Roles of motility and flagellar structure in pathogenicity of Vibrio cholerae: analysis of motility mutants in three animal models

Purification processes of polymeric nanoparticles: How to improve their clinical translation?

Polymeric nanoparticles, as revolutionary nanomedicines, have offered a new class of diagnostic and therapeutic solutions for a multitude of diseases. With its immense potential, the world witnesses the new age of nanotechnology after the COVID-19 vaccines were developed based on nanotechnology. Even though there are countless benchtop research studies in the nanotechnology world, their integration into commercially available technologies is still restricted. The post-pandemic world demands a surge of research in the domain, which leaves us with the fundamental question: why is the clinical translation of therapeutic nanoparticles so restricted? Complications in nanomedicine purification, among other things, are to blame for the lack of transference. Polymeric nanoparticles, owing to their ease of manufacture, biocompatibility, and enhanced efficiency, are one of the more explored domains in organic-based nanomedicines. Purification of nanoparticles can be challenging and necessitates tailoring the available methods in accordance with the polymeric nanoparticle and impurities involved. Though a number of techniques have been described, there are no available guidelines that help in selecting the method to better suit our requirements. We encountered this difficulty while compiling articles for this review and looking for methods to purify polymeric nanoparticles. The currently accessible bibliography for purification techniques only provides approaches for a specific type of nanomaterial or sometimes even procedures for bulk materials, that are not fully relevant to nanoparticles. In our research, we tried to summarize the available purification techniques using the approach of A.F. Armington. We divided the purification systems into two major classes, namely: phase separation-based techniques (based on the physical differences between the phases) and matter exchange-based techniques (centered on physicochemical induced transfer of materials and compounds). The phase separation methods are based on either using nanoparticle size differences to retain them on a physical barrier (filtration techniques) or using their densities to segregate them (centrifugation techniques). The matter exchange separation methods rely on either transferring the molecules or impurities across a barrier using simple physicochemical phenomena, like the concentration gradients (dialysis method) or partition coefficients (extraction technique). After describing the methods in detail, we highlight their advantages and limitations, mainly focusing on preformed polymer-based nanoparticles. Tailoring a purification strategy takes into account the nanoparticle structure and its integrity, the method selected should be suited for preserving the integrity of the particles, in addition to conforming to the economical, material and productivity considerations. In the meantime, we advocate the use of a harmonized international regulatory framework to define the adequate physicochemical and biological characterization of nanomedicines. An appropriate purification strategy serves as the backbone to achieving desired characteristics, in addition to reducing variability. As a result, the present review aspires to serve as a comprehensive guide for researchers, who are new to the domain, as well as a synopsis of purification strategies and analytical characterization methods used in preclinical studies.

Vibrio cholerae, classification, pathogenesis, immune response, and trends in vaccine development

Vibrio cholerae is the causative agent of cholera, a highly contagious diarrheal disease affecting millions worldwide each year. Cholera is a major public health problem, primarily in countries with poor sanitary conditions and regions affected by natural disasters, where access to safe drinking water is limited. In this narrative review, we aim to summarize the current understanding of the evolution of virulence and pathogenesis of V. cholerae as well as provide an overview of the immune response against this pathogen. We highlight that V. cholerae has a remarkable ability to adapt and evolve, which is a global concern because it increases the risk of cholera outbreaks and the spread of the disease to new regions, making its control even more challenging. Furthermore, we show that this pathogen expresses several virulence factors enabling it to efficiently colonize the human intestine and cause cholera. A cumulative body of work also shows that V. cholerae infection triggers an inflammatory response that influences the development of immune memory against cholera. Lastly, we reviewed the status of licensed cholera vaccines, those undergoing clinical evaluation, and recent progress in developing next-generation vaccines. This review offers a comprehensive view of V. cholerae and identifies knowledge gaps that must be addressed to develop more effective cholera vaccines.

A polyvalent multiepitope protein cross-protects against Vibrio cholerae infection in rabbit colonization and passive protection models

Using epitope- and structure-based multiepitope fusion antigen vaccinology platform, we constructed a polyvalent protein immunogen that presents antigenic domains (epitopes) of Vibrio cholerae toxin-coregulated pilus A, cholera toxin (CT), sialidase, hemolysin A, flagellins (B, C, and D), and peptides mimicking lipopolysaccharide O-antigen on a flagellin B backbone. Mice and rabbits immunized intramuscularly with this polyvalent protein immunogen developed antibodies to all of the virulence factors targeted by the immunogen except lipopolysaccharide. Mouse and rabbit antibodies exhibited functional activities against CT enterotoxicity, CT binding to GM₁ ganglioside, bacterial motility, and in vitro adherence of V. cholerae O1, O139, and non-O1/non-O139 serogroup strains. When challenged orogastrically with V. cholerae O1 El Tor N16961 or a non-O1/non-O139 strain, rabbits IM immunized with the immunogen showed a 2-log (99%) reduction in V. cholerae colonization of small intestines. Moreover, infant rabbits born to the mother immunized with the protein immunogen acquired antibodies passively and were protected from bacterial intestinal colonization (>2-log reduction), severe diarrhea (100%), and mild diarrhea (88%) after infection with V. cholerae O1 El Tor (N16961), O1 classical (O395), O139 (Bengal), or a non-O1/non-O139 strain. This study demonstrated that this polyvalent cholera protein is broadly immunogenic and cross-protective, and an adult rabbit colonization model and an infant rabbit passive protection model fill a gap in preclinical efficacy assessment in cholera vaccine development.

The Assembly of Flagella in Enteropathogenic Escherichia coli Requires the Presence of a Functional Type III Secretion System

In enteropathogenic Escherichia coli (EPEC), the production of flagella and the type III secretion system (T3SS) is activated in the presence of host cultured epithelial cells. The goal of this study was to investigate the relationship between expression of flagella and the T3SS. Mutants deficient in assembling T3SS basal and translocon components (ΔespA, ΔespB, ΔespD, ΔescC, ΔescN, and ΔescV), and in secreting effector molecules (ΔsepD and ΔsepL) were tested for flagella production under several growth conditions. The ΔespA mutant did not produce flagella in any condition tested, although fliC was transcribed. The remaining mutants produced different levels of flagella upon growth in LB or in the presence of cells but were significantly diminished in flagella production after growth in Dulbecco's minimal essential medium. We also investigated the role of virulence and global regulator genes in expression of flagella. The ΔqseB and ΔqseC mutants produced abundant flagella only when growing in LB and in the presence of HeLa cells, indicating that QseB and QseC act as negative regulators of fliC transcription. The ΔgrlR, ΔperA, Δler, Δhns, and Δfis mutants produced low levels of flagella, suggesting these regulators are activators of fliC expression. These data suggest that the presence of an intact T3SS is required for assembly of flagella highlighting the existence in EPEC of a cross-talk between these two virulence-associated T3SSs.

