Protein-peptidoglycan interactions modulate the assembly of the needle complex in the Salmonella invasion-associated type III secretion system

Morphogenetic penicillin-binding proteins control virulence-associated type III secretion systems in Salmonella

Type III protein secretion systems (T3SSs) function as multiprotein devices that span the envelope of Gram-negative bacteria using the peptidoglycan (PG) layer as scaffold. This spatial arrangement explains why modifications in PG structure can alter T3SS activity. In Salmonella, incorporation of non-canonical D-amino acids in the PG was shown to decrease the activity of the T3SS encoded by the pathogenicity island-1 (SPI-1) without affecting other T3SS, like the flagellum apparatus. Enigmatically, following invasion of host cell Salmonella enterica serovar Typhimurium modifies PG synthesis by upregulating two pathogen-specific enzymes, the penicillin-binding proteins PBP2SAL and PBP3SAL, with roles in cell elongation and division, respectively. In the mouse typhoid model, the amount of PBP2SAL and PBP3SAL produced by the pathogen exceeds by large those of the canonical enzymes PBP2 and PBP3. This change responds to acidity and high osmolarity, the same cues that intra-phagosomal S. Typhimurium perceives to switch the SPI-1 T3SS by that encoded in SPI-2. Using isogenic mutants lacking each of the four morphogenetic PBPs, we tested whether their activities and those of the T3SS encoded by SPI-1 and SPI-2, are interconnected. Our data show that PBP2 is required for proper function of SPI-1 T3SS but dispensable for motility, whereas the lack of any of the morphogenetic PBPs increases SPI-2 T3SS activity. The positive control exerted by PBP2 on SPI-1 takes place via the SPI-1-specific regulators HilA and InvF. To our knowledge, these findings provide the first evidence linking morphogenetic enzymes that synthesize PG with T3SS associated to virulence.

Development of recombinant subunit vaccine targeting InvH protein of Salmonella Typhimurium and evaluation of its immunoprotective efficacy against salmonellosis

Salmonella Typhimurium is the most prevalent non-host specific Salmonella serovars and a major concern for both human and animal health systems worldwide contributing to significant economic loss. Type 3 secretion system (T3SS) of Salmonella plays an important role in bacterial adherence and entry into the host epithelial cells. The product of invH gene of Salmonella is an important component of the needle complex of the type 3 secretion system. Hence, the present study was undertaken to clone and express the 15 kDa InvH surface protein of Salmonella Typhimurium in an E. coli host and to evaluate its immune potency in mice. The purified recombinant InvH (r-InvH) protein provoked a significant (p < 0.01) rise in IgG in the inoculated mice. The immunized mice were completely (100%) protected against the challenge dose of 107.5 LD50, while protection against challenge with the same dose of heterologous serovars was 90%. The bacterin-vaccinated group showed homologous protection of 60% against all three serovars. Findings in this study suggest the potential of the r-InvH protein of S. Typhimurium as an effective vaccine candidate against Salmonella infections.

Scaffolding Protein GspB/OutB Facilitates Assembly of the Dickeya dadantii Type 2 Secretion System by Anchoring the Outer Membrane Secretin Pore to the Inner Membrane and to the Peptidoglycan Cell Wall

The phytopathogenic proteobacterium Dickeya dadantii secretes an array of plant cell wall-degrading enzymes and other virulence factors via the type 2 secretion system (T2SS). T2SSs are widespread among important plant, animal, and human bacterial pathogens. This multiprotein complex spans the double membrane cell envelope and secretes fully folded proteins through a large outer membrane pore formed by 15 subunits of the secretin GspD. Secretins are also found in the type 3 secretion system and the type 4 pili. Usually, specialized lipoproteins termed pilotins assist the targeting and assembly of secretins into the outer membrane. Here, we show that in D. dadantii, the pilotin acts in concert with the scaffolding protein GspB. Deletion of gspB profoundly impacts secretin assembly, pectinase secretion, and virulence. Structural studies reveal that GspB possesses a conserved periplasmic homology region domain that interacts directly with the N-terminal secretin domain. Site-specific photo-cross-linking unravels molecular details of the GspB-GspD complex in vivo. We show that GspB facilitates outer membrane targeting and assembly of the secretin pores and anchors them to the inner membrane while the C-terminal extension of GspB provides a scaffold for the secretin channel in the peptidoglycan cell wall. Phylogenetic analysis shows that in other bacteria, GspB homologs vary in length and domain composition and act in concert with either a cognate ATPase GspA or the pilotin GspS.

