derived from the Hsp 60 molecule of Salmonella that is presented by the major histocompatibility complex (MHC) class Ib molecule Qa-1 to CD8(+) cytotoxic T cells (CTLs) was described. In the present study we investigated the Salmonella-induced processing and presentation pathway for generating this Qa-1-restricted epitope. Live bacteria and, to a lesser extent, opsonized heat-killed bacteria are able to sensitize target cells for lysis by Salmonella-specific CTL. In contrast, heat-killed bacteria cannot sensitize target cells. Presentation of the Hsp 60 epitope appears independent of bacterial internalization, because cytochalasin D does not affect presentation. Moreover, Salmonella strains defective in the InvA or InvE operon, two critical components of the type III secretion pathway, are as efficient as wild-type Salmonella enterica serovar Typhimurium in sensitizing infected targets to lysis. Collectively, these results suggest the existence of a novel antigen-processing pathway in which exogenous antigens gain access to the cytosolic MHC class I processing machinery. Considering the abundant nature of bacterial Hsp 60 and the upregulation of this protein after Salmonella infection of eukaryotic cells, this mode of antigen presentation may be particularly relevant to understanding the host defense mechanisms against gram-negative bacteria.

Mycobacterium avium-superoxide dismutase binds to epithelial cell aldolase, glyceraldehyde-3-phosphate dehydrogenase and cyclophilin A

Mycobacterium avium complex (MAC) adheres, invades and multiplies inside epithelial cells. Earlier, we demonstrated two MAC protein adhesins, 25 and 31 kDa, binding with HEp-2 cells. The 25 kDa MAC adhesin was found to be superoxide dismutase (SOD). In this study, epithelial cell (HEp-2 and A549) ligands for MAC-SOD were identified by probing two-dimensional western blots of epithelial extracts with MAC proteins followed by monoclonal anti-MAC-SOD antibodies. Three epithelial cell proteins with molecular masses 43, 40 and 18 kDa, present in both membrane and cytosolic fractions, were found to bind with MAC-SOD. Based on the N-terminal amino acid sequences, the 43, 40 and 18 kDa epithelial proteins were identified as aldolase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and cyclophilin A (CypA), respectively. Furthermore, MAC-SOD was found to bind to purified rabbit muscle aldolase, GAPDH and recombinant CypA in western blotting.

M1 protein triggers a phosphoinositide cascade for group A Streptococcus invasion of epithelial cells

Invasion of nonphagocytic cells by bacteria provides a favorable niche for persistence and evasion of host defenses and antibiotics. M protein is a major virulence factor because it promotes high-frequency invasion of epithelial cells by group A Streptococcus (GAS) and also renders the bacterium resistant to phagocytosis. In this study, we investigated the role of M1 protein from serotype M1 strain 90-226 in regulating mammalian signal transduction and cytoskeletal rearrangement for bacterial entry. LY294002 and wortmannin, which are inhibitors of phosphatidylinositol 3-kinase (PI 3-K) blocked invasion of epithelial cells by GAS by 75 and 80%, respectively, but failed to inhibit invasion by Salmonella enterica serovar Typhimurium. Also, epithelial cells transiently transfected with dominant negative p85 and p110 genes, the regulatory and catalytic subunits of PI 3-K, respectively, were less able to be invaded by GAS. To separate the influence of other streptococcal virulence factors from M protein, Lactococcus lactis was engineered to express M1 protein on its surface. L. lactis(pLM1) invaded epithelial cells efficiently in vitro, and PI 3-K inhibitors blocked 90% of this invasion. Purified soluble M1 protein stimulated the formation of stress fibers and actin tuffs on epithelial cells. LY294002 and wortmannin inhibited these cellular changes. A phosphoinositide analogue also inhibited the invasion of epithelial cells by GAS. Therefore, M1 protein, either directly or via bound fibronectin, initiates signals that depend on the lipid kinase PI 3-K pathway, which paves the way for cytoskeletal rearrangement that internalize the bacterium.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Proinflammatory signalling stimulated by the type III translocation factor YopB is counteracted by multiple effectors in epithelial cells infected with Yersinia pseudotuberculosis