Brucella and Its Hidden Flagellar System

Brucella, a Gram-negative bacterium with a high infective capacity and a wide spectrum of hosts in the animal world, is found in terrestrial and marine mammals, as well as amphibians. This broad spectrum of hosts is closely related to the non-classical virulence factors that allow this pathogen to establish its replicative niche, colonizing epithelial and immune system cells, evading the host's defenses and defensive response. While motility is the primary role of the flagellum in most bacteria, in Brucella, the flagellum is involved in virulence, infectivity, cell growth, and biofilm formation, all of which are very important facts in a bacterium that to date has been described as a non-motile organism. Evidence of the expression of these flagellar proteins that are present in Brucella makes it possible to hypothesize certain evolutionary aspects as to where a free-living bacterium eventually acquired genetic material from environmental microorganisms, including flagellar genes, conferring on it the ability to reach other hosts (mammals), and, under selective pressure from the environment, can express these genes, helping it to evade the immune response. This review summarizes relevant aspects of the presence of flagellar proteins and puts into context their relevance in certain functions associated with the infective process. The study of these flagellar genes gives the genus Brucella a very high infectious versatility, placing it among the main organisms in urgent need of study, as it is linked to human health by direct contact with farm animals and by eventual transmission to the general population, where flagellar genes and proteins are of great relevance.

The Infectious Dose Shapes Vibrio cholerae Within-Host Dynamics

During infection, the rates of pathogen replication, death, and migration affect disease progression, dissemination, transmission, and resistance evolution. Here, we follow the population dynamics of Vibrio cholerae in a mouse model by labeling individual bacteria with one of >500 unique, fitness-neutral genomic tags. Using the changes in tag frequencies and CFU numbers, we inform a mathematical model that describes the within-host spatiotemporal bacterial dynamics. This allows us to disentangle growth, death, forward, and retrograde migration rates continuously during infection. Our model has robust predictive power across various experimental setups. The population dynamics of V. cholerae shows substantial spatiotemporal heterogeneity in replication, death, and migration. Importantly, we find that the niche available to V. cholerae in the host increases with inoculum size, suggesting cooperative effects during infection. Therefore, it is not enough to consider just the likelihood of exposure (50% infectious dose) but rather the magnitude of exposure to predict outbreaks. IMPORTANCE Determining the rates of bacterial migration, replication, and death during infection is important for understanding how infections progress. Separately measuring these rates is often difficult in systems where multiple processes happen simultaneously. Here, we use next-generation sequencing to measure V. cholerae migration, replication, death, and niche size along the mouse gastrointestinal tract. We show that the small intestine of the mouse is a heterogeneous environment, and the population dynamic characteristics change substantially between adjacent gut sections. Our approach also allows us to characterize the effect of inoculum size on these processes. We find that the niche size in mice increases with the infectious dose, hinting at cooperative effects in larger inocula. The dose-response relationship between inoculum size and final pathogen burden is important for the infected individual and is thought to influence the progression of V. cholerae epidemics.

Alkaline pH Increases Swimming Speed and Facilitates Mucus Penetration for Vibrio cholerae

The diarrheal disease cholera is still a burden for populations in developing countries with poor sanitation. To develop effective vaccines and prevention strategies against Vibrio cholerae, we must understand the initial steps of infection leading to the colonization of the small intestine.

Intestinal mucus is the first line of defense against intestinal pathogens. It acts as a physical barrier between epithelial tissues and the lumen that enteropathogens must overcome to establish a successful infection. We investigated the motile behavior of two Vibrio cholerae strains (El Tor C6706 and Classical O395) in mucus using single-cell tracking in unprocessed porcine intestinal mucus. We determined that V. cholerae can penetrate mucus using flagellar motility and that alkaline pH increases swimming speed and, consequently, improves mucus penetration. Microrheological measurements indicate that changes in pH between 6 and 8 (the physiological range for the human small intestine) had little effect on the viscoelastic properties of mucus. Finally, we determined that acidic pH promotes surface attachment by activating the mannose-sensitive hemagglutinin (MshA) pilus in V. cholerae El Tor C6706 without a measurable change in the total cellular concentration of the secondary messenger cyclic dimeric GMP (c-di-GMP). Overall, our results support the hypothesis that pH is an important factor affecting the motile behavior of V. cholerae and its ability to penetrate mucus. Therefore, changes in pH along the human small intestine may play a role in determining the preferred site for V. cholerae during infection.

IMPORTANCE The diarrheal disease cholera is still a burden for populations in developing countries with poor sanitation. To develop effective vaccines and prevention strategies against Vibrio cholerae, we must understand the initial steps of infection leading to the colonization of the small intestine. To infect the host and deliver the cholera toxin, V. cholerae has to penetrate the mucus layer protecting the intestinal tissues. However, the interaction of V. cholerae with intestinal mucus has not been extensively investigated. In this report, we demonstrated using single-cell tracking that V. cholerae can penetrate intestinal mucus using flagellar motility. In addition, we observed that alkaline pH improves the ability of V. cholerae to penetrate mucus. This finding has important implications for understanding the dynamics of infection, because pH varies significantly along the small intestine, between individuals, and between species. Blocking mucus penetration by interfering with flagellar motility in V. cholerae, reinforcing the mucosa, controlling intestinal pH, or manipulating the intestinal microbiome will offer new strategies to fight cholera.