MagC is a NplC/P60-like member of the α-2-macroglobulin Mag complex of Pseudomonas aeruginosa that interacts with peptidoglycan

Bacterial α-2 macroglobulins (A2Ms) structurally resemble the large spectrum protease inhibitors of the eukaryotic immune system. In Pseudomonas aeruginosa, MagD acts as an A2M and is expressed within a six-gene operon encoding the MagA-F proteins. In this work, we employ isothermal calorimetry (ITC), analytical ultracentrifugation (AUC), and X-ray crystallography to investigate the function of MagC and show that MagC associates with the macroglobulin complex and with the peptidoglycan (PG). However, the catalytic residues of MagC display an inactive conformation that could suggest that it binds to PG but does not degrade it. We hypothesize that MagC could serve as an anchor between the MagD macroglobulin and the PG and could provide stabilization and/or regulation for the entire complex.

Antibacterial activity and effects of Colla corii asini on Salmonella typhimurium invasion in vitro and in vivo

Background

Salmonella enterica serovar Typhimurium is a foodborne pathogen that triggers inflammatory responses in the intestines of humans and livestock. Colla corii asini is a traditional medicine used to treat gynecologic and chronic diseases in Korea and China. However, the antibacterial activity of Colla corii asini has been unknown. In this study, we investigated the antibacterial activity and effects of Colla corii asini extract on Salmonella typhimurium invasion.

Methods

To tested for antibacterial effects of Colla corii asini extracts, we confirmed the agar diffusion using Luria solid broth medium. Also, we determined the MIC (minimum inhibitory concentration) and the MBC (minimum bactericidal concentration) value of the Colla corii asini ethanol extract (CEE) by using two-fold serial dilution methods. We evaluated the expression of salmonella invasion proteins including SipA, SipB and SipC by using Western blot and qPCR at the concentration of CEE without inhibition of bacterial growth. In vitro and vivo, we determined the inhibitory effect of invasion of S. typhimurium on CEE by using gentamicin assay and S. typhimurium-infected mice.

Results

CEE significantly inhibited the growth of Salmonella typhimurium in an agar diffuse assay and had an MIC of 0.78 mg/ml and an MBC of 1.56 mg/ml. Additionally, CEE reduced Salmonella typhimurium cell invasion via the inhibition of Salmonella typhimurium invasion proteins, such as SipA, SipB and SipC. Furthermore, CEE significantly suppressed invasion in the small intestines (ilea) of mice injected with Salmonella typhimurium.

Conclusion

These findings show that Colla corii asini exerts antibacterial activity and suppresses Salmonella typhimurium invasion in vitro and in vivo. Together, these findings demonstrate that Colla corii asini is a potentially useful therapeutic herbal medicine for treating salmonella-mediated diseases.

Functional characterization of glucosamine-6-phosphate synthase (GlmS) in Salmonella enterica serovar Enteritidis

Salmonella is a threat to public health due to consumption of contaminated food. Screening of a transposon library identified a unique mutant that was growth and host cell binding deficient. The objective of this study was to determine the functional role of glucosamine-6-phosphate synthase (GlmS) in the biology and pathogenesis of Salmonella. To examine this, we created a glmS mutant (ΔglmS) of Salmonella and examined the effect on cell envelope integrity, growth, metabolism, and pathogenesis. Our data indicated ΔglmS was defective in growth unless media were supplemented with D-glucosamine (D-GlcN). Examination of the bacterial cell envelope revealed that ΔglmS was highly sensitive to detergents, hydrophobic antibiotics, and bile salts compared to the wild type (WT). A release assay indicated that ΔglmS secreted higher amounts of β-lactamase than the WT in culture supernatant fractions. Furthermore, ΔglmS was attenuated in cell culture models of Salmonella infection. Taken together, this study determined an important role for GlmS in the pathogenesis and biology of Salmonella.

Structural analysis of a specialized type III secretion system peptidoglycan-cleaving enzyme

The Gram-negative bacterium enteropathogenic Escherichia coli uses a syringe-like type III secretion system (T3SS) to inject virulence or "effector" proteins into the cytoplasm of host intestinal epithelial cells. To assemble, the T3SS must traverse both bacterial membranes, as well as the peptidoglycan layer. Peptidoglycan is made of repeating N-acetylmuramic acid and N-acetylglucosamine disaccharides cross-linked by pentapeptides to form a tight mesh barrier. Assembly of many macromolecular machines requires a dedicated peptidoglycan lytic enzyme (PG-lytic enzyme) to locally clear peptidoglycan. Here we have solved the first structure of a T3SS-associated PG-lytic enzyme, EtgA from enteropathogenic E. coli. Unexpectedly, the active site of EtgA has features in common with both lytic transglycosylases and hen egg white lysozyme. Most notably, the β-hairpin region resembles that of lysozyme and contains an aspartate that aligns with lysozyme Asp-52 (a residue critical for catalysis), a conservation not observed in other previously characterized lytic transglycosylase families to which the conserved T3SS enzymes had been presumed to belong. Mutation of the EtgA catalytic glutamate, Glu-42, conserved across lytic transglycosylases and hen egg white lysozyme, and this differentiating aspartate diminishes type III secretion in vivo, supporting its essential role in clearing the peptidoglycan for T3SS assembly. Finally, we show that EtgA forms a 1:1 complex with the building block of the polymerized T3SS inner rod component, EscI, and that this interaction enhances PG-lytic activity of EtgA in vitro, collectively providing the necessary strict localization and regulation of the lytic activity to prevent overall cell lysis.