Type III secretion systems are used by several pathogens to translocate effector proteins into host cells. Yersinia pseudotuberculosis delivers several Yop effectors (e.g. YopH, YopE and YopJ) to counteract signalling responses during infection. YopB, YopD and LcrV are components of the translocation machinery. Here, we demonstrate that a type III translocation protein stimulates proinflammatory signalling in host cells, and that multiple effector Yops counteract this response. To examine proinflammatory signalling by the type III translocation machinery, HeLa cells infected with wild-type or Yop-Y. pseudotuberculosis strains were assayed for interleukin (IL)-8 production. HeLa cells infected with a YopEHJ- triple mutant released significantly more IL-8 than HeLa cells infected with isogenic wild-type, YopE-, YopH- or YopJ- bacteria. Complementation analysis demonstrated that YopE, YopH or YopJ are sufficient to counteract IL-8 production. IL-8 production required YopB, but did not require YopD, pore formation or invasin-mediated adhesion. In addition, YopB was required for activation of nuclear factor kappa B, the mitogen-activated protein kinases ERK and JNK and the small GTPase Ras in HeLa cells infected with the YopEHJ- mutant. We conclude that interaction of the Yersinia type III translocator factor YopB with the host cell triggers a proinflammatory signalling response that is counteracted by multiple effectors in host cells.

Rho GTPase is activated by cytotoxic necrotizing factor 1 in peripheral blood T lymphocytes: potential cytotoxicity for intestinal epithelial cells

Some strains of Escherichia coli related to acute cystitis or colitis produce a toxin named cytotoxic necrotizing factor 1 (CNF-1). CNF-1 mediates its effects on epithelial cells or phagocytes via the permanent activation of small GTP-binding proteins, caused by the toxin-induced deamidation of Glu(63) of p21 Rho. The behavior of peripheral blood T lymphocytes during the acute phase of bacterial colitis has been poorly investigated. Our study was conducted to test whether (i) peripheral blood T lymphocytes can be activated by CNF-1 and (ii) CNF-1-activated T lymphocytes are cytotoxic against intestinal epithelial cells. Activation of T lymphocytes by CNF-1 was assessed by electrophoresis, flow cytometry, confocal microscopy, and electron microscopy studies. Assays for migration and adherence of CNF-1-treated T lymphocytes were performed in Transwell chambers with T84 intestinal epithelial cells grown on polycarbonate semipermeable filters. CNF-1 induced a decrease in the electrophoretic mobility of the GTP-binding protein Rho in treated T lymphocytes. CNF-1 provoked an increase in the content of actin stress fibers and pseudopodia in T lymphocytes. Several adherence molecules were clustered into cytoplasmic projections in CNF-1-treated T lymphocytes and adherence of such lymphocytes on the basolateral pole of T84 was increased, resulting in cytotoxicity toward epithelial cells. Such enhanced adherence in response to CNF-1 was dependent on p42-44(MAP) kinase activation of T lymphocytes. Taken together, these results suggest that CNF-1, by acting on T lymphocytes, may increase in an important fashion the virulence of certain strains of E. coli against the intestinal epithelia.

Conversion of PtdIns(4,5)P(2) into PtdIns(5)P by the S.flexneri effector IpgD reorganizes host cell morphology

Phosphoinositides play a central role in the control of several cellular events including actin cytoskeleton organization. Here we show that, upon infection of epithelial cells with the Gram-negative pathogen Shigella flexneri, the virulence factor IpgD is translocated directly into eukaryotic cells and acts as a potent inositol 4-phosphatase that specifically dephosphorylates phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] into phosphatidylinositol 5-monophosphate [PtdIns(5)P] that then accumulates. Transfection experiments indicate that the transformation of PtdIns(4,5)P(2) into PtdIns(5)P by IpgD is responsible for dramatic morphological changes of the host cell, leading to a decrease in membrane tether force associated with membrane blebbing and actin filament remodelling. These data provide the molecular basis for a new mechanism employed by a pathogenic bacterium to promote membrane ruffling at the entry site.