A mutagenic screen reveals NspS residues important for regulation of Vibrio cholerae biofilm formation

Biofilm formation in the human intestinal pathogen Vibrio cholerae is in part regulated by norspermidine, spermidine and spermine. V. cholerae senses these polyamines through a signalling pathway consisting of the periplasmic protein, NspS, and the integral membrane c-di-GMP phosphodiesterase MbaA. NspS and MbaA belong to a proposed class of novel signalling systems composed of periplasmic ligand-binding proteins and membrane-bound c-di-GMP phosphodiesterases containing both GGDEF and EAL domains. In this signal transduction pathway, NspS is hypothesized to interact with MbaA in the periplasm to regulate its phosphodiesterase activity. Polyamine binding to NspS likely alters this interaction, leading to the activation or inhibition of biofilm formation depending on the polyamine. The purpose of this study was to determine the amino acids important for NspS function. We performed random mutagenesis of the nspS gene, identified mutant clones deficient in biofilm formation, determined their responsiveness to norspermidine and mapped the location of these residues onto NspS homology models. Single mutants clustered on two lobes of the NspS model, but the majority were found on a single lobe that appeared to be more mobile upon norspermidine binding. We also identified residues in the putative ligand-binding site that may be important for norspermidine binding and interactions with MbaA. Ultimately, our results provide new insights into this novel signalling pathway in V. cholerae and highlight differences between periplasmic binding proteins involved in transport versus signal transduction.

Humans Surviving Cholera Develop Antibodies against Vibrio cholerae O-Specific Polysaccharide That Inhibit Pathogen Motility

Cholera is a severe dehydrating illness of humans caused by Vibrio cholerae. V. cholerae is a highly motile bacterium that has a single flagellum covered in lipopolysaccharide (LPS) displaying O-specific polysaccharide (OSP), and V. cholerae motility correlates with its ability to cause disease. The mechanisms of protection against cholera are not well understood; however, since V. cholerae is a noninvasive intestinal pathogen, it is likely that antibodies that bind the pathogen or its products in the intestinal lumen contribute to protection from infection. Here, we demonstrate that OSP-specific antibodies isolated from humans surviving cholera in Bangladesh inhibit V. cholerae motility and are associated with protection against challenge in a motility-dependent manner.

The mechanism of protection against cholera afforded by previous illness or vaccination is currently unknown. We have recently shown that antibodies targeting O-specific polysaccharide (OSP) of Vibrio cholerae correlate highly with protection against cholera. V. cholerae is highly motile and possesses a flagellum sheathed in OSP, and motility of V. cholerae correlates with virulence. Using high-speed video microscopy and building upon previous animal-related work, we demonstrate that sera, polyclonal antibody fractions, and OSP-specific monoclonal antibodies recovered from humans surviving cholera block V. cholerae motility at both subagglutinating and agglutinating concentrations. This antimotility effect is reversed by preadsorbing sera and polyclonal antibody fractions with purified OSP and is associated with OSP-specific but not flagellin-specific monoclonal antibodies. Fab fragments of OSP-specific polyclonal antibodies do not inhibit motility, suggesting a requirement for antibody-mediated cross-linking in motility inhibition. We show that OSP-specific antibodies do not directly affect V. cholerae viability, but that OSP-specific monoclonal antibody highly protects against death in the murine cholera model. We used in vivo competitive index studies to demonstrate that OSP-specific antibodies impede colonization and survival of V. cholerae in intestinal tissues and that this impact is motility dependent. Our findings suggest that the impedance of motility by antibodies targeting V. cholerae OSP contributes to protection against cholera.

Proteomics profiles of Cronobacter sakazakii and a fliF mutant: Adherence and invasion in mouse neuroblastoma cells

Cronobacter sakazakii is an opportunistic foodborne pathogen associated with necrotizing enterocolitis, bacteremia, and meningitis in infants. A comparative proteomic study of C. sakazakii ATCC BAA-894 (CS WT) and a fliF::Tn5 mutant was performed, including the ability of both strains to adhere to and invade N1E-115 cells. To achieve this goal, a nonmotile C. sakazakii‬ ATCC BAA-894 fliF::Tn5 (CS fliF::Tn5) strain was generated using an EZ-Tn5 <KAN-2>Tnp Transposome kit. Analysis of differential protein expression showed that 81.49% (361/443) of the proteins were expressed in both strains, 8.35% (37/443) were exclusively expressed in the CS WT strain, and 10.16% (45/443) were exclusively expressed in the CS fliF::Tn5 strain. The main exclusively expressed proteins in the CS WT strain were classified into the "cell motility" and "signal transduction mechanisms" subcategories. The proteins exclusively expressed in the CS fliF::Tn5 strain were classified into the following subcategories: "intracellular trafficking, secretion, and vesicular transport", "replication, recombination, and repair", "nucleotide transport and metabolism", "carbohydrate transport and metabolism", "coenzyme transport and metabolism", and "lipid transport and metabolism". Expression of the Cpa protein was detected in both strains, but Cpa was more abundant in the CS WT strain than in the CS fliF::Tn5 strain. A significant increase (p = 0.0001) in adherence to N1E-115 cells was observed in the nonmotile CS fliF::Tn5 strain (31.3 × 10⁶ CFU/mL) compared to the CS WT strain (14.5 × 10⁶ CFU/mL). Additionally, the CS WT strain showed a 0.17% invasion frequency in N1E-115 cells, which was significantly higher (p = 0.01) than that of the nonmotile CS fliF::Tn5 strain. In conclusion, the proteins involved in the motility were mainly identified by proteomic analysis in the CS WT strain compared to the CS fliF::Tn5 strain. Our data indicate that flagella are required to promote the invasion of N1E-115 cells and that the absence of flagella significantly increases the adherence to N1E-115 cells.

The Secretion of Toxins and Other Exoproteins of Cronobacter: Role in Virulence, Adaption, and Persistence

: Cronobacter species are considered an opportunistic group of foodborne pathogenic bacteria capable of causing both intestinal and systemic human disease. This review describes common virulence themes shared among the seven Cronobacter species and describes multiple exoproteins secreted by Cronobacter, many of which are bacterial toxins that may play a role in human disease. The review will particularly concentrate on the virulence factors secreted by C. sakazakii, C. malonaticus, and C. turicensis, which are the primary human pathogens of interest. It has been discovered that various species-specific virulence factors adversely affect a wide range of eukaryotic cell processes including protein synthesis, cell division, and ion secretion. Many of these factors are toxins which have been shown to also modulate the host immune response. These factors are encoded on a variety of mobile genetic elements such as plasmids and transposons; this genomic plasticity implies ongoing re-assortment of virulence factor genes which has complicated our efforts to categorize Cronobacter into sharply defined genomic pathotypes.