Bile-induced peptidoglycan remodelling in Salmonella enterica

Changes in the peptidoglycan (PG) structure of Salmonella enterica are detected in the presence of a sublethal concentration of sodium deoxycholate (DOC): (i) lower proportions of Braun lipoprotein (Lpp)-bound muropeptides; (ii) reduced levels of muropeptides cross-linked by L(meso)-diaminopimelyl-D(meso)-diaminopimelic acid (L-D) peptide bridges (3-3 cross-links). Similar structural changes are found in S. enterica cultures adapted to grow in the presence of a lethal concentration of DOC, suggesting that reduced anchoring of Braun protein to PG and low occurrence of 3-3 cross-links may increase S. enterica resistance to bile. This view is further supported by additional observations: (i) A triple mutant lacking L,D-transpeptidases YbiS, ErfK, and YcfS, which does not contain Lpp anchored to PG, is hyper-resistant to bile; (ii) enhanced 3-3 cross-linking upon overexpression of YnhG transpeptidase causes a decrease in bile resistance. These observations suggest that remodelling of the cell wall may be added to the list of adaptive responses that permit survival of S. enterica in the presence of bile.

The Pseudomonas aeruginosa periplasmic protease CtpA can affect systems that impact its ability to mount both acute and chronic infections

Proteases play important roles in the virulence of Pseudomonas aeruginosa. Some are exported to act on host targets and facilitate tissue destruction and bacterial dissemination. Others work within the bacterial cell to process virulence factors and regulate virulence gene expression. Relatively little is known about the role of one class of bacterial serine proteases known as the carboxyl-terminal processing proteases (CTPs). The P. aeruginosa genome encodes two CTPs annotated as PA3257/Prc and PA5134/CtpA in strain PAO1. Prc degrades mutant forms of the anti-sigma factor MucA to promote mucoidy in some cystic fibrosis lung isolates. However, nothing is known about the role or importance of CtpA. We have now found that endogenous CtpA is a soluble periplasmic protein and that a ctpA null mutant has specific phenotypes consistent with an altered cell envelope. Although a ctpA null mutation has no major effect on bacterial growth in the laboratory, CtpA is essential for the normal function of the type 3 secretion system (T3SS), for cytotoxicity toward host cells, and for virulence in a mouse model of acute pneumonia. Conversely, increasing the amount of CtpA above its endogenous level induces an uncharacterized extracytoplasmic function sigma factor regulon, an event that has been reported to attenuate P. aeruginosa in a rat model of chronic lung infection. Therefore, a normal level of CtpA activity is critical for T3SS function and acute virulence, whereas too much activity can trigger an apparent stress response that is detrimental to chronic virulence.

Genome expression analysis of nonproliferating intracellular Salmonella enterica serovar Typhimurium unravels an acid pH-dependent PhoP-PhoQ response essential for dormancy

Genome-wide expression analyses have provided clues on how Salmonella proliferates inside cultured macrophages and epithelial cells. However, in vivo studies show that Salmonella does not replicate massively within host cells, leaving the underlying mechanisms of such growth control largely undefined. In vitro infection models based on fibroblasts or dendritic cells reveal limited proliferation of the pathogen, but it is presently unknown whether these phenomena reflect events occurring in vivo. Fibroblasts are distinctive, since they represent a nonphagocytic cell type in which S. enterica serovar Typhimurium actively attenuates intracellular growth. Here, we show in the mouse model that S. Typhimurium restrains intracellular growth within nonphagocytic cells positioned in the intestinal lamina propria. This response requires a functional PhoP-PhoQ system and is reproduced in primary fibroblasts isolated from the mouse intestine. The fibroblast infection model was exploited to generate transcriptome data, which revealed that ∼2% (98 genes) of the S. Typhimurium genome is differentially expressed in nongrowing intracellular bacteria. Changes include metabolic reprogramming to microaerophilic conditions, induction of virulence plasmid genes, upregulation of the pathogenicity islands SPI-1 and SPI-2, and shutdown of flagella production and chemotaxis. Comparison of relative protein levels of several PhoP-PhoQ-regulated functions (PagN, PagP, and VirK) in nongrowing intracellular bacteria and extracellular bacteria exposed to diverse PhoP-PhoQ-inducing signals denoted a regulation responding to acidic pH. These data demonstrate that S. Typhimurium restrains intracellular growth in vivo and support a model in which dormant intracellular bacteria could sense vacuolar acidification to stimulate the PhoP-PhoQ system for preventing intracellular overgrowth.

