FbpA, a novel multifunctional Listeria monocytogenes virulence factor

Influence of temperature on regulation of key virulence and stress response genes in Listeria monocytogenes biofilms

Temperature is a major determinant of Listeria (L.) monocytogenes adherence and biofilm formation on abiotic surfaces. However, its role on gene regulation of L. monocytogenes mature biofilms has not been investigated. In the present study, we aimed to evaluate the impact of temperature up- and down-shift on L. monocytogenes biofilms gene transcription. L. monocytogenes strain EGD-e biofilms were first developed on stainless steel surfaces in Brain Heart Infusion broth at 20 °C for 48 h. Then, nutrient broth was renewed, and mature biofilms were exposed to 10 °C, 20 °C or 37 °C for 24 h. Biofilm cells were harvested and RNA levels of plcA, prfA, hly, mpl, plcB, sigB, bapL, fbpA, fbpB, lmo2178, lmo0880, lmo0160, lmo1115, lmo 2089, lmo2576, lmo0159 and lmo0627 were evaluated by quantitative RT-PCR. The results revealed an over-expression of all genes tested in biofilm cells compared to planktonic cells. When biofilms were further allowed to proliferate at 20 °C for 24 h, the transcription levels of key virulence, stress response and putative binding proteins genes plcA, sigB, fbpA, fbpB, lmo1115, lmo0880 and lmo2089 decreased. A temperature-dependent transcription for sigB, plcA, hly, and lmo2089 genes was observed after biofilm proliferation at 10 °C or 37 °C. Our findings suggest that temperature differentially affects gene regulation of L. monocytogenes mature biofilms, thus modulating attributes such as virulence, stress response and pathogenesis.

Antibiofilm, AntiAdhesive and Anti-Invasive Activities of Bacterial Lysates Extracted from Pediococcus acidilactici against Listeria monocytogenes

This study aimed to investigate whether bacterial lysates (BLs) extracted from Pediococcus acidilactici reduce Listeria monocytogenes biofilm formation, as well as adhesion to and invasion of human intestinal epithelial cells. Pretreatment with P. acidilactici BLs (20, 40, and 80 μg/mL) significantly inhibited L. monocytogenes biofilm formation on the surface of polystyrene (p < 0.05). Fluorescence and scanning-electron-microscopic analyses indicated that L. monocytogenes biofilm comprised a much less dense layer of more-dispersed cells in the presence of P. acidilactici BLs. Moreover, biofilm-associated genes, such as flaA, fliG, flgE, motB, degU, agrA, and prfA, were significantly downregulated in the presence of P. acidilactici BLs (p < 0.05), suggesting that P. acidilactici BLs prevent L. monocytogenes biofilm development by suppressing biofilm-associated genes. Although P. acidilactici BLs did not dose-dependently inhibit L. monocytogenes adhesion to and invasion of intestinal epithelial cells, the BLs effectively inhibited adhesion and invasion at 40 and 80 μg/mL (p < 0.05). Supporting these findings, P. acidilactici BLs significantly downregulated L. monocytogenes transcription of genes related to adhesion and invasion, specifically fbpA, ctaP, actA, lapB, ami, and inlA. Collectively, these results suggest that P. acidilactici BLs have the potential to reduce health risks from L. monocytogenes.

Listeria monocytogenes Pathogenesis: The Role of Stress Adaptation

Adaptive stress tolerance responses are the driving force behind the survival ability of Listeria monocytogenes in different environmental niches, within foods, and ultimately, the ability to cause human infections. Although the bacterial stress adaptive responses are primarily a necessity for survival in foods and the environment, some aspects of the stress responses are linked to bacterial pathogenesis. Food stress-induced adaptive tolerance responses to acid and osmotic stresses can protect the pathogen against similar stresses in the gastrointestinal tract (GIT) and, thus, directly aid its virulence potential. Moreover, once in the GIT, the reprogramming of gene expression from the stress survival-related genes to virulence-related genes allows L. monocytogenes to switch from an avirulent to a virulent state. This transition is controlled by two overlapping and interlinked transcriptional networks for general stress response (regulated by Sigma factor B, (SigB)) and virulence (regulated by the positive regulatory factor A (PrfA)). This review explores the current knowledge on the molecular basis of the connection between stress tolerance responses and the pathogenesis of L. monocytogenes. The review gives a detailed background on the currently known mechanisms of pathogenesis and stress adaptation. Furthermore, the paper looks at the current literature and theories on the overlaps and connections between the regulatory networks for SigB and PrfA.

Deep impact of the inactivation of the SecA2-only protein export pathway on the proteosurfaceome of Listeria monocytogenes

Listeria monocytogenes presents a dimorphism associated to the SecA2 activity with cells having a normal rod shape or a dysmorphic elongated filamentous form. Besides variation of the cell and colony morphotype, this cell differentiation has profound ecophysiological and physiopathological implications with collateral effects on virulence and pathogenicity, biotope colonisation, bacterial adhesion and biofilm formation. This suggests the SecA2-only protein export could influence the listerial cell surface, which was investigated first by characterising its properties in L. monocytogenes wt and ΔsecA2. The degree of hydrophilicity and Lewis acid-base properties appeared significantly affected upon SecA2 inactivation. As modification of electrostatic properties would owe to modification in the composition of cell-surface proteins, the proteosurfaceome was further investigated by shotgun label-free proteomic analysis with a comparative relative quantitative approach. Following secretomic analysis, the protein secretion routes of the identified proteins were mapped considering the cognate transport and post-translocational maturation systems, as well as protein categories and subcellular localisation. Differential protein abundance profiles coupled to network analysis revealed the SecA2 dependence of 48 proteins, including some related to cell envelope biogenesis, translation and protein export, which could account for modifications of adhesion and surface properties of L. monocytogenes upon SecA2 inactivation. This investigation unravelled the profound influence of SecA2 activity on the cell surface properties and proteosurfaceome of L. monocytogenes, which provides advanced insights about its ecophysiopathology. SIGNIFICANCE: L. monocytogenes is a foodborne zoonotic pathogen and etiological agent of human listeriosis. This species presents a cellular dimorphism associated to the SecA2 activity that has profound physiopathological and ecophysiological implications with collateral effects on bacterial virulence and colonisation. To explore the influence of the SecA2-only protein export on the listerial cell, the surface properties of L. monocytogenes expressing or depleted of SecA2 was characterised by microelectrophoresis, microbial affinity to solvents and contact angles analyses. As modifications of hydrophilicity and Lewis acid-base electrostatic properties would owe to modification in the composition of cell-surface proteins, the proteinaceous subset of the surfaceome, i.e. the proteosurfaceome, was investigated further by shotgun label-free proteomic analysis. This subproteome appeared quite impacted upon SecA2 inactivation with the identification of proteins accounting for modifications in the cell surface properties. The profound influence of SecA2 activity on the cell surface of L. monocytogenes was unravelled, which provides advanced insights about its ecophysiopathology.