Vibrio Flagellar Synthesis

Vibrio spp. are highly motile Gram-negative bacteria, ubiquitously found in aquatic environments. Some Vibrios are responsible for disease and morbidity of marine invertebrates and humans, while others are studied for their symbiotic interactions. Vibrio spp. are motile due to synthesis of flagella that rotate and propel the bacteria. Many Vibrio spp. synthesize monotrichous polar flagella (e.g., V. cholerae, V. alginolyticus); however, some synthesize peritrichous or lophotrichous flagella. Flagellar-mediated motility is intimately connected to biological and cellular processes such as chemotaxis, biofilm formation, colonization, and virulence of Vibrio spp. This review focuses on the polar flagellum and its regulation in regard to Vibrio virulence and environmental persistence.

A Critical Region in the FlaA Flagellin Facilitates Filament Formation of the Vibrio cholerae Flagellum

Vibrio cholerae is a Gram-negative bacterium with a monotrichous flagellum that causes the human disease cholera. Flagellum-mediated motility is an integral part of the bacterial life cycle inside the host and in the aquatic environment. The V. cholerae flagellar filament is composed of five flagellin subunits (FlaA, FlaB, FlaC, FlaD, and FlaE); however, only FlaA is necessary and sufficient for filament synthesis. flaA is transcribed from a class III flagellar promoter, whereas the other four flagellins are transcribed from class IV promoters. However, expressing flaA from a class IV promoter still facilitated motility in a strain that was otherwise lacking all five flagellins (ΔflaA-E). Furthermore, FlaA from V. parahaemolyticus (FlaAVP; 77% identity) supported motility of the V. cholerae ΔflaA-E strain, whereas FlaA from V. vulnificus (FlaAVV; 75% identity) did not, indicating that FlaA amino acid sequence is responsible for its critical role in flagellar synthesis. Chimeric proteins composed of different domains of FlaAVC and FlaD or FlaAVV revealed that the N-terminal D1 domain (D1N) contains an important region required for FlaA function. Further analyses of chimeric FlaAVC-FlaD proteins identified a lysine residue present at position 145 of the other flagellins but absent from FlaAVC that can prevent monofilament formation. Moreover, the D1N region of amino acids 87 to 153 of FlaAVV inserted into FlaAVC allows monofilament formation but not motility, apparently due to the lack of filament curvature. These results identify residues within the D1N domain that allow FlaAVC to fold into a functional filament structure and suggest that FlaAVC assists correct folding of the other flagellins.IMPORTANCEV. cholerae causes the severe diarrheal disease cholera. Its ability to swim is mediated by rotation of a polar flagellum, and this motility is integral to its ability to cause disease and persist in the environment. The current studies illuminate how one specific flagellin (FlaA) within a multiflagellin structure mediates formation of the flagellar filament, thus allowing V. cholerae to swim. This knowledge can lead to safer vaccines and potential therapeutics to inhibit cholera.

Regulation of Escherichia coli Pathogenesis by Alternative Sigma Factor N

σ^N (also σ⁵⁴) is an alternative sigma factor subunit of the RNA polymerase complex that regulates the expression of genes from many different ontological groups. It is broadly conserved in the Eubacteria with major roles in nitrogen metabolism, membrane biogenesis, and motility. σ^N is encoded as the first gene of a five-gene operon including rpoN (σ^N), ptsN, hpf, rapZ, and npr that has been genetically retained among species of Escherichia, Shigella, and Salmonella. In an increasing number of bacteria, σ^N has been implicated in the control of genes essential to pathogenic behavior, including those involved in adherence, secretion, immune subversion, biofilm formation, toxin production, and resistance to both antimicrobials and biological stressors. For most pathogens how this is achieved is unknown. In enterohemorrhagic Escherichia coli (EHEC) O157, Salmonella enterica, and Borrelia burgdorferi, regulation of virulence by σ^N requires another alternative sigma factor, σ^S, yet the model by which σ^N-σ^S virulence regulation is predicted to occur is varied in each of these pathogens. In this review, the importance of σ^N to bacterial pathogenesis is introduced, and common features of σ^N-dependent virulence regulation discussed. Emphasis is placed on the molecular mechanisms underlying σ^N virulence regulation in E. coli O157. This includes a review of the structure and function of regulatory pathways connecting σ^N to virulence expression, predicted input signals for pathway stimulation, and the role for cognate σ^N activators in initiation of gene systems determining pathogenic behavior.

Pseudoalteromonas piratica strain OCN003 is a coral pathogen that causes a switch from chronic to acute Montipora white syndrome in Montipora capitata

Reports of mass coral mortality from disease have increased over the last two decades. Montipora white syndrome (MWS) is a tissue loss disease that has negatively impacted populations of the coral Montipora capitata in Kāne'ohe Bay, Hawai'i. Two types of MWS have been documented; a progressive disease termed chronic MWS (cMWS), that can be caused by Vibrio owensii strain OCN002, and a comparatively faster disease termed acute MWS (aMWS), that can be caused by Vibrio coralliilyticus strain OCN008. M. capitata colonies exhibiting cMWS can spontaneously switch to aMWS in the field. In this study, a novel Pseudoalteromonas species, P. piratica strain OCN003, fulfilled Koch's postulates of disease causation as another etiological agent of aMWS. Additionally, OCN003 induced a switch from cMWS to aMWS on M. capitata in laboratory infection trials. A comparison of OCN003 and Vibrio coralliilyticus strain OCN008, showed that OCN003 was more effective at inducing the cMWS to aMWS switch in M. capitata than OCN008. This study is the first to demonstrate that similar disease signs on one coral species (aMWS on M. capitata) can be caused by multiple pathogens, and describes the first Pseudoalteromonas species that infects coral.