Association of ActA to peptidoglycan revealed by cell wall proteomics of intracellular Listeria monocytogenes

Listeria monocytogenes is a Gram-positive intracellular bacterial pathogen that colonizes the cytosol of eukaryotic cells. Recent transcriptomic studies have revealed that intracellular L. monocytogenes alter expression of genes encoding envelope components. However, no comparative global analysis of this cell wall remodeling process is yet known at the protein level. Here, we used high resolution mass spectrometry to define the cell wall proteome of L. monocytogenes growing inside epithelial cells. When compared with extracellular bacteria growing in a nutrient-rich medium, a major difference found in the proteome was the presence of the actin assembly-inducing protein ActA in peptidoglycan purified from intracellular bacteria. ActA was also identified in the peptidoglycan of extracellular bacteria growing in a chemically defined minimal medium. In this condition, ActA maintains its membrane anchoring domain and promotes efficient bacterial entry into nonphagocytic host cells. Unexpectedly, Internalin-A, which mediates entry of extracellular L. monocytogenes into eukaryotic cells, was identified at late infection times (6 h) as an abundant protein in the cell wall of intracellular bacteria. Other surface proteins covalently bound to the peptidoglycan, as Lmo0514 and Lmo2085, were detected exclusively in intracellular and extracellular bacteria, respectively. Altogether, these data provide the first insights into the changes occurring at the protein level in the L. monocytogenes cell wall as the pathogen transits from the extracellular environment to an intracytosolic lifestyle inside eukaryotic cells. Some of these changes include alterations in the relative amount and the mode of association of certain surface proteins.

The muramidase EtgA from enteropathogenic Escherichia coli is required for efficient type III secretion

Enteropathogenic Escherichia coli (EPEC) is an important cause of infectious diarrhoea. It colonizes human intestinal epithelial cells by delivering effector proteins into the host cell cytoplasm via a type III secretion system (T3SS) encoded within the chromosomal locus of enterocyte effacement (LEE). The LEE pathogenicity island also encodes a lytic transglycosylase (LT) homologue named EtgA. In the present work we investigated the significance of EtgA function in type III secretion (T3S). Purified recombinant EtgA was found to have peptidoglycan lytic activity in vitro. Consistent with this function, signal peptide processing and bacterial cell fractionation revealed that EtgA is a periplasmic protein. EtgA possesses the conserved glutamate characteristic of the LT family, and we show here that it is essential for enzymic activity. Overproduction of EtgA in EPEC inhibits bacterial growth and induces cell lysis unless the predicted catalytic glutamate is mutated. An etgA mutant is attenuated for T3S, red blood cell haemolysis and EspA filamentation. BfpH, a plasmid-encoded putative LT, was not able to functionally replace EtgA. Overall, our results indicate that the muramidase activity of EtgA is not critical but makes a significant contribution to the efficiency of the T3S process.

The SciZ protein anchors the enteroaggregative Escherichia coli Type VI secretion system to the cell wall

Type VI secretion systems (T6SS) are multi-component machines encoded within the genomes of most Gram-negative bacteria that associate with plant, animal and/or human cells, and therefore are considered as potential virulence factors. We recently launched a study on the Sci-1 T6SS of enteroaggregative Escherichia coli (EAEC). The Sci-1 T6SS is composed of all or a subset of the 21 gene products encoded within the cluster, 13 of which are shared by all T6SS identified so far. In the present work, we focussed our attention on the SciZ protein. We first showed that SciZ is required for the release of the Hcp protein in the culture supernatant and for efficient biofilm formation, demonstrating that SciZ is necessary for EAEC T6SS function. Indeed, SciZ forms a complex with SciP, SciS and SciN, three core components of the transport apparatus. Fractionation and topology studies showed that SciZ is a polytopic inner membrane protein with three trans-membrane segments. Computer analyses identified a motif shared by peptidoglycan binding proteins of the OmpA family in the SciZ periplasmic domain. Using in vivo and in vitro binding assays, we showed that this motif anchors the SciZ protein to the cell wall and is required for T6SS function.