Lysosomes and the plasma membrane: trypanosomes reveal a secret relationship

Studies of the cell invasion mechanism of the parasite Trypanosoma cruzi led to a series of novel findings, which revealed a previously unsuspected ability of conventional lysosomes to fuse with the plasma membrane. This regulated exocytic process, previously regarded mostly as a specialization of certain cell types, was recently shown to play an important role in the mechanism by which cells reseal their plasma membrane after injury.

Proteolytic inhibition of Salmonella enterica serovar typhimurium-induced activation of the mitogen-activated protein kinases ERK and JNK in cultured human intestinal cells

Bromelain, a mixture of cysteine proteases from pineapple stems, blocks signaling by the mitogen-activated protein (MAP) kinases extracellular regulated kinase 1 (ERK-1) and ERK-2, inhibits inflammation, and protects against enterotoxigenic Escherichia coli infection. In this study, we examined the effect of bromelain on Salmonella enterica serovar Typhimurium infection, since an important feature of its pathogenesis is its ability to induce activation of ERK-1 and ERK-2, which leads to internalization of bacteria and induction of inflammatory responses. Our results show that bromelain dose dependently blocks serovar Typhimurium-induced ERK-1, ERK-2, and c-Jun NH(2)-terminal kinase (JNK) activation in Caco-2 cells. Bromelain also blocked signaling induced by carbachol and anisomycin, pharmacological MAP kinase agonists. Despite bromelain inhibition of serovar Typhimurium-induced MAP kinase signaling, it did not prevent subsequent invasion of the Caco-2 cells by serovar Typhimurium or alter serovar Typhimurium -induced decreases in resistance across Caco-2 monolayers. Surprisingly, bromelain also did not block serovar Typhimurium-induced interleukin-8 (IL-8) secretion but synergized with serovar Typhimurium to enhance IL-8 production. We also found that serovar Typhimurium does not induce ERK phosphorylation in Caco-2 cells in the absence of serum but that serovar Typhimurium-induced invasion and decreases in monolayer resistance are unaffected. Collectively, these data indicate that serovar Typhimurium-induced invasion of Caco-2 cells, changes in the resistance of epithelial cell monolayers, and IL-8 production can occur independently of the ERK and JNK signaling pathways. Data also confirm that bromelain is a novel inhibitor of MAP kinase signaling pathways and suggest a novel role for proteases as inhibitors of signal transduction pathways in intestinal epithelial cells.

Three-dimensional tissue assemblies: novel models for the study of Salmonella enterica serovar Typhimurium pathogenesis

The lack of readily available experimental systems has limited knowledge pertaining to the development of Salmonella-induced gastroenteritis and diarrheal disease in humans. We used a novel low-shear stress cell culture system developed at the National Aeronautics and Space Administration in conjunction with cultivation of three-dimensional (3-D) aggregates of human intestinal tissue to study the infectivity of Salmonella enterica serovar Typhimurium for human intestinal epithelium. Immunohistochemical characterization and microscopic analysis of 3-D aggregates of the human intestinal epithelial cell line Int-407 revealed that the 3-D cells more accurately modeled human in vivo differentiated tissues than did conventional monolayer cultures of the same cells. Results from infectivity studies showed that Salmonella established infection of the 3-D cells in a much different manner than that observed for monolayers. Following the same time course of infection with Salmonella, 3-D Int-407 cells displayed minimal loss of structural integrity compared to that of Int-407 monolayers. Furthermore, Salmonella exhibited significantly lower abilities to adhere to, invade, and induce apoptosis of 3-D Int-407 cells than it did for infected Int-407 monolayers. Analysis of cytokine expression profiles of 3-D Int-407 cells and monolayers following infection with Salmonella revealed significant differences in expression of interleukin 1alpha (IL-1alpha), IL-1beta, IL-6, IL-1Ra, and tumor necrosis factor alpha mRNAs between the two cultures. In addition, uninfected 3-D Int-407 cells constitutively expressed higher levels of transforming growth factor beta1 mRNA and prostaglandin E2 than did uninfected Int-407 monolayers. By more accurately modeling many aspects of human in vivo tissues, the 3-D intestinal cell model generated in this study offers a novel approach for studying microbial infectivity from the perspective of the host-pathogen interaction.