Treatment with Entinostat Heals Experimental Cholera by Affecting Physical and Chemical Barrier Functions of Intestinal Epithelia

We have shown previously that oral treatment with sodium butyrate or phenylbutyrate in an experimental model of shigellosis improves clinical outcomes and induces the expression of the antimicrobial peptide CAP-18 in the large intestinal epithelia. In a subsequent study, we found that entinostat, an aroylated phenylenediamine compound, has similar therapeutic potential against shigellosis. In this study, we aimed to evaluate entinostat as a potential candidate for host-directed therapy against cholera in an experimental model. Vibrio cholerae-infected rabbits were treated with two different dose regimens of entinostat: either 0.5 mg twice daily for 2 days or 1 mg once daily for 2 days. The effects of treatment on clinical outcomes and V. cholerae shedding (CFU count in stool) were observed. Immunohistochemical analysis was carried out to assess CAP-18 expression in ileal and jejunal mucosae. The serum zonulin level was measured by an enzyme-linked immunosorbent assay (ELISA) to evaluate gut permeability. Infection of rabbits with V. cholerae downregulated CAP-18 expression in the ileal epithelium; the expression was replenished by oral treatment with entinostat at either dose regimen. The level of zonulin, a marker of gut permeability, in serum was upregulated after infection, and this upregulation was counteracted after treatment with entinostat. Entinostat treatment also led to recovery from cholera and a decline in the V. cholerae count in stool. In conclusion, the improved clinical outcome of cholera for rabbits treated with entinostat is associated with the induction of CAP-18 and the reduction of gut epithelial permeability.

A unique role of flagellar function in Aliivibrio salmonicida pathogenicity not related to bacterial motility in aquatic environments

Aliivibrio salmonicida is the causative agent of cold-water vibriosis, a septicemia of farmed salmonid fish. The mechanisms of disease are not well described, and few virulence factors have been identified. However, a requirement for motility in the pathogenesis has been reported. Al. salmonicida is motile by the means of lophotrichous polar flagella, consisting of multiple flagellin subunits that are expressed simultaneously. Here we show that flagellin subunit FlaA, but not FlaD, is of major importance for motility in Al. salmonicida. Deletion of flaA resulted in 62% reduction in motility, as well as a reduction in the fraction of flagellated cells and number of flagella per cell. Similarly, deletion of the gene encoding motor protein motA gave rise to an aflagellate phenotype and cessation of motility. Surprisingly, we found that Al. salmonicida does not require motility for invasion of Atlantic salmon. Nevertheless, in-frame deletion mutants defective of motA and flaA were less virulent in Atlantic salmon challenged by immersion, whereas an effect on virulence after i.p. challenge was only seen for the latter. Our results indicate a complex requirement for motility and/or flagellation in the pathogenesis of cold-water vibriosis, but the mechanisms involved remain unknown. We hypothesize that the differences in virulence observed after immersion and i.p. challenge are related to the immune response of the host.

A Periplasmic Polymer Curves Vibrio cholerae and Promotes Pathogenesis

Pathogenic Vibrio cholerae remains a major human health concern. V. cholerae has a characteristic curved rod morphology, with a longer outer face and a shorter inner face. The mechanism and function of this curvature were previously unknown. Here, we identify and characterize CrvA, the first curvature determinant in V. cholerae. CrvA self-assembles into filaments at the inner face of cell curvature. Unlike traditional cytoskeletons, CrvA localizes to the periplasm and thus can be considered a periskeletal element. To quantify how curvature forms, we developed QuASAR (quantitative analysis of sacculus architecture remodeling), which measures subcellular peptidoglycan dynamics. QuASAR reveals that CrvA asymmetrically patterns peptidoglycan insertion rather than removal, causing more material insertions into the outer face than the inner face. Furthermore, crvA is quorum regulated, and CrvA-dependent curvature increases at high cell density. Finally, we demonstrate that CrvA promotes motility in hydrogels and confers an advantage in host colonization and pathogenesis.

Cyclic diguanylate signaling in Gram-positive bacteria

The nucleotide second messenger 3'-5' cyclic diguanylate monophosphate (c-di-GMP) is a central regulator of the transition between motile and non-motile lifestyles in bacteria, favoring sessility. Most research investigating the functions of c-di-GMP has focused on Gram-negative species, especially pathogens. Recent work in Gram-positive species has revealed that c-di-GMP plays similar roles in Gram-positives, though the precise targets and mechanisms of regulation may differ. The majority of bacterial life exists in a surface-associated state, with motility allowing bacteria to disseminate and colonize new environments. c-di-GMP signaling regulates flagellum biosynthesis and production of adherence factors and appears to be a primary mechanism by which bacteria sense and respond to surfaces. Ultimately, c-di-GMP influences the ability of a bacterium to alter its transcriptional program, physiology and behavior upon surface contact. This review discusses how bacteria are able to sense a surface via flagella and type IV pili, and the role of c-di-GMP in regulating the response to surfaces, with emphasis on studies of Gram-positive bacteria.

Vibrio cholerae Biofilms and Cholera Pathogenesis

Vibrio cholerae can switch between motile and biofilm lifestyles. The last decades have been marked by a remarkable increase in our knowledge of the structure, regulation, and function of biofilms formed under laboratory conditions. Evidence has grown suggesting that V. cholerae can form biofilm-like aggregates during infection that could play a critical role in pathogenesis and disease transmission. However, the structure and regulation of biofilms formed during infection, as well as their role in intestinal colonization and virulence, remains poorly understood. Here, we review (i) the evidence for biofilm formation during infection, (ii) the coordinate regulation of biofilm and virulence gene expression, and (iii) the host signals that favor V. cholerae transitions between alternative lifestyles during intestinal colonization, and (iv) we discuss a model for the role of V. cholerae biofilms in pathogenicity.

The NAIP-NLRC4 inflammasome in innate immune detection of bacterial flagellin and type III secretion apparatus

Bacterial flagella and type III secretion system (T3SS) are evolutionarily related molecular transport machineries. Flagella mediate bacterial motility; the T3SS delivers virulence effectors to block host defenses. The inflammasome is a cytosolic multi-protein complex that activates caspase-1. Active caspase-1 triggers interleukin-1β (IL-1β)/IL-18 maturation and macrophage pyroptotic death to mount an inflammatory response. Central to the inflammasome is a pattern recognition receptor that activates caspase-1 either directly or through an adapter protein. Studies in the past 10 years have established a NAIP-NLRC4 inflammasome, in which NAIPs are cytosolic receptors for bacterial flagellin and T3SS rod/needle proteins, while NLRC4 acts as an adapter for caspase-1 activation. Given the wide presence of flagella and the T3SS in bacteria, the NAIP-NLRC4 inflammasome plays a critical role in anti-bacteria defenses. Here, we review the discovery of the NAIP-NLRC4 inflammasome and further discuss recent advances related to its biochemical mechanism and biological function as well as its connection to human autoinflammatory disease.