Salmonella enterica serovar typhimurium invades fibroblasts by multiple routes differing from the entry into epithelial cells

Fibroblasts are ubiquitous cells essential to tissue homeostasis. Despite their nonphagocytic nature, fibroblasts restrain replication of intracellular bacterial pathogens such as Salmonella enterica serovar Typhimurium. The extent to which the entry route of the pathogen determines this intracellular response is unknown. Here, we analyzed S. Typhimurium invasion in fibroblasts obtained from diverse origins, including primary cultures and stable nontransformed cell lines derived from normal tissues. Features distinct to the invasion of epithelial cells were found in all fibroblasts tested. In some fibroblasts, bacteria lacking the type III secretion system encoded in the Salmonella pathogenicity island 1 displayed significant invasion rates and induced the formation of lamellipodia and filopodia at the fibroblast-bacteria contact site. Other bacterial invasion traits observed in fibroblasts were the requirement of phosphatidylinositol 3-kinase, mitogen-activated protein kinase MEK1, and both actin filaments and microtubules. RNA interference studies showed that different Rho family GTPases are targeted by S. Typhimurium to enter into distinct fibroblasts. Rac1 and Cdc42 knockdown affected invasion of normal rat kidney fibroblasts, whereas none of the GTPases tested (Rac1, Cdc42, RhoA, or RhoG) was essential for invasion of immortalized human foreskin fibroblasts. Collectively, these data reveal a marked diversity in the modes used by S. Typhimurium to enter into fibroblasts.

Type VI secretion modulates quorum sensing and stress response in Vibrio anguillarum

Type VI protein secretion systems (T6SS) are essential for virulence of several Gram-negative bacteria. In this study, we identified a T6SS in Vibrio anguillarum, a marine bacterium that causes a hemorrhagic septicemia in fish. A partial operon vtsA-H (vibrio type six secretion) was sequenced and shown to encode eight proteins. VtsE-H are signature proteins found in other T6SSs, while VtsA-D are not associated with T6SS studied so far. In-frame deletions were made in each gene. Secretion of a haemolysin-co-regulated-like protein (Hcp), a protein secreted by all studied T6SSs, was decreased in VtsE-H. Unexpectedly, VtsA, VtsC and VtsD activated while VtsB and VtsE-H repressed hcp expression. The T6SS proteins also regulated expression of two extracellular proteases, EmpA and PrtV, but inversely to Hcp expression. This regulation was indirect as T6S positively regulated expression of the stress-response regulator RpoS and the quorum-sensing regulator VanT, which positively regulate protease expression. Moreover, VtsA-H proteins were not needed for virulence but did play a role in various stress responses. Thus, these data characterize a new role for T6S in the ecology of bacteria and we hypothesize this role to be a signal sensing mechanism that modulates the expression of regulators of the general stress response.

Signature-tagged mutagenesis of Edwardsiella ictaluri identifies virulence-related genes, including a salmonella pathogenicity island 2 class of type III secretion systems

Edwardsiella ictaluri is the leading cause of mortality in channel catfish culture, but little is known about its pathogenesis. The use of signature-tagged mutagenesis in a waterborne infection model resulted in the identification of 50 mutants that were unable to infect/survive in catfish. Nineteen had minitransposon insertions in miscellaneous genes in the chromosome, 10 were in genes that matched to hypothetical proteins, and 13 were in genes that had no significant matches in the NCBI databases. Eight insertions were in genes encoding proteins associated with virulence in other pathogens, including three in genes involved in lipopolysaccharide biosynthesis, three in genes involved in type III secretion systems (TTSS), and two in genes involved in urease activity. With the use of a sequence from a lambda clone carrying several TTSS genes, Blastn analysis of the partially completed E. ictaluri genome identified a 26,135-bp pathogenicity island containing 33 genes of a TTSS with similarity to the Salmonella pathogenicity island 2 class of TTSS. The characterization of a TTSS apparatus mutant indicated that it retained its ability to invade catfish cell lines and macrophages but was defective in intracellular replication. The mutant also invaded catfish tissues in numbers equal to those of invading wild-type E. ictaluri bacteria but replicated poorly and was slowly cleared from the tissues, while the wild type increased in number.