Comparative transcriptome analysis reveals that deletion of CheY influences gene expressions of ABC transports and metabolism in Haemophilus parasuis

Haemophilus (Glaesserella) parasuis is a commensal bacterium that causes Glässer's disease (GD) in swine. As a global transcriptional factor, CheY regulates the expression of hundreds of genes in H. parasuis. In this study, we measured changes in gene expression at the whole transcriptome level using RNAseq. We identified 2058 co-expressed genes, and found 624 differentially expressed genes (q < 0.05) in ΔcheY and SC1401. Several important GO annotations and signaling pathways were identified. RNA-seq results were assembled according to the reference genome, compared with the annotated gene model, and 12 new transcriptional regions were found. Finally, q-PCR results validated the RNA-seq results with 8 randomly selected genes. The present study indicated that CheY is mainly involved in the regulation of ABC transport, oxidative phosphorylation, and β-Lactam resistance. We draw the regulatory network of CheY, which offers greater insight into the regulatory mechanism of CheY in H.parasuis.

Ribosome-Associated Quality Control in Bacteria

Translation of the genetic information into proteins, performed by the ribosome, is a key cellular process in all organisms. Translation usually proceeds smoothly, but, unfortunately, undesirable events can lead to stalling of translating ribosomes. To rescue these faulty arrested ribosomes, bacterial cells possess three well-characterized quality control systems, tmRNA, ArfA, and ArfB. Recently, an additional ribosome rescue mechanism has been discovered in Bacillus subtilis. In contrast to the "canonical" systems targeting the 70S bacterial ribosome, this latter mechanism operates by first splitting the ribosome into the small (30S) and large (50S) subunits to then clearing the resultant jammed large subunit from the incomplete nascent polypeptide. Here, I will discuss the recent microbiological, biochemical, and structural data regarding functioning of this novel rescue system.

RqcH and RqcP catalyze processive poly-alanine synthesis in a reconstituted ribosome-associated quality control system

In the cell, stalled ribosomes are rescued through ribosome-associated protein quality-control (RQC) pathways. After splitting of the stalled ribosome, a C-terminal polyalanine 'tail' is added to the unfinished polypeptide attached to the tRNA on the 50S ribosomal subunit. In Bacillus subtilis, polyalanine tailing is catalyzed by the NEMF family protein RqcH, in cooperation with RqcP. However, the mechanistic details of this process remain unclear. Here we demonstrate that RqcH is responsible for tRNAAla selection during RQC elongation, whereas RqcP lacks any tRNA specificity. The ribosomal protein uL11 is crucial for RqcH, but not RqcP, recruitment to the 50S subunit, and B. subtilis lacking uL11 are RQC-deficient. Through mutational mapping, we identify critical residues within RqcH and RqcP that are important for interaction with the P-site tRNA and/or the 50S subunit. Additionally, we have reconstituted polyalanine-tailing in vitro and can demonstrate that RqcH and RqcP are necessary and sufficient for processivity in a minimal system. Moreover, the in vitro reconstituted system recapitulates our in vivo findings by reproducing the importance of conserved residues of RqcH and RqcP for functionality. Collectively, our findings provide mechanistic insight into the role of RqcH and RqcP in the bacterial RQC pathway.

Lactobacillus Cell Surface Proteins Involved in Interaction with Mucus and Extracellular Matrix Components

The gut microbiota is a complex microbial ecosystem where bacteria, through mutual interactions, cooperate in maintaining of wellbeing and health. Lactobacilli are among the most important constituents of human and animal intestinal microbiota and include many probiotic strains. Their presence ensures protection from invasion of pathogens, as well as stimulation of the immune system and protection of the intestinal flora, often exerted through the ability to interact with mucus and extracellular matrix components. The main factors responsible for mediating adhesion of pathogens and commensals to the gut are cell surface proteins that recognize host targets, as mucus layer and extracellular matrix proteins. In the last years, several adhesins have been reported to be involved in lactobacilli–host interaction often miming the same mechanism used by pathogens.

Characterization and pathogenicity of fibronectin binding protein FbpI of Streptococcus intermedius

Streptococcus intermedius is a causative agent of brain or liver abscesses. S. intermedius produces intermedilysin that plays a pivotal role in pathogenicity. We identified other pathogenic factors and described a fibronectin binding protein (FBP) homolog of S. intermedius (FbpI) that mediated bacterial adhesion to epithelial cells and virulence for mice. The amino acid sequence of FbpI is similar to that of atypical FBPs, which do not possess a conventional secretion signal and an anchoring motif. A full-length recombinant FbpI (rFbpI) bound to immobilized fibronectin in a dose-dependent manner. The fibronectin binding activity of an N-terminal construct of rFbpI comprising the translation initiation methionine of the open reading frame to lysine 265 (rFbpI-N) bound immobilized fibronectin to a much lesser extent compared with rFbpI. A construct comprising the C-terminal domain (alanine 266 to methionine 549; rFbpI-C) bound immobilized fibronectin equivalently to rFbpI. Adherence of the isogenic mutant ΔfbpI to cultured epithelial cells and immobilized fibronectin was significantly lower than that of the wild-type strain. Abscess formation of ΔfbpI reduced in a mouse infection model compared with that in the wild-type. Thus, FbpI may play a role in bacterial adhesion to host cells and represent a critical pathogenic factor of S. intermedius.

Genomic dissection of the most prevalent Listeria monocytogenes clone, sequence type ST87, in China

Background

Listeria monocytogenes consists of four lineages that occupy a wide variety of ecological niches. Sequence type (ST) 87 (serotype 1/2b), belonging to lineage I, is one of the most common STs isolated from food products, food associated environments and sporadic listeriosis in China. Here, we performed a comparative genomic analysis of the L. monocytogenes ST87 clone by sequencing 71 strains representing a diverse range of sources, different geographical locations and isolation years.

Results

The core genome and pan genome of ST87 contained 2667 genes and 3687 genes respectively. Phylogenetic analysis based on core genome SNPs divided the 71 strains into 10 clades. The clinical strains were distributed among multiple clades. Four clades contained strains from multiple geographic regions and showed high genetic diversity. The major gene content variation of ST87 genomes was due to putative prophages, with eleven hotspots of the genome that harbor prophages. All strains carry an intact CRISRP/Cas system. Two major CRISPR spacer profiles were found which were not clustered phylogenetically. A large plasmid of about 90 Kb, which carried heavy metal resistance genes, was found in 32.4% (23/71) of the strains. All ST87 strains harbored the Listeria pathogenicity island (LIPI)-4 and a unique 10-open read frame (ORF) genomic island containing a novel restriction-modification system.

Conclusion

Whole genome sequence analysis of L. monocytogenes ST87 enabled a clearer understanding of the population structure and the evolutionary history of ST87 L. monocytogenes in China. The novel genetic elements identified may contribute to its virulence and adaptation to different environmental niches. Our findings will be useful for the development of effective strategies for the prevention and treatment of listeriosis caused by this prevalent clone.