Non-O1, non-O139 Vibrio cholerae bacteremic skin and soft tissue infections

BACKGROUND

Non-O1, non-O139 Vibrio cholerae can cause sporadic cases of gastroenteritis and extra-intestinal invasive infections, following exposure to contaminated seawater or freshwater or after consumption of raw seafood. Bacteremic infections with skin and soft tissue manifestations are uncommon and in most cases are associated with liver cirrhosis, haematologic malignancies, diabetes mellitus and other immunosuppressed conditions.

METHODS

The medical literature was reviewed and we found 47 published cases of non-O1, non-O139 Vibrio cholerae bacteremic skin and soft tissue infections. A fatal case of bacteremia with bullous cellulitis in a 43-year-old patient with liver cirrhosis is described, which is the first reported in Greece.

RESULTS

From January 1974 to May 2015, a total of 48 patients with non-O1, non-O139 Vibrio cholerae bacteremia with skin and soft tissue infections were reported. Males predominated. Liver cirrhosis, chronic liver disease and alcohol abuse were common comorbidities. The soft tissue lesions most commonly described were localised cellulitis, with or without bullous and haemorrhagic lesions (66.7%), while necrotising fasciitis was more rare (29.2%). Of the 48 patients with non-O1, non-O139 V. cholerae bacteremic skin and soft tissue infections, 20 (41.7%) died despite treatment.

CONCLUSION

Although rarely encountered, non-O1, non-O139 Vibrio cholerae should be included in the differential diagnosis of bacteremic skin and soft tissue infections in patients with underlying illnesses and epidemiologic risk factors. Timely and appropriate antibiotic and surgical treatments are important in the management of the infection.

The role of flagella in Clostridium difficile pathogenicity

Clostridium difficile is widely publicised as a problem in the health-care system. Disruption of the normal gut microbiota by antibiotic therapy allows C. difficile to colonise the colon. On colonisation, C. difficile produces two toxins that lead to disease, with symptoms ranging from mild-to-severe diarrhoea, to fulminant and often fatal pseudomembranous colitis (PMC). How C. difficile establishes initial colonisation of the host is an area of active investigation. Recently there has been increased research into the role of C. difficile flagella in colonisation and adherence. Novel research has also elucidated a more complex role of flagella in C. difficile virulence pertaining to the regulation of toxin gene expression. This review focuses on new insights into the specific role of C. difficile flagella in colonisation and toxin gene expression.

Structure, gene regulation and environmental response of flagella in Vibrio

Vibrio species are Gram-negative, rod-shaped bacteria that live in aqueous environments. Several species, such as V. harveyi, V. alginotyticus, and V. splendidus, are associated with diseases in fish or shellfish. In addition, a few species, such as V. cholerae and V. parahaemolyticus, are risky for humans due to infections from eating raw shellfish infected with these bacteria or from exposure of wounds to the marine environment. Bacterial flagella are not essential to live in a culture medium. However, most Vibrio species are motile and have rotating flagella which allow them to move into favorable environments or to escape from unfavorable environments. This review summarizes recent studies about the flagellar structure, function, and regulation of Vibrio species, especially focused on the Na⁺-driven polar flagella that are principally responsible for motility and sensing the surrounding environment, and discusses the relationship between flagella and pathogenicity.

The second messenger cyclic Di-GMP regulates Clostridium difficile toxin production by controlling expression of sigD

The Gram-positive obligate anaerobe Clostridium difficile causes potentially fatal intestinal diseases. How this organism regulates virulence gene expression is poorly understood. In many bacterial species, the second messenger cyclic di-GMP (c-di-GMP) negatively regulates flagellar motility and, in some cases, virulence. c-di-GMP was previously shown to repress motility of C. difficile. Recent evidence indicates that flagellar gene expression is tightly linked with expression of the genes encoding the two C. difficile toxins TcdA and TcdB, which are key virulence factors for this pathogen. Here, the effect of c-di-GMP on expression of the toxin genes tcdA and tcdB was determined, and the mechanism connecting flagellar and toxin gene expressions was examined. In C. difficile, increasing c-di-GMP levels reduced the expression levels of tcdA and tcdB, as well as that of tcdR, which encodes an alternative sigma factor that activates tcdA and tcdB expression. We hypothesized that the C. difficile orthologue of the flagellar alternative sigma factor SigD (FliA; σ(28)) mediates regulation of toxin gene expression in response to c-di-GMP. Indeed, ectopic expression of sigD in C. difficile resulted in increased expression levels of tcdR, tcdA, and tcdB. Furthermore, sigD expression enhanced toxin production and increased the cytopathic effect of C. difficile on cultured fibroblasts. Finally, evidence is provided that SigD directly activates tcdR expression and that SigD cannot activate tcdA or tcdB expression independent of TcdR. Taken together, these data suggest that SigD positively regulates toxin genes in C. difficile and that c-di-GMP can inhibit both motility and toxin production via SigD, making this signaling molecule a key virulence gene regulator in C. difficile.

Genetic determinants of swimming motility in the squid light-organ symbiont Vibrio fischeri

Bacterial flagellar motility is a complex cellular behavior required for the colonization of the light-emitting organ of the Hawaiian bobtail squid, Euprymna scolopes, by the beneficial bioluminescent symbiont Vibrio fischeri. We characterized the basis of this behavior by performing (i) a forward genetic screen to identify mutants defective in soft-agar motility, as well as (ii) a transcriptional analysis to determine the genes that are expressed downstream of the flagellar master regulator FlrA. Mutants with severe defects in soft-agar motility were identified due to insertions in genes with putative roles in flagellar motility and in genes that were unexpected, including those predicted to encode hypothetical proteins and cell division-related proteins. Analysis of mutants for their ability to enter into a productive symbiosis indicated that flagellar motility mutants are deficient, while chemotaxis mutants are able to colonize a subset of juvenile squid to light-producing levels. Thirty-three genes required for normal motility in soft agar were also downregulated in the absence of FlrA, suggesting they belong to the flagellar regulon of V. fischeri. Mutagenesis of putative paralogs of the flagellar motility genes motA, motB, and fliL revealed that motA1, motB1, and both fliL1 and fliL2, but not motA2 and motB2, likely contribute to soft-agar motility. Using these complementary approaches, we have characterized the genetic basis of flagellar motility in V. fischeri and furthered our understanding of the roles of flagellar motility and chemotaxis in colonization of the juvenile squid, including identifying 11 novel mutants unable to enter into a productive light-organ symbiosis.