Proteomic analysis of Salmonella enterica serovar typhimurium isolated from RAW 264.7 macrophages: identification of a novel protein that contributes to the replication of serovar typhimurium inside macrophages

To evade host resistance mechanisms, Salmonella enterica serovar Typhimurium (STM), a facultative intracellular pathogen, must alter its proteome following macrophage infection. To identify new colonization and virulence factors that mediate STM pathogenesis, we have isolated STM cells from RAW 264.7 macrophages at various time points following infection and used a liquid chromatography-mass spectrometry-based proteomic approach to detect the changes in STM protein abundance. Because host resistance to STM infection is strongly modulated by the expression of a functional host-resistant regulator, i.e. natural resistance-associated macrophage protein 1 (Nramp1, also called Slc11a1), we have also examined the effects of Nramp1 activity on the changes of STM protein abundances. A total of 315 STM proteins have been identified from isolated STM cells, which are largely housekeeping proteins whose abundances remain relatively constant during the time course of infection. However, 39 STM proteins are strongly induced after infection, suggesting their involvement in modulating colonization and infection. Of the 39 induced proteins, 6 proteins are specifically modulated by Nramp1 activity, including STM3117, as well as STM3118-3119 whose time-dependent abundance changes were confirmed using Western blot analysis. Deletion of the gene encoding STM3117 resulted in a dramatic reduction in the ability of STM to colonize wild-type RAW 264.7 macrophages, demonstrating a critical involvement of STM3117 in promoting the replication of STM inside macrophages. The predicted function common for STM3117-3119 is biosynthesis and modification of the peptidoglycan layer of the STM cell wall.

Interactions between peptidoglycan and the ExeAB complex during assembly of the type II secretin of Aeromonas hydrophila

Aeromonas hydrophila transports extracellular protein toxins via the type II secretion system, an export mechanism comprised of numerous proteins that spans both the inner and outer membranes. Two components of this secretion system, ExeA and ExeB, form a complex in the inner membrane that functions to locate and/or assemble the ExeD secretin in the outer membrane. In the studies reported here, two-codon insertion mutagenesis of exeA revealed that an insertion at amino acid 495 in the C-terminal region of ExeA did not alter ExeAB complex formation yet completely abrogated its involvement in ExeD secretin assembly and thus rendered the bacteria secretion negative. In silico analysis of protein motifs with similar amino acid profiles revealed that this amino acid is located within a putative peptidoglycan (PG) binding motif in the periplasmic domain of ExeA. Substitution mutations of three highly conserved amino acids in the motif were constructed. In cells expressing each of these mutants, the ability to assemble the ExeD secretin or secrete aerolysin was lost, while ExeA retained the ability to form a complex with ExeB. In in vivo cross-linking experiments, wild-type ExeA could be cross-linked to PG, whereas the three substitution mutants of ExeA could not. These data indicate that PG binding and/or remodelling plays a role in the function of the ExeAB complex during assembly of the ExeD secretin.

Identification of genes associated with survival of Salmonella enterica serovar Enteritidis in chicken egg albumen

Salmonella enterica consists of over 2,000 serovars that are major causes of morbidity and mortality associated with contaminated food. Despite similarities among serovars of Salmonella enterica, many demonstrate unique host specificities, epidemiological characteristics, and clinical manifestations. One of the unique epidemiological characteristics of the serovar Enteritidis is that it is the only bacterium routinely transmitted to humans through intact chicken eggs. Therefore, Salmonella enterica serovar Enteritidis must be able to persist inside chicken eggs to be transmitted to humans, and its survival in egg is important for its transmission to the human population. The ability of Salmonella enterica serovar Enteritidis to survive in and transmit through eggs may have contributed to its drastically increased prevalence in the 1980s and 1990s. In the present study, using transposon-mediated mutagenesis, we have identified genes important for the association of Salmonella enterica serovar Enteritidis with chicken eggs. Our results indicate that genes involved in cell wall structural and functional integrity, and nucleic acid and amino acid metabolism are important for Salmonella enterica serovar Enteritidis to persist in egg albumen. Two regions unique to Salmonella enterica serovar Enteritidis were also identified, one of which enhanced the survival of a Salmonella enterica serovar Typhimurium isolate in egg albumen. The implication of our results to the serovar specificity of Salmonella enterica is also explored in the present study.

Increased excision of the Salmonella prophage ST64B caused by a deficiency in Dam methylase

Salmonella enterica mutants defective in Dam methylase are strongly attenuated in virulence and release a large amount of proteins to the extracellular medium. The extent to which these two phenotypes are linked is unknown. Using a proteomic approach, we identified Sb6, Sb13, and Sb36 as proteins present in larger amounts in culture supernatants of an S. enterica serovar Typhimurium dam mutant than in those of the wild-type strain. These three proteins are encoded in the Salmonella prophage ST64B. Higher amounts of ST64B phage DNA and tailless viral capsids were also detected in supernatant extracts of the dam mutant, suggesting that Dam methylation negatively regulates the excision of ST64B. Reverse transcription-PCR analysis revealed that the expression of two ST64B genes encoding a putative antirepressor and a phage replication protein increases in the dam mutant. The SOS response also augments the excision of ST64B. Infection assays performed with phage-cured strains demonstrated that ST64B does not carry genes required for virulence in the mouse model. Evidence was also obtained discarding a relationship between the high excision of ST64B and the envelope instability or virulence attenuation phenotype. Taken together, these data indicate that ST64B excises at a high rate in dam mutants due to the loss of repression exerted by Dam on phage genes and induction of the SOS response characteristic of these mutants. The exacerbated excision of ST64B does not however contribute to the incapacity of dam mutants to cause disease.