Fibronectin and Its Role in Human Infective Diseases

Fibronectin is a multidomain glycoprotein ubiquitously detected in extracellular fluids and matrices of a variety of animal and human tissues where it functions as a key link between matrices and cells. Fibronectin has also emerged as the target for a large number of microorganisms, particularly bacteria. There are clear indications that the binding of microorganism’ receptors to fibronectin promotes attachment to and infection of host cells. Each bacterium may use different receptors which recognize specific fibronectin domains, mostly the N-terminal domain and the central cell-binding domain. In many cases, fibronectin receptors have actions over and above that of simple adhesion: In fact, adhesion is often the prerequisite for invasion and internalization of microorganisms in the cells of colonized tissues. This review updates the current understanding of fibronectin receptors of several microorganisms with emphasis on their biochemical and structural properties and the role they can play in the onset and progression of host infection diseases. Furthermore, we describe the antigenic profile and discuss the possibility of designing adhesion inhibitors based on the structure of the fibronectin-binding site in the receptor or the receptor-binding site in fibronectin.

Mycobacterium tuberculosis Primary Infection and Dissemination: A Critical Role for Alveolar Epithelial Cells

Globally, tuberculosis (TB) has reemerged as a major cause of morbidity and mortality, despite the use of the Mycobacterium bovis BCG vaccine and intensive attempts to improve upon BCG or develop new vaccines. Two lacunae in our understanding of the Mycobacterium tuberculosis (M. tb)-host pathogenesis have mitigated the vaccine efforts; the bacterial-host interaction that enables successful establishment of primary infection and the correlates of protection against TB. The vast majority of vaccine efforts are based on the premise that cell-mediated immunity (CMI) is the predominating mode of protection against TB. However, studies in animal models and in humans demonstrate that post-infection, a period of several weeks precedes the initiation of CMI during which the few inhaled bacteria replicate dramatically and disseminate systemically. The “Trojan Horse” mechanism, wherein M. tb is phagocytosed and transported across the alveolar barrier by infected alveolar macrophages has been long postulated as the sole, primary M. tb:host interaction. In the current review, we present evidence from our studies of transcriptional profiles of M. tb in sputum as it emerges from infectious patients where the bacteria are in a quiescent state, to its adaptations in alveolar epithelial cells where the bacteria transform to a highly replicative and invasive phenotype, to its maintenance of the invasive phenotype in whole blood to the downregulation of invasiveness upon infection of epithelial cells at an extrapulmonary site. Evidence for this alternative mode of infection and dissemination during primary infection is supported by in vivo, in vitro cell-based, and transcriptional studies from multiple investigators in recent years. The proposed alternative mechanism of primary infection and dissemination across the alveolar barrier parallels our understanding of infection and dissemination of other Gram-positive pathogens across their relevant mucosal barriers in that barrier-specific adhesins, toxins, and enzymes synergize to facilitate systemic establishment of infection prior to the emergence of CMI. Further exploration of this M. tb:non-phagocytic cell interaction can provide alternative approaches to vaccine design to prevent infection with M. tb and not only decrease clinical disease but also decrease the overwhelming reservoir of latent TB infection.

Alanine Tails Signal Proteolysis in Bacterial Ribosome-Associated Quality Control

In ribosome-associated quality control (RQC), Rqc2/NEMF closely supports the E3 ligase Ltn1/listerin in promoting ubiquitylation and degradation of aberrant nascent-chains obstructing large (60S) ribosomal subunits-products of ribosome stalling during translation. However, while Ltn1 is eukaryote-specific, Rqc2 homologs are also found in bacteria and archaea; whether prokaryotic Rqc2 has an RQC-related function has remained unknown. Here, we show that, as in eukaryotes, a bacterial Rqc2 homolog (RqcH) recognizes obstructed 50S subunits and promotes nascent-chain proteolysis. Unexpectedly, RqcH marks nascent-chains for degradation in a direct manner, by appending C-terminal poly-alanine tails that act as degrons recognized by the ClpXP protease. Furthermore, RqcH acts redundantly with tmRNA/ssrA and protects cells against translational and environmental stresses. Our results uncover a proteolytic-tagging mechanism with implications toward the function of related modifications in eukaryotes and suggest that RQC was already active in the last universal common ancestor (LUCA) to help cope with incomplete translation.

Analysis of host cell binding specificity mediated by the Tp0136 adhesin of the syphilis agent Treponema pallidum subsp. pallidum

Background

Syphilis affects approximately 11 million people each year globally, and is the third most prevalent sexually transmitted bacterial infection in the United States. Inability to independently culture and genetically manipulate Treponema pallidum subsp. pallidum, the causative agent of this disease, has hindered our understanding of the molecular mechanisms of syphilis pathogenesis. Here, we used the non-infectious and poorly adherent B314 strain of the Lyme disease-causing spirochete, Borrelia burgdorferi, to express two variants of a known fibronectin-binding adhesin, Tp0136, from T. pallidum SS14 and Nichols strains. Using this surrogate system, we investigated the ability of Tp0136 in facilitating differential binding to mammalian cell lines offering insight into the possible role of this virulence factor in colonization of specific tissues by T. pallidum during infection.

Principal findings

Expression of Tp0136 could be detected on the surface of B. burgdorferi by indirect immunofluorescence assay using sera from a secondary syphilis patient that does not react with intact B314 spirochetes transformed with the empty vector. Increase in Tp0136-mediated adherence of B314 strain to human epithelial HEK293 cells was observed with comparable levels of binding exhibited by both Tp0136 alleles. Adherence of Tp0136-expressing B314 was highest to epithelial HEK293 and C6 glioma cells. Gain in binding of B314 strain expressing Tp0136 to purified fibronectin and poor binding of these spirochetes to the fibronectin-deficient cell line (HEp-2) indicated that Tp0136 interaction with this host receptor plays an important role in spirochetal attachment to mammalian cells. Furthermore, preincubation of these cell lines with fibronectin-binding peptide from Staphylococcus aureus FnbA-2 protein significantly inhibited binding of B314 expressing Tp0136.

Conclusions

Our results show that Tp0136 facilitates differential level of binding to cell lines representing various host tissues, which highlights the importance of this protein in colonization of human organs by T. pallidum and resulting syphilis pathogenesis.

Syphilis is one of the most prevalent sexually transmitted infections that affect millions of people around the world. The causative bacterium, Treponema pallidum subsp. pallidum, can be transmitted from mother to fetus during maternal infection, resulting in adverse pregnancy outcomes. Although timely treatment of syphilis is highly effective, untreated infection causes late syphilis that affects virtually every organ and leads to serious clinical manifestations. Therefore, syphilis remains a serious healthcare problem. T. pallidum cannot be grown in laboratory using traditional methods, which has slowed the progress in understanding this pathogen biology and pathogenesis. We employed a novel approach of using a related bacterium, Borrelia burgdorferi, to express Tp0136 protein from two different T. pallidum isolates to study the function of this protein. This strategy enabled us to demonstrate the ability of this protein to bind to fibronectin and laminin receptors present on the surface of various host cells. We showed that Tp0136 facilitates binding to only those host cells that produce fibronectin. In addition, we found that Tp0136-mediated binding is not equivalent in all host cell types, suggesting that the protein could help in colonization of specific human organs and tissues during infection by T. pallidum.