A bacterial type III secretion system inhibits actin polymerization to prevent pore formation in host cell membranes

The bacterial pathogen Yersinia pseudotuberculosis uses type III secretion machinery to translocate Yop effector proteins through host cell plasma membranes. A current model suggests that a type III translocation channel is inserted into the plasma membrane, and if Yops are not present to fill the channel, the channel will form a pore. We examined the possibility that Yops act within the host cell to prevent pore formation. Yop- mutants of Y.pseudotuberculosis were assayed for pore-forming activity in HeLa cells. A YopE- mutant exhibited high levels of pore-forming activity. The GTPase-downregulating function of YopE was required to prevent pore formation. YopE+ bacteria had increased pore-forming activity when HeLa cells expressed activated Rho GTPases. Pore formation by YopE- bacteria required actin polymerization. F-actin was concentrated at sites of contact between HeLa cells and YopE- bacteria. The data suggest that localized actin polymerization, triggered by the type III machinery, results in pore formation in cells infected with YopE- bacteria. Thus, translocated YopE inhibits actin polymerization to prevent membane damage to cells infected with wild-type bacteria.

Unusual molecular architecture of the Yersinia pestis cytotoxin YopM: a leucine-rich repeat protein with the shortest repeating unit

Many Gram-negative bacterial pathogens employ a contact-dependent (type III) secretion system to deliver effector proteins into the cytosol of animal or plant cells. Collectively, these effectors enable the bacteria to evade the immune response of the infected organism by modulating host-cell functions. YopM, a member of the leucine-rich repeat protein superfamily, is an effector produced by the bubonic plague bacterium, Yersinia pestis, that is essential for virulence. Here, we report crystal structures of YopM at 2.4 and 2.1 A resolution. Among all leucine-rich repeat family members whose atomic coordinates have been reported, the repeating unit of YopM has the least canonical secondary structure. In both crystals, four YopM monomers form a hollow cylinder with an inner diameter of 35 A. The domain that targets YopM for translocation into eukaryotic cells adopts a well-ordered, alpha-helical conformation that packs tightly against the proximal leucine-rich repeat module. A similar alpha-helical domain can be identified in virulence-associated leucine-rich repeat proteins produced by Salmonella typhimurium and Shigella flexneri, and in the conceptual translation products of several open reading frames in Y. pestis.

Microarrays for studying the host transcriptional response to microbial infection and for the identification of host drug targets

It is essential to understand the molecular basis of a host's response to microbial infection so that disease and tissue damage can be prevented. Modulation of host RNA expression is a critical set of molecular changes that occur upon infection. Global analysis of gene expression should provide an understanding of host RNA transcriptional changes that occur upon host-pathogen interaction. This series of articles focuses on the use of microarrays for analyzing the transcriptional response of a host to microbial infection and for drug target identification.

Platelet activation by a relapsing fever spirochaete results in enhanced bacterium-platelet interaction via integrin alphaIIbbeta3 activation

Borrelia hermsii, a spirochaete responsible for relapsing fever in humans, grows to high density in the bloodstream and causes thrombocytopenia. We show here that B. hermsii binds to human platelets. Extended culture in bacteriological medium resulted in both diminished infectivity in vivo and diminished platelet binding in vitro. Platelet binding was promoted by the platelet integrin alphaIIbbeta3: the bacterium bound to purified integrin alphaIIbbeta3, and bacterial binding to platelets was diminished by alphaIIbbeta3 antagonists or by a genetic defect in this integrin. Integrin alphaIIbbeta3 undergoes a conformational change upon platelet activation, and bacteria bound more efficiently to activated rather than resting platelets. Nevertheless, B. hermsii bound at detectable levels to preparations of resting platelets. The bacterium did not recognize a point mutant of alphaIIbbeta3 that cannot acquire an active conformation. Rather, B. hermsii was capable of triggering platelet and integrin alphaIIbbeta3 activation, as indicated by the expression of the platelet activation marker P-selectin and integrin alphaIIbbeta3 in its active conformation. The degree of platelet activation varied depending upon bacterial strain and growth conditions. Prostacyclin I2, an inhibitor of platelet activation, diminished bacterial attachment, indicating that activation enhanced bacterial binding. Thus, B. hermsii signals the host cell to activate a critical receptor for the bacterium, thereby promoting high-level bacterial attachment.