Strong decrease in invasive ability and outer membrane vesicle release in Crohn's disease-associated adherent-invasive Escherichia coli strain LF82 with the yfgL gene deleted

Adherent-invasive Escherichia coli strain LF82 recovered from a chronic lesion of a patient with Crohn's disease is able to invade cultured intestinal epithelial cells. Three mutants with impaired ability to invade epithelial cells had the Tn5phoA transposon inserted in the yfgL gene encoding the YfgL lipoprotein. A yfgL- negative isogenic mutant showed a marked decrease both in its ability to invade Intestine-407 cells and in the amount of the outer membrane proteins OmpA and OmpC in the culture supernatant, as shown by analysis of the culture supernatant protein contents by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization-time of flight mass spectrometry. Transcomplementation of the LF82-DeltayfgL isogenic mutant with the cloned yfgL gene restored invasion ability and outer membrane protein release in the culture supernatant. The outer membrane proteins in the culture supernatant of strain LF82 resulted from the formation of vesicles. This was shown by Western blot analysis of periplasmic and outer membrane fraction markers typically found in outer membrane vesicles and by transmission electron microscopic analysis of ultracentrifuged cell-free LF82 supernatant pellets, indicating the presence of vesicles with a bilayered structure surrounding a central electron-dense core. Thus, deletion of the yfgL gene in strain LF82 resulted in a decreased ability to invade intestinal epithelial cells and a decreased release of outer membrane vesicles.

Bioinformatics analysis of the locus for enterocyte effacement provides novel insights into type-III secretion

Background

Like many other pathogens, enterohaemorrhagic and enteropathogenic strains of Escherichia coli employ a type-III secretion system to translocate bacterial effector proteins into host cells, where they then disrupt a range of cellular functions. This system is encoded by the locus for enterocyte effacement. Many of the genes within this locus have been assigned names and functions through homology with the better characterised Ysc-Yop system from Yersinia spp. However, the functions and homologies of many LEE genes remain obscure.

Results

We have performed a fresh bioinformatics analysis of the LEE. Using PSI-BLAST we have been able to identify several novel homologies between LEE-encoded and Ysc-Yop-associated proteins: Orf2/YscE, Orf5/YscL, rORF8/EscI, SepQ/YscQ, SepL/YopN-TyeA, CesD2/LcrR. In addition, we highlight homology between EspA and flagellin, and report many new homologues of the chaperone CesT.

Conclusion

We conclude that the vast majority of LEE-encoded proteins do indeed possess homologues and that homology data can be used in combination with experimental data to make fresh functional predictions.

Type III protein secretion mechanism in mammalian and plant pathogens

The type III protein secretion system (TTSS) is a complex organelle in the envelope of many Gram-negative bacteria; it delivers potentially hundreds of structurally diverse bacterial virulence proteins into plant and animal cells to modulate host cellular functions. Recent studies have revealed several basic features of this secretion system, including assembly of needle/pilus-like secretion structures, formation of putative translocation pores in the host membrane, recognition of N-terminal/5' mRNA-based secretion signals, and requirement of small chaperone proteins for optimal delivery and/or expression of effector proteins. Although most of our knowledge about the TTSS is derived from studies of mammalian pathogenic bacteria, similar and unique features are learned from studies of plant pathogenic bacteria. Here, we summarize the most salient aspects of the TTSS, with special emphasis on recent findings.

Murein (peptidoglycan) binding property of the essential cell division protein FtsN from Escherichia coli

The binding of the essential cell division protein FtsN of Escherichia coli to the murein (peptidoglycan) sacculus was studied. Soluble truncated variants of FtsN, including the complete periplasmic part of the protein as well as a variant containing only the C-terminal 77 amino acids, did bind to purified murein sacculi isolated from wild-type cells. FtsN variants lacking this C-terminal region showed reduced or no binding to murein. Binding of FtsN was severely reduced when tested against sacculi isolated either from filamentous cells with blocked cell division or from chain-forming cells of a triple amidase mutant. Binding experiments with radioactively labeled murein digestion products revealed that the longer murein glycan strands (>25 disaccharide units) showed a specific affinity to FtsN, but neither muropeptides, peptides, nor short glycan fragments bound to FtsN. In vivo FtsN could be cross-linked to murein with the soluble disulfide bridge containing cross-linker DTSSP. Less FtsN, but similar amounts of OmpA, was cross-linked to murein of filamentous or of chain-forming cells compared to levels in wild-type cells. Expression of truncated FtsN variants in cells depleted in full-length FtsN revealed that the presence of the C-terminal murein-binding domain was not required for cell division under laboratory conditions. FtsN was present in 3,000 to 6,000 copies per cell in exponentially growing wild-type E. coli MC1061. We discuss the possibilities that the binding of FtsN to murein during cell division might either stabilize the septal region or might have a function unrelated to cell division.