Listeria monocytogenes: cell biology of invasion and intracellular growth

The Gram-positive pathogen Listeria monocytogenes is able to promote its entry into a diverse range of mammalian host cells by triggering plasma membrane remodeling, leading to bacterial engulfment. Upon cell invasion, L. monocytogenes disrupts its internalization vacuole and translocates to the cytoplasm, where bacterial replication takes place. Subsequently, L. monocytogenes uses an actin-based motility system that allows bacterial cytoplasmic movement and cell-to-cell spread. L. monocytogenes therefore subverts host cell receptors, organelles and the cytoskeleton at different infection steps, manipulating diverse cellular functions that include ion transport, membrane trafficking, post-translational modifications, phosphoinositide production, innate immune responses as well as gene expression and DNA stability.

Prevalence and Characterization of Listeria monocytogenes Isolated from Beef and Sheep Carcasses in Turkey with Characterization of Locally Isolated Listeriophages as a Control Measure

Swab samples from cattle and sheep carcasses (120 of each) were tested for Listeria monocytogenes, and 120 slaughterhouse wastewater samples were tested for listeriophages over 12 months (10 samples per month) to note the seasonal distribution. L. monocytogenes and bacteriophage isolates were characterized, and the biocontrol of L. monocytogenes was investigated in meatballs with a phage cocktail. L. monocytogenes was found in 3.4 and 2.5% of cattle and sheep carcasses, respectively. All the isolates were found to harbor hlyA, actA, inlA, inlB, inlC, inlJ, plcA, plcB, fbpA, and fri genes with varied mRNA expression levels by real-time reverse transcriptase PCR analysis. Five isolates did not harbor the vip gene. According to enterobacterial repetitive intergenic consensus PCR, L. monocytogenes isolates were classified into four different groups based on their DNA patterns. The L. monocytogenes isolates were characterized for antibiotic susceptibility; one strain was found to be resistant to five different antibiotic classes. Of 11 lytic listeriophages, two were selected for the cocktail based on their DNA restriction profiles, efficiency of plating, transmission electron microscopy, and in vitro and in vivo analyses. In the biocontrol study, we used a food model that consisted of a novel bacteriophage cocktail in raw meatballs. The highest reduction of L. monocytogenes was recorded as 2.2 log CFU/g at a multiplicity of cellular infection of 4.7 at the end of 1 h. In conclusion, the new bacteriophage cocktail in this study can be considered an efficient biocontrol agent of L. monocytogenes in meatballs.

Surfaceome and Proteosurfaceome in Parietal Monoderm Bacteria: Focus on Protein Cell-Surface Display

The cell envelope of parietal monoderm bacteria (archetypal Gram-positive bacteria) is formed of a cytoplasmic membrane (CM) and a cell wall (CW). While the CM is composed of phospholipids, the CW is composed at least of peptidoglycan (PG) covalently linked to other biopolymers, such as teichoic acids, polysaccharides, and/or polyglutamate. Considering the CW is a porous structure with low selective permeability contrary to the CM, the bacterial cell surface hugs the molecular figure of the CW components as a well of the external side of the CM. While the surfaceome corresponds to the totality of the molecules found at the bacterial cell surface, the proteinaceous complement of the surfaceome is the proteosurfaceome. Once translocated across the CM, secreted proteins can either be released in the extracellular milieu or exposed at the cell surface by associating to the CM or the CW. Following the gene ontology (GO) for cellular components, cell-surface proteins at the CM can either be integral (GO: 0031226), i.e., the integral membrane proteins, or anchored to the membrane (GO: 0046658), i.e., the lipoproteins. At the CW (GO: 0009275), cell-surface proteins can be covalently bound, i.e., the LPXTG-proteins, or bound through weak interactions to the PG or wall polysaccharides, i.e., the cell wall binding proteins. Besides monopolypeptides, some proteins can associate to each other to form supramolecular protein structures of high molecular weight, namely the S-layer, pili, flagella, and cellulosomes. After reviewing the cell envelope components and the different molecular mechanisms involved in protein attachment to the cell envelope, perspectives in investigating the proteosurfaceome in parietal monoderm bacteria are further discussed.

Genomic characterization, phylogenetic analysis, and identification of virulence factors in Aerococcus sanguinicola and Aerococcus urinae strains isolated from infection episodes

Aerococcus sanguinicola and Aerococcus urinae are emerging pathogens in clinical settings mostly being causative agents of urinary tract infections (UTIs), urogenic sepsis and more seldomly complicated infective endocarditis (IE). Limited knowledge exists concerning the pathogenicity of these two species. Eight clinical A. sanguinicola (isolated from 2009 to 2015) and 40 clinical A. urinae (isolated from 1984 to 2015) strains from episodes of UTIs, bacteremia, and IE were whole-genome sequenced (WGS) to analyze genomic diversity and characterization of virulence genes involved in the bacterial pathogenicity. A. sanguinicola genome sizes were 2.06-2.12 Mb with 47.4-47.6% GC-contents, and 1783-1905 genes were predicted whereof 1170 were core-genes. In case of A. urinae strains, the genome sizes were 1.93-2.44 Mb with 41.6-42.6% GC-contents, and 1708-2256 genes of which 907 were core-genes. Marked differences were observed within A. urinae strains with respect to the average genome sizes, number and sequence identity of core-genes, proteome conservations, phylogenetic analysis, and putative capsular polysaccharide (CPS) loci sequences. Strains of A. sanguinicola showed high degree of homology. Phylogenetic analyses showed the 40 A. urinae strains formed two clusters according to two time periods: 1984-2004 strains and 2010-2015 strains. Genes that were homologs to virulence genes associated with bacterial adhesion and antiphagocytosis were identified by aligning A. sanguinicola and A. urinae pan- and core-genes against Virulence Factors of Bacterial Pathogens (VFDB). Bacterial adherence associated gene homologs were present in genomes of A. sanguinicola (htpB, fbpA, lmb, and ilpA) and A. urinae (htpB, lap, lmb, fbp54, and ilpA). Fifteen and 11-16 CPS gene homologs were identified in genomes of A. sanguinicola and A. urinae strains, respectively. Analysis of these genes identified one type of putative CPS locus within all A. sanguinicola strains. In A. urinae genomes, five different CPS loci types were identified with variations in CPS locus sizes, genetic content, and structural organization. In conclusion, this is the first study dealing with WGS and comparative genomics of clinical A. sanguinicola and A. urinae strains from episodes of UTIs, bacteremia, and IE. Gene homologs associated with antiphagocytosis and bacterial adherence were identified and genetic variability was observed within A. urinae genomes. These findings contribute with important knowledge and basis for future molecular and experimental pathogenicity study of UTIs, bacteremia, and IE causing A. sanguinicola and A. urinae strains.