A Salmonella inositol polyphosphatase acts in conjunction with other bacterial effectors to promote host cell actin cytoskeleton rearrangements and bacterial internalization

A central feature of Salmonella pathogenicity is the bacterium's ability to enter into non-phagocytic cells. Bacterial internalization is the consequence of cellular responses characterized by Cdc42- and Rac-dependent actin cytoskeleton rearrangements. These responses are triggered by the co-ordinated function of bacterial proteins delivered into the host cell by a specialized protein secretion system termed type III. We report here that SopB, a Salmonella inositol polyphosphatase delivered to the host cell by this secretion system, mediates actin cytoskeleton rearrangements and bacterial entry in a Cdc42-dependent manner. SopB exhibits overlapping functions with two other effectors of bacterial entry, the Rho family GTPase exchange factors SopE and SopE2. Thus, Salmonella strains deficient in any one of these proteins can enter into cells at high efficiency, whereas a strain lacking all three effectors is completely defective for entry. Consistent with an important role for inositol phosphate metabolism in Salmonella-induced cellular responses, a catalytically defective mutant of SopB failed to stimulate actin cytoskeleton rearrangements and bacterial entry. Furthermore, bacterial infection of intestinal cells resulted in a marked increase in Ins(1,4,5,6)P4, a consumption of InsP5 and the activation of phospholipase C. In agreement with the in vivo findings, purified SopB specifically dephosphorylated InsP5 to Ins(1,4,5,6)P4 in vitro. Surprisingly, the inositol phosphate fluxes induced by Salmonella were not caused exclusively by SopB. We show that the SopB-independent inositol phosphate fluxes are the consequence of the SopE-dependent activation of an endogenous inositol phosphatase. The ability of Salmonella to stimulate Rho GTPases signalling and inositol phosphate metabolism through alternative mechanisms is an example of the remarkable ability of this bacterial pathogen to manipulate host cellular functions.

Identification of functional regions within invasion plasmid antigen C (IpaC) of Shigella flexneri

Shigella flexneri causes bacillary dysentery with symptoms resulting from the inflammation that accompanies bacterial entry into the cells of the colonic epithelium. The effectors of S. flexneri invasion are the Ipa proteins, particularly IpaB and IpaC, which are secreted at the host-pathogen interface following bacterial contact with a host cell. Of the purified Ipa proteins, only IpaC has been shown to possess quantifiable in vitro activities that are related to cellular invasion. In this study, ipaC deletion mutants were generated to identify functional regions within the IpaC protein. From these data, we now know that the N-terminus and an immunogenic central region are not required for IpaC-dependent enhancement of cellular invasion by S. flexneri. However, to restore invasiveness to an ipaC null mutant of S. flexneri, the N-terminus is essential, because IpaC mutants lacking the N-terminus are not secreted by the bacterium. Deletion of the central hydrophobic region eliminates IpaC's ability to interact with phospholipid membranes, and fusion of this region to a modified form of green fluorescent protein converts it into an efficient membrane-associating protein. Meanwhile, deletion of the C-terminus eliminates the mutant protein's ability to establish protein-protein contacts with full-length IpaC. Interestingly, the mutant form of ipaC that restores partial invasiveness to the S. flexneri ipaC null mutant also restores full contact-mediated haemolysis activity to this bacterium. These data support a model in which IpaC possesses a distinct functional organization that is important for bacterial invasion. This information will be important in defining the precise role of IpaC in S. flexneri pathogenesis and in exploring the potential effects of purified IpaC at mucosal surfaces.