An analysis of type-III secretion gene clusters in Chromobacterium violaceum

Chromobacterium violaceum is an environmental Gram-negative bacterium that is common in soil and water in tropical and sub-tropical regions. It is also a model organism for studying quorum-sensing and is a rare but deadly human pathogen. Recent completion of the genome sequence of C. violaceum strain ATCC 12472 revealed the presence of genes associated with type-III secretion systems (TTSSs). One of these systems resembles the Spi-1 system found in Salmonella enterica, whereas another is similar to the Spi-2 system from the same organism. Here, we present a detailed analysis and a fresh annotation of the two gene clusters. Moreover, we highlight the presence of several genes encoding putative type-III effector proteins that lead us to predict that this organism can manipulate vesicular trafficking, the actin cytoskeleton and apoptotic pathways within mammalian cells to its own advantage.

Repression of the RcsC-YojN-RcsB phosphorelay by the IgaA protein is a requisite for Salmonella virulence

Bacterial pathogenesis relies on regulators that activate virulence genes. Some of them act, in addition, as repressors of specific genes. Intracellular-growth-attenuator-A (IgaA) is a Salmonella enterica membrane protein that prevents overactivation of the RcsC-YojN-RcsB regulatory system. This negative control is critical for growth because disruption of the igaA gene is only possible in rcsC, yojN or rcsB strains. In this work, we examined the contribution of this regulatory circuit to virulence. Viable igaA point mutant alleles were isolated and characterized. These alleles encode IgaA variants leading to different levels of activation of the RcsC-YojN-RcsB system. IgaA-mediated repression of the RcsB-YojN-RcsC system occurred at the post-translational level, as shown by chromosomal epitope tagging of the rcsC, yojN and rcsB genes. The activity of the RcsC-YojN-RcsB system, monitored with the product of a tagged gmd-3xFLAG gene (positively regulated by RcsC-YojN-RcsB), was totally abolished by wild-type bacteria in mouse target organs. Such tight repression occurred only in vivo and was mediated by IgaA. Shutdown of the RcsC-YojN-RcsB system is a requisite for Salmonella virulence since all igaA point mutant strains were highly attenuated. The degree of attenuation correlated to that of the activation status of RcsC-YojN-RcsB. In some cases, the attenuation recorded was unprecedented, with competitive index (CI) values as low as 10(-6). Strikingly, IgaA is a protein absolutely dispensable for virulence in mutant strains having a non-functional RcsC-YojN-RcsB system. To our knowledge, IgaA exemplifies the first protein that contributes to virulence by exclusively acting as a negative regulator upon host colonization.

XopC and XopJ, two novel type III effector proteins from Xanthomonas campestris pv. vesicatoria

Pathogenicity of the gram-negative plant pathogen Xanthomonas campestris pv. vesicatoria depends on a type III secretion (TTS) system which translocates bacterial effector proteins into the plant cell. Previous transcriptome analysis identified a genome-wide regulon of putative virulence genes that are coexpressed with the TTS system. In this study, we characterized two of these genes, xopC and xopJ. Both genes encode Xanthomonas outer proteins (Xops) that were shown to be secreted by the TTS system. In addition, type III-dependent translocation of both proteins into the plant cell was demonstrated using the AvrBs3 effector domain as a reporter. XopJ belongs to the AvrRxv/YopJ family of effector proteins from plant and animal pathogenic bacteria. By contrast, XopC does not share significant homology to proteins in the database. Sequence analysis revealed that the xopC locus contains several features that are reminiscent of pathogenicity islands. Interestingly, the xopC region is flanked by 62-bp inverted repeats that are also associated with members of the Xanthomonas avrBs3 effector family. Besides xopC, a second gene of the locus, designated hpaJ, was shown to be coexpressed with the TTS system. hpaJ encodes a protein with similarity to transglycosylases and to the Pseudomonas syringae pv. maculicola protein HopPmaG. HpaJ secretion and translocation by the X. campestris pv. vesicatoria TTS system was not detectable, which is consistent with its predicted Sec signal and a putative function as transglycosylase in the bacterial periplasm.