The accessory Sec system (SecY2A2) in Streptococcus pneumoniae is involved in export of pneumolysin toxin, adhesion and biofilm formation

In Streptococcus pneumoniae TIGR4, genes encoding a SecY2A2 accessory Sec system are present within a locus encoding a serine-rich repeat surface protein PsrP. Mutant strains deleted in secA2 or psrP were deficient in biofilm formation, while the ΔsecA2 mutant was reduced in binding to airway epithelial cells. Cell wall protein (CWP) fractions from the ΔsecA2 mutant, but not from the ΔpsrP mutant, were reduced in haemolytic (pneumolysin) activity. Contact-dependent pneumolysin (Ply) activity of wild type TIGR4 cells was ten-fold greater than that of ΔsecA2 mutant cells suggesting that Ply was not active at the ΔsecA2 cell surface. Ply protein was found to be present in the CWP fraction from the ΔsecA2 mutant, but showed aberrant electrophoretic migration indicative of protein modification. Proteomic analyses led to the discovery that the ΔsecA2 mutant CWP fraction was deficient in two glycosidases as well as other enzymes involved in carbohydrate metabolism. Taken collectively the results suggest that positioning of Ply into the cell wall compartment in active form, together with glycosyl hydrolases and adhesins, requires a functional accessory Sec system.

Multimer recognition and secretion by the non-classical secretion pathway in Bacillus subtilis

Non-classical protein secretion in bacteria is a common phenomenon. However, the selection principle for non-classical secretion pathways remains unclear. Here, our experimental data, to our knowledge, are the first to show that folded multimeric proteins can be recognized and excreted by a non-classical secretion pathway in Bacillus subtilis. We explored the secretion pattern of a typical cytoplasmic protein D-psicose 3-epimerase from Ruminococcus sp. 5_1_39BFAA (RDPE), and showed that its non-classical secretion is not simply due to cell lysis. Analysis of truncation variants revealed that the C- and N-terminus, and two hydrophobic domains, are required for structural stability and non-classical secretion of RDPE. Alanine scanning mutagenesis of the hydrophobic segments of RDPE revealed that hydrophobic residues mediated the equilibrium between its folded and unfolded forms. Reporter mCherry and GFP fusions with RDPE regions show that its secretion requires an intact tetrameric protein complex. Using cross-linked tetramers, we show that folded tetrameric RDPE can be secreted as a single unit. Finally, we provide evidence that the non-classical secretion pathway has a strong preference for multimeric substrates, which accumulate at the poles and septum region. Altogether, these data show that a multimer recognition mechanism is likely applicable across the non-classical secretion pathway.

InlP, a New Virulence Factor with Strong Placental Tropism

Intrauterine infection is a major detriment for maternal-child health and occurs despite local mechanisms that protect the maternal-fetal interface from a wide variety of pathogens. The bacterial pathogen Listeria monocytogenes causes spontaneous abortion, stillbirth, and preterm labor in humans and serves as a model for placental pathogenesis. Given the unique immunological environment of the maternal-fetal interface, we hypothesized that virulence determinants with placental tropism are required for infection of this tissue. We performed a genomic screen in pregnant guinea pigs that led to the identification of 201 listerial genes important for infection of the placenta but not maternal liver. Among these genes was lmrg1778 (lmo2470), here named inlP, predicted to encode a secreted protein that belongs to the internalin family. InlP is conserved in virulent L. monocytogenes strains but absent in Listeria species that are nonpathogenic for humans. The intracellular life cycle of L. monocytogenes deficient in inlP (ΔinlP) was not impaired. In guinea pigs and mice, InlP increased the placental bacterial burden by a factor of 3 log₁₀ while having only a minor role in other maternal organs. Furthermore, the ΔinlP strain was attenuated in intracellular growth in primary human placental organ cultures and trophoblasts. InlP is a novel virulence factor for listeriosis with a strong tropism for the placenta. This virulence factor represents a tool for the development of new modalities to prevent and treat infection-related pregnancy complications.

Stuck in the Middle: Fibronectin-Binding Proteins in Gram-Positive Bacteria

Fibronectin is a multidomain glycoprotein found ubiquitously in human body fluids and extracellular matrices of a variety of cell types from all human tissues and organs, including intestinal epithelial cells. Fibronectin plays a major role in the regulation of cell migration, tissue repair, and cell adhesion. Importantly, fibronectin also serves as a common target for bacterial adhesins in the gastrointestinal tract. Fibronectin-binding proteins (FnBPs) have been identified and characterized in a wide variety of host-associated bacteria. Single bacterial species can contain multiple, diverse FnBPs. In pathogens, some FnBPs contribute to virulence via host cell attachment, invasion, and interference with signaling pathways. Although FnBPs in commensal and probiotic strains are not sufficient to confer virulence, they are essential for attachment to their ecological niches. Here we describe the interaction between human fibronectin and bacterial adhesins by highlighting the FnBPs of Gram-positive pathogens and commensals. We provide an overview of the occurrence and diversity of FnBPs with a focus on the model pathogenic organisms in which FnBPs are most characterized. Continued investigation of FnBPs is needed to fully understand their divergence and specificity in both pathogens and commensals.

Fibronectin-binding protein TDE1579 affects cytotoxicity of Treponema denticola

While FbpA, a family of bacterial fibronectin (FN) binding proteins has been studied in several gram-positive bacteria, the gram-negative Treponema denticola, an anaerobic periodontal pathogen, also has an overlooked fbp gene (tde1579). In this research, we confirm that recombinant Fbp protein (rFbp) of T. denticola binds human FN with a Kdapp of 1.5 × 10(-7) M and blocks the binding of T. denticola to FN in a concentration-dependent manner to a level of 42%. The fbp gene was expressed in T. denticola. To reveal the roles of fbp in T. denticola pathogenesis, an fbp isogenic mutant was constructed. The fbp mutant had 51% reduced binding ability to human gingival fibroblasts (hGF). When hGF were challenged with T. denticola, the fbp mutant caused less cell morphology change, had 50% reduced cytotoxicity to hGF, and had less influence on the growth of hGF cells.

Listeriosis in animals, its public health significance (food-borne zoonosis) and advances in diagnosis and control: a comprehensive review

Listeriosis is an infectious and fatal disease of animals, birds, fish, crustaceans and humans. It is an important food-borne zoonosis caused by Listeria monocytogenes, an intracellular pathogen with unique potential to spread from cell to cell, thereby crossing blood-brain, intestinal and placental barriers. The organism possesses a pile of virulence factors that help to infect the host and evade from host immune machinery. Though disease occurrence is sporadic throughout the world, it can result in severe damage during an outbreak. Listeriosis is characterized by septicaemia, encephalitis, meningitis, meningoencephalitis, abortion, stillbirth, perinatal infections and gastroenteritis with the incubation period varying with the form of infection. L. monocytogenes has been isolated worldwide from humans, animals, poultry, environmental sources like soil, river, decaying plants, and food sources like milk, meat and their products, seafood and vegetables. Since appropriate vaccines are not available and infection is mainly transmitted through foods in humans and animals, hygienic practices can prevent its spread. The present review describes etiology, epidemiology, transmission, clinical signs, post-mortem lesions, pathogenesis, public health significance, and advances in diagnosis, vaccines and treatment of this disease. Special attention has been given to novel as well as prospective emerging therapies that include bacteriophage and cytokine therapy, avian egg yolk antibodies and herbal therapy. Various vaccines, including advances in recombinant and DNA vaccines and their modes of eliciting immune response, are also discussed. Due focus has also been given regarding appropriate prevention and control strategies to be adapted for better management of this zoonotic disease.