Vibrio cholerae VttR(A) and VttR(B) regulatory influences extend beyond the type 3 secretion system genomic island

A subset of non-O1/non-O139 serogroup strains of Vibrio cholerae cause disease using type 3 secretion system (T3SS)-mediated mechanisms. An ∼50-kb genomic island carries genes encoding the T3SS structural apparatus, effector proteins, and two transmembrane transcriptional regulators, VttR(A) and VttR(B), which are ToxR homologues. Previous experiments demonstrated that VttR(A) and VttR(B) are necessary for colonization in vivo and promote bile-dependent T3SS gene expression in vitro. To better understand the scope of genes that are potential targets of VttR(A) and VttR(B) regulation, we performed deep RNA sequencing using O39 serogroup strain AM-19226 and derivatives carrying deletions in vttR(A) and vttR(B) grown in bile. Comparison of the transcript profiles from ΔvttR(A) and ΔvttR(B) mutant strains to the isogenic parent strain confirmed that VttR(A) and VttR(B) regulate expression of some T3SS island genes and provided additional information about relative expression levels and operon organization. Interestingly, the data also suggested that additional genes, located outside the T3SS island and encoding functions involved in motility, chemotaxis, type 6 secretion, transcriptional regulation, and stress responses, may also by regulated by VttR(A) and VttR(B). We verified transcript levels for selected genes by quantitative reverse transcription (RT)-PCR and then focused additional studies on motility and biofilm formation. The results suggest that VttR(A) and VttR(B) act as part of a complex transcriptional network that coordinates virulence gene expression with multiple cellular phenotypes. VttR(A) and VttR(B) therefore represent horizontally acquired transcriptional regulators with the ability to influence global gene expression in addition to modulating gene expression within the T3SS genomic island.

Defects in D-rhamnosyl residue biosynthetic genes affect lipopolysaccharide structure, motility, and cell-surface hydrophobicity in Pseudomonas syringae pathovar glycinea race 4

D-rhamnose (D-Rha) residue is a major component of lipopolysaccharides (LPSs) in strains of the phytopathogen Pseudomonas syringae pathovar glycinea. To investigate the effects of a deficiency in GDP-D-rhamnose biosynthetic genes on LPS structure and pathogenicity, we generated three mutants defective in D-Rha biosynthetic genes, encoding proteins GDP-D-mannose 4,6-dehydratase (GMD), GDP-4-keto-6-deoxy-D-mannose reductase (RMD), and a putative α-D-rhamnosyltransferase (WbpZ) in P. syringae pv. glycinea race 4. The Δgmd, Δrmd, and ΔwbpZ mutants had a reduced O-antigen polysaccharide consisting of D-Rha residues as compared with the wild type (WT). The swarming motility of the Δgmd, Δrmd, and ΔwbpZ mutant strains decreased and hydrophobicity and adhesion ability increased as compared with WT. Although the mutants had truncated O-antigen polysaccharides, and altered surface properties, they showed virulence to soybean, as WT did.

Comparative study of four flagellins of Vibrio anguillarum: vaccine potential and adjuvanticity

Vibrio anguillarum is the etiological agent of vibriosis, an aquaculture disease that affects a wide range of farmed fish. The genome of V. anguillarum contains five flagellin genes, i.e. flaA, flaB, flaC, flaD, and flaE. In this study, we analyzed the vaccine potential and adjuvanticity of FlaA, FlaB, FlaD, and FlaE in a model of Japanese flounder (Paralichthys olivaceus). For this purpose, recombinant FlaA, FlaB, FlaD, and FlaE were expressed in and purified from Escherichia coli. In vivo immunogenicity analysis showed that antibodies against rFlaA, rFlaB, rFlaD, and rFlaE were detected in rat antiserum raised against live V. anguillarum, with the highest antibody level being that against rFlaB. When administered into flounder via intraperitoneal injection, rFlaA, rFlaD, and rFlaE induced comparable relative percent survival (RPS) rates, which were significantly lower than that induced by rFlaB. Specific serum antibodies were induced by all flagellins, however, the antibody level induced by rFlaB was significantly higher than those induced by other three flagellins. Compared to sera from fish vaccinated with rFlaA, rFlaD, and rFlaE, serum from fish vaccinated with rFlaB significantly reduced the infectivity of V. anguillarum against host cells. To examine the potential adjuvant effect of the flagellins, flounder were immunized with rEsa1, a D15-like surface antigen that induces protective immunity as a subunit vaccine, in the presence or absence of rFlaA, rFlaB, rFlaD, and rFlaE respectively. The results showed that rFlaE, but not other three flagellins, significantly increased the RPS of rEsa1. Compared to fish vaccinated with rEsa1, fish vaccinated with rEsa1 plus rFlaE exhibited a significantly higher level of serum antibodies and enhanced expression of the genes involved in innate and adaptive immunity. Taken together, these results indicate that FlaA, FlaB, FlaD, and FlaE have different immunological properties and, as a result, differ in vaccine and adjuvant potentials.

A novel dnaJ family gene, sflA, encodes an inhibitor of flagellation in marine Vibrio species

The marine bacterium Vibrio alginolyticus has a single polar flagellum. Formation of that flagellum is regulated positively and negatively by FlhF and by FlhG, respectively. The ΔflhF mutant makes no flagellum, whereas the ΔflhFG double-deletion mutant usually lacks a flagellum. However, the ΔflhFG mutant occasionally reverts to become motile by forming peritrichous flagella. We have isolated a suppressor pseudorevertant from the ΔflhFG strain (ΔflhFG-sup). The suppressor strain forms peritrichous flagella in the majority of cells. We identified candidate suppressor mutations by comparing the genome sequence of the parental strain, VIO5, with the genome sequences of the suppressor strains. Two mutations were mapped to a gene, named sflA (suppressor of ΔflhFG), at the VEA003730 locus of the Vibrio sp. strain EX25 genome. This gene is specific for Vibrio species and is predicted to encode a transmembrane protein with a DnaJ domain. When the wild-type gene was introduced into the suppressor strain, motility was impaired. Introducing a mutant version of the sflA gene into the ΔflhFG strain conferred the suppressor phenotype. Thus, we conclude that loss of the sflA gene is responsible for the suppressor phenotype and that the wild-type SflA protein plays a role in preventing polar-type flagella from forming on the lateral cell wall.