Evaluation of cell binding activities of Leptospira ECM adhesins

Pathogenic spirochetes of the genus Leptospira are the causative agents of leptospirosis, a zoonotic infection that occurs globally. The bacteria colonize the renal proximal tubules of many animals and are shed in the urine. Contact with the urine, or with water contaminated with the urine of infected animals can cause infection of new host animals, including humans. Mechanisms of colonization of the proximal tubule and other tissues are not known, but specific interactions between bacterial adhesins and host substrates are likely to be critical in this process. Several extracellular matrix (ECM) adhesins have been previously identified, but more recently, it has been shown that Leptospira bind more efficiently to cells than ECM. In this work, recombinant forms of five putative Leptospira ECM adhesins, namely LipL32, Loa22, OmpL1, p31/LipL45, and LenA were evaluated for binding to cells as well as an expanded variety of ECM components. Reproducible and significant adhesin activity was demonstrated only for OmpL1, which bound to both mammalian cell lines tested and to glycosaminoglycans (GAGs). While determination of biologically significant bacterial adhesion activity will require generation of site-directed mutant strains, our results suggest that OmpL1 is a strong candidate for future evaluation regarding the roles of the adhesin activity of the protein during L. interrogans infection.

Leptospirosis is the most widespread zoonotic infection in the world and represents a major public health problem, especially in tropical climates. The processes by which some Leptospira species cause infection, disease, and colonization of carrier animals remains poorly understood. Specific binding of Leptospira molecules and host molecules are likely important for infection and colonization. To identify Leptospira molecules that mediate attachment to host substrates, prior studies have evaluated Leptospira membrane proteins for binding to extracellular matrix (ECM) proteins. More recent data, however, show that Leptospira bind more efficiently to cells than to ECM. In search of adhesins mediating the latter activity, our study evaluated the direct cell binding activity of recombinant forms of a group of previously reported Leptospira ECM adhesins. Only one of these proteins, OmpL1, demonstrated reproducible direct cell binding activity. Further work will focus on identification of the mammalian receptor for OmpL1 and determining the biological significance of this activity during infection.

Glycosaminoglycan binding by Borrelia burgdorferi adhesin BBK32 specifically and uniquely promotes joint colonization

Microbial pathogens that colonize multiple tissues commonly produce adhesive surface proteins that mediate attachment to cells and/or extracellular matrix in target organs. Many of these 'adhesins' bind to multiple ligands, complicating efforts to understand the role of each ligand-binding activity. Borrelia burgdorferi, the causative agent of Lyme disease, produces BBK32, first identified as a fibronectin-binding adhesin that promotes skin and joint colonization. BBK32 also binds to glycosaminoglycan (GAG), which, like fibronectin is ubiquitously present on cell surfaces. To determine which binding activity is relevant for BBK32-promoted infectivity, we generated a panel of BBK32 truncation and internal deletion mutants, and identified variants specifically defective for binding to either fibronectin or GAG. These variants promoted bacterial attachment to different mammalian cell types in vitro, suggesting that fibronectin and GAG binding may play distinct roles during infection. Intravenous inoculation of mice with a high-passage non-infectious B. burgdorferi strain that produced wild-type BBK32 or BBK32 mutants defective for GAG or fibronectin binding, revealed that only GAG-binding activity was required for significant localization to joints at 60 min post-infection. An otherwise infectious B. burgdorferi strain producing BBK32 specifically deficient in fibronectin binding was fully capable of both skin and joint colonization in the murine model, whereas a strain producing BBK32 selectively attenuated for GAG binding colonized the inoculation site but not knee or tibiotarsus joints. Thus, the BBK32 fibronectin- and GAG-binding activities are separable in vivo, and BBK32-mediated GAG binding, but not fibronectin binding, contributes to joint colonization.

Excretion of cytosolic proteins (ECP) in bacteria

Excretion of cytosolic proteins (ECP) has been reported in bacteria and eukaryotes. As none of the classical signal peptide (SP) dependent or SP-independent pathways could be associated with ECP, it has been also referred to as 'non-classical protein export'. When microbiologists first began to study this subject in 1990, mainly singular cytoplasmic proteins were investigated, such as GAPDH at the cell surface and in the supernatant of pathogenic streptococci or glutamine synthetase (GlnA) as a major extracellular protein in pathogenic mycobacteria. Later, with the rising popularity of proteomics, it became obvious that the secretome of most bacteria contained a copious amount of cytosolic proteins. In particular ancient proteins such as glycolytic enzymes, chaperones, translation factors or enzymes involved in detoxification of reactive oxygen were found in the supernatants. As the excreted proteins do not possess a common motive, the most widespread opinion is that ECP is due to cell lysis. Indeed, upregulation of autolysins or distortion of the murein structure increased ECP, suggesting that enhanced ECP is some sort of survival strategy to counteract osmotic stress. However, in the meantime there are mounting evidences and hints that speak against cell lysis as a primary mechanism for ECP. Very likely, ECP belongs to the normal life cycle of bacteria and involves a programmed process. This review provides a brief overview of the 'non-classical protein export'.

Surface adhesins and exopolymers of selected foodborne pathogens

The ability of bacteria to bind different compounds and to adhere to biotic and abiotic surfaces provides them with a range of advantages, such as colonization of various tissues, internalization, avoidance of an immune response, and survival and persistence in the environment. A variety of bacterial surface structures are involved in this process and these promote bacterial adhesion in a more or less specific manner. In this review, we will focus on those surface adhesins and exopolymers in selected foodborne pathogens that are involved mainly in primary adhesion. Their role in biofilm development will also be considered when appropriate. Both the clinical impact and the implications for food safety of such adhesion will be discussed.

How Listeria monocytogenes organizes its surface for virulence

Listeria monocytogenes is a Gram-positive pathogen responsible for the manifestation of human listeriosis, an opportunistic foodborne disease with an associated high mortality rate. The key to the pathogenesis of listeriosis is the capacity of this bacterium to trigger its internalization by non-phagocytic cells and to survive and even replicate within phagocytes. The arsenal of virulence proteins deployed by L. monocytogenes to successfully promote the invasion and infection of host cells has been progressively unveiled over the past decades. A large majority of them is located at the cell envelope, which provides an interface for the establishment of close interactions between these bacterial factors and their host targets. Along the multistep pathways carrying these virulence proteins from the inner side of the cytoplasmic membrane to their cell envelope destination, a multiplicity of auxiliary proteins must act on the immature polypeptides to ensure that they not only maturate into fully functional effectors but also are placed or guided to their correct position in the bacterial surface. As the major scaffold for surface proteins, the cell wall and its metabolism are critical elements in listerial virulence. Conversely, the crucial physical support and protection provided by this structure make it an ideal target for the host immune system. Therefore, mechanisms involving fine modifications of cell envelope components are activated by L. monocytogenes to render it less recognizable by the innate immunity sensors or more resistant to the activity of antimicrobial effectors. This review provides a state-of-the-art compilation of the mechanisms used by L. monocytogenes to organize its surface for virulence, with special focus on those proteins that work “behind the frontline”, either supporting virulence effectors or ensuring the survival of the bacterium within its host.

A highly conserved family of domains related to the DNA-glycosylase fold helps predict multiple novel pathways for RNA modifications

A protein family including mammalian NEMF, Drosophila caliban, yeast Tae2, and bacterial FpbA-like proteins was first defined over a decade ago and found to be universally distributed across the three domains/superkingdoms of life. Since its initial characterization, this family of proteins has been tantalizingly linked to a wide range of biochemical functions. Tapping the enormous wealth of genome information that has accumulated since the initial characterization of these proteins, we perform a detailed computational analysis of the family, identifying multiple conserved domains. Domains identified include an enzymatic domain related to the formamidopyrimidine (Fpg), MutM, and Nei/EndoVIII family of DNA glycosylases, a novel, predicted RNA-binding domain, and a domain potentially mediating protein–protein interactions. Through this characterization, we predict that the DNA glycosylase-like domain catalytically operates on double-stranded RNA, as part of a hitherto unknown base modification mechanism that probably targets rRNAs. At least in archaea, and possibly eukaryotes, this pathway might additionally include the AMMECR1 family of proteins. The predicted RNA-binding domain associated with this family is also observed in distinct architectural contexts in other proteins across phylogenetically diverse prokaryotes. Here it is predicted to play a key role in a new pathway for tRNA 4-thiouridylation along with TusA-like sulfur transfer proteins.

Listeria monocytogenes: a promising vehicle for neonatal vaccination

Vaccination as a medical intervention has proven capable of greatly reducing the suffering from childhood infectious disease. However, newborns and infants in particular are age groups for whom adequate vaccine-mediated protection is still largely lacking. With the challenges that the neonatal immune system faces and the required highest level of stringency for safety, designing vaccines for early life in general and the newborn in particular poses great difficulty. Nevertheless, recent advances in our understanding of neonatal immunity and its responses to vaccines and adjuvants suggest that neonatal vaccination is a task fully within reach. Among the most promising developments in neonatal vaccination is the use of Listeria monocytogenes (Lm) as a delivery platform. In this review, we will outline key properties of Lm that make it such an ideal neonatal and early life vaccine vehicle, and also discuss potential constraints of Lm as a vaccine delivery platform.

Characterization of betaine aldehyde dehydrogenase (BetB) as an essential virulence factor of Brucella abortus

The pathogenic mechanisms of Brucellosis used to adapt to the harsh intracellular environment of the host cell are not fully understood. The present study investigated the in vitro and in vivo characteristics of B. abortus betaine aldehyde dehydrogenase (BetB) (Gene Bank ID: 006932) using a betB deletion mutant constructed from virulent B. abortus 544. In test under stress conditions, including osmotic- and acid stress-resistance, the betB mutant had a lower osmotic-resistance than B. abortus wild-type. In addition, the betB mutant showed higher internalization rates compared to the wild-type strain; however, it also displayed replication failures in HeLa cells and RAW 264.7 macrophages. During internalization, compared to the wild-type strain, the betB mutant was more adherent to the host surface and showed enhanced phosphorylation of protein kinases, two processes that promote phagocytic activity, in host cells. During intracellular trafficking, colocalization of B. abortus-containing phagosomes with LAMP-1 was elevated in betB mutant-infected cells compared to the wild-type cells. In mice, the betB mutant was predominantly cleared from spleens compared to the wild-type strain after 2 weeks post-infection, and the vaccination test with the live betB mutant showed effective protection against challenge infection with the virulent wild-type strain. These findings suggested that the B. abortus betB gene substantially affects the phagocytic pathway in human phagocytes and in host cells in mice. Furthermore, this study highlights the potential use of the B. abortus betB mutant as a live vaccine for the control of brucellosis.

Acid shock of Listeria monocytogenes at low environmental temperatures induces prfA, epithelial cell invasion, and lethality towards Caenorhabditis elegans

BACKGROUND

The saprophytic pathogen Listeria monocytogenes has to cope with a variety of acidic habitats during its life cycle. The impact of low-temperature coupled with pH decrease for global gene expression and subsequent virulence properties, however, has not been elucidated.

RESULTS

qRT-PCR revealed for the first time a transient, acid triggered prfA induction of approximately 4-fold, 5.7-fold, 7-fold and 9.3-fold 60 to 90 min after acid shock of L. monocytogenes at 37°C, 25°C, 18°C, and 10°C, respectively. Comparable data were obtained for seven different L. monocytogenes strains, demonstrating that prfA induction under these conditions is a general response of L. monocytogenes. Transcriptome analysis revealed that the in vivo-relevant genes bsh, clpP, glpD, hfq, inlA, inlB, inlE, lisR, and lplA1 as well as many other genes with a putative role during infection are transiently induced upon acid shock conducted at 25°C and 37°C. Twenty-five genes repressed upon acid shock are known to be down regulated during intracellular growth or by virulence regulators. These data were confirmed by qRT-PCR of twelve differentially regulated genes and by the identification of acid shock-induced genes influenced by σB. To test if up regulation of virulence genes at temperatures below 37°C correlates with pathogenicity, the capacity of L. monocytogenes to invade epithelial cells after acid shock at 25°C was measured. A 12-fold increased number of intracellular bacteria was observed (acid shock, t = 60 min) that was reduced after adaptation to the level of the unshocked control. This increased invasiveness was shown to be in line with the induction of inlAB. Using a nematode infection assay, we demonstrated that Caenorhabditis elegans fed with acid-shocked L. monocytogenes exhibits a shorter time to death of 50% (TD50) of the worms (6.4 days) compared to infection with unshocked bacteria (TD50 = 10.2 days).

CONCLUSIONS

PrfA and other listerial virulence genes are induced by an inorganic acid in a temperature-dependent manner. The data presented here suggest that low pH serves as a trigger for listerial pathogenicity at environmental temperatures.

Leaving home ain't easy: protein export systems in Gram-positive bacteria

Transport of proteins into or across biological membranes is catalyzed by membrane-bound transport machineries. In Gram-positive bacteria, the vast majority of proteins are exported out of the cytosol by the conserved general secretion (Sec) system or, alternatively, by the twin-arginine translocation (Tat) system, that closely resemble their well-studied counterparts in Gram-negative bacteria. Besides these common major export routes, additional unique protein export systems (such as accessory Sec2 systems and/or type VII/WXG100 secretion systems) exist in some Gram-positive bacteria that are specifically involved in the secretion of limited subsets of proteins.

Exoproteomic analysis of the SecA2-dependent secretion in Listeria monocytogenes EGD-e

As part of the Sec translocase, the accessory ATPase SecA2 is present in some pathogenic Gram-positive bacteria. In Listeria monocytogenes, deletion of secA2 results in filamentous cells that form rough colonies and have lower virulence. However, only a few proteins have been identified that are secreted by this pathway. This investigation aims to provide the first exoproteomic analysis of the SecA2-dependent secretion in L. monocytogenes EGD-e. By using media and temperatures relevant to bacterial physiology, we demonstrated that the rough colony and elongated bacterial cell morphotypes are highly dependent on growth conditions. Subsequently, comparative exoproteomic analyses of the ΔsecA2 versus wt strains were performed in chemically defined medium at 20°C and 37°C. Analyzing the proteomic data following the secretomics-based method, part of the proteins appeared routed towards the Sec pathway and exhibited an N-terminal signal peptide. For another significant part, they were primarily cytoplasmic proteins, thus lacking signal peptide and with no predictable secretion pathway. In total, 13 proteins were newly identified as secreted via SecA2, which were essentially associated with cell-wall metabolism, adhesion and/or biofilm formation. From this comparative exoproteomic analysis, new insights into the L. monocytogenes physiology are discussed in relation to its saprophytic and pathogenic lifestyle.

Entry of Listeria monocytogenes in mammalian epithelial cells: an updated view

Listeria monocytogenes is a bacterial pathogen that promotes its internalization into host epithelial cells. Interaction between the bacterial surface molecules InlA and InlB and their cellular receptors E-cadherin and Met, respectively, triggers the recruitment of endocytic effectors, the subversion of the phosphoinositide metabolism, and the remodeling of the actin cytoskeleton that lead to bacterial engulfment. Additional bacterial surface and secreted virulence factors also contribute to entry, albeit to a lesser extent. Here we review the increasing number of signaling effectors that are reported as being subverted by L. monocytogenes during invasion of cultured cell lines. We also update the current knowledge of the early steps of in vivo cellular infection, which, as shown recently, challenges previous concepts generated from in vitro data.

HmgR, a key enzyme in the mevalonate pathway for isoprenoid biosynthesis, is essential for growth of Listeria monocytogenes EGDe

Isoprenoids may be synthesized via one of two pathways, the classical mevalonate pathway or the alternative 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. While the majority of bacteria utilize a single pathway for isoprenoid biosynthesis, Listeria monocytogenes is unusual in possessing the complete set of genes for both pathways. Here, we utilized new molecular tools to create precise gene deletions in selected genes encoding enzymes of both pathways, gcpE, lytB (encoding proteins in the MEP pathway) and hmgR (encoding a protein in the mevalonate pathway). We demonstrate that the hmgR gene can only be deleted when the growth medium is supplemented with exogenous mevalonate. Furthermore, full growth of the mutant in the absence of mevalonate was only possible when the intact hmgR gene was supplied in trans using an IPTG-inducible expression system. Murine competitive index assays performed via the oral and intraperitoneal routes of infection revealed that the mevalonate hmgR mutant could not be recovered from livers and spleens 3 days post-infection. We propose that HmgR in L. monocytogenes EGDe is involved in essential metabolic functions and that an intact MEP pathway is not capable of sustaining growth.

Signature tagged mutagenesis in the functional genetic analysis of gastrointestinal pathogens

Signature tagged mutagenesis is a genetic approach that was developed to identify novel bacterial virulence factors. It is a negative selection method in which unique identification tags allow analysis of pools of mutants in mixed populations. The approach is particularly well suited to functional genetic analysis of the gastrointestinal phase of infection in foodborne pathogens and has the capacity to guide the development of novel vaccines and therapeutics. In this review we outline the technical principles underpinning signature-tagged mutagenesis as well as novel sequencing-based approaches for transposon mutant identification such as TraDIS (transposon directed insertion-site sequencing). We also provide an analysis of screens that have been performed in gastrointestinal pathogens which are a global health concern (Escherichia coli, Listeria monocytogenes, Helicobacter pylori, Vibrio cholerae and Salmonella enterica). The identification of key virulence loci through the use of signature tagged mutagenesis in mice and relevant larger animal models is discussed.

Group B streptococcus GAPDH is released upon cell lysis, associates with bacterial surface, and induces apoptosis in murine macrophages

Glyceraldehyde 3-phosphate dehydrogenases (GAPDH) are cytoplasmic glycolytic enzymes that, despite lacking identifiable secretion signals, have been detected at the surface of several prokaryotic and eukaryotic organisms where they exhibit non-glycolytic functions including adhesion to host components. Group B Streptococcus (GBS) is a human commensal bacterium that has the capacity to cause life-threatening meningitis and septicemia in newborns. Electron microscopy and fluorescence-activated cell sorter (FACS) analysis demonstrated the surface localization of GAPDH in GBS. By addressing the question of GAPDH export to the cell surface of GBS strain NEM316 and isogenic mutant derivatives of our collection, we found that impaired GAPDH presence in the surface and supernatant of GBS was associated with a lower level of bacterial lysis. We also found that following GBS lysis, GAPDH can associate to the surface of many living bacteria. Finally, we provide evidence for a novel function of the secreted GAPDH as an inducer of apoptosis of murine macrophages.

The pore-forming toxin listeriolysin O mediates a novel entry pathway of L. monocytogenes into human hepatocytes

Intracellular pathogens have evolved diverse strategies to invade and survive within host cells. Among the most studied facultative intracellular pathogens, Listeria monocytogenes is known to express two invasins-InlA and InlB-that induce bacterial internalization into nonphagocytic cells. The pore-forming toxin listeriolysin O (LLO) facilitates bacterial escape from the internalization vesicle into the cytoplasm, where bacteria divide and undergo cell-to-cell spreading via actin-based motility. In the present study we demonstrate that in addition to InlA and InlB, LLO is required for efficient internalization of L. monocytogenes into human hepatocytes (HepG2). Surprisingly, LLO is an invasion factor sufficient to induce the internalization of noninvasive Listeria innocua or polystyrene beads into host cells in a dose-dependent fashion and at the concentrations produced by L. monocytogenes. To elucidate the mechanisms underlying LLO-induced bacterial entry, we constructed novel LLO derivatives locked at different stages of the toxin assembly on host membranes. We found that LLO-induced bacterial or bead entry only occurs upon LLO pore formation. Scanning electron and fluorescence microscopy studies show that LLO-coated beads stimulate the formation of membrane extensions that ingest the beads into an early endosomal compartment. This LLO-induced internalization pathway is dynamin-and F-actin-dependent, and clathrin-independent. Interestingly, further linking pore formation to bacteria/bead uptake, LLO induces F-actin polymerization in a tyrosine kinase-and pore-dependent fashion. In conclusion, we demonstrate for the first time that a bacterial pathogen perforates the host cell plasma membrane as a strategy to activate the endocytic machinery and gain entry into the host cell.