Modulation of toxin production by the flagellar regulon in Clostridium difficile

We show in this study that toxin production in Clostridium difficile is altered in cells which can no longer form flagellar filaments. The impact of inactivation of fliC, CD0240, fliF, fliG, fliM, and flhB-fliR flagellar genes upon toxin levels in culture supernatants was assessed using cell-based cytotoxicity assay, proteomics, immunoassay, and immunoblotting approaches. Each of these showed that toxin levels in supernatants were significantly increased in a fliC mutant compared to that in the C. difficile 630 parent strain. In contrast, the toxin levels in supernatants secreted from other flagellar mutants were significantly reduced compared with that in the parental C. difficile 630 strain. Transcriptional analysis of the pathogenicity locus genes (tcdR, tcdB, tcdE, and tcdA) revealed a significant increase of all four genes in the fliC mutant strain, while transcription of all four genes was significantly reduced in fliM, fliF, fliG, and flhB-fliR mutants. These results demonstrate that toxin transcription in C. difficile is modulated by the flagellar regulon. More significantly, mutant strains showed a corresponding change in virulence compared to the 630 parent strain when tested in a hamster model of C. difficile infection. This is the first demonstration of differential flagellum-related transcriptional regulation of toxin production in C. difficile and provides evidence for elaborate regulatory networks for virulence genes in C. difficile.

The transcriptional regulator, CosR, controls compatible solute biosynthesis and transport, motility and biofilm formation in Vibrio cholerae

Vibrio cholerae inhabits aquatic environments and colonizes the human digestive tract to cause the disease cholera. In these environments, V. cholerae copes with fluctuations in salinity and osmolarity by producing and transporting small, organic, highly soluble molecules called compatible solutes, which counteract extracellular osmotic pressure. Currently, it is unclear how V. cholerae regulates the expression of genes important for the biosynthesis or transport of compatible solutes in response to changing salinity or osmolarity conditions. Through a genome-wide transcriptional analysis of the salinity response of V. cholerae, we identified a transcriptional regulator we name CosR for compatible solute regulator. The expression of cosR is regulated by ionic strength and not osmolarity. A transcriptome analysis of a ΔcosR mutant revealed that CosR represses genes involved in ectoine biosynthesis and compatible solute transport in a salinity-dependent manner. When grown in salinities similar to estuarine environments, CosR activates biofilm formation and represses motility independently of its function as an ectoine regulator. This is the first study to characterize a compatible solute regulator in V. cholerae and couples the regulation of osmotic tolerance with biofilm formation and motility.

LitR of Vibrio salmonicida is a salinity-sensitive quorum-sensing regulator of phenotypes involved in host interactions and virulence

Vibrio (Aliivibrio) salmonicida is the causal agent of cold-water vibriosis, a fatal bacterial septicemia primarily of farmed salmonid fish. The molecular mechanisms of invasion, colonization, and growth of V. salmonicida in the host are still largely unknown, and few virulence factors have been identified. Quorum sensing (QS) is a cell-to-cell communication system known to regulate virulence and other activities in several bacterial species. The genome of V. salmonicida LFI1238 encodes products presumably involved in several QS systems. In this study, the gene encoding LitR, a homolog of the master regulator of QS in V. fischeri, was deleted. Compared to the parental strain, the litR mutant showed increased motility, adhesion, cell-to-cell aggregation, and biofilm formation. Furthermore, the litR mutant produced less cryptic bioluminescence, whereas production of acylhomoserine lactones was unaffected. Our results also indicate a salinity-sensitive regulation of LitR. Finally, reduced mortality was observed in Atlantic salmon infected with the litR mutant, implying that the fish were more susceptible to infection with the wild type than with the mutant strain. We hypothesize that LitR inhibits biofilm formation and favors planktonic growth, with the latter being more adapted for pathogenesis in the fish host.

Migration of chemotactic bacteria in soft agar: role of gel concentration

We study the migration of chemotactic wild-type Escherichia coli populations in semisolid (soft) agar in the concentration range C = 0.15-0.5% (w/v). For C≲0.35%, expanding bacterial colonies display characteristic chemotactic rings. At C = 0.35%, however, bacteria migrate as broad circular bands rather than sharp rings. These are growth/diffusion waves arising because of suppression of chemotaxis by the agar and have not been previously reported experimentally to our knowledge. For C = 0.4-0.5%, expanding colonies do not span the depth of the agar and develop pronounced front instabilities. The migration front speed is weakly dependent on agar concentration at C < 0.25%, but decreases sharply above this value. We discuss these observations in terms of an extended Keller-Segel model for which we derived novel transport parameter expressions accounting for perturbations of the chemotactic response by collisions with the agar. The model makes it possible to fit the observed front speed decay in the range C = 0.15-0.35%, and its solutions qualitatively reproduce the observed transition from chemotactic to growth/diffusion bands. We discuss the implications of our results for the study of bacteria in porous media and for the design of improved bacteriological chemotaxis assays.

Vibrio salmonicida pathogenesis analyzed by experimental challenge of Atlantic salmon (Salmo salar)

Cold-water vibriosis (CV) is a bacterial septicemia of farmed salmonid fish and cod caused by the Gram-negative bacterium Vibrio (Aliivibrio) salmonicida. To study the pathogenesis of this marine pathogen, Atlantic salmon was experimentally infected by immersion challenge with wild type V. salmonicida and the bacterial distribution in different organs was investigated at different time points. V. salmonicida was identified in the blood as early as 2 h after challenge demonstrating a rapid establishment of bacteremia without an initial period of colonization of the host. Two days after immersion challenge, only a few V. salmonicida were identified in the intestines, but the amount increased with time. In prolonged CV cases, V. salmonicida was the dominating bacterium of the gut microbiota causing a release of the pathogen to the water. We hypothesize that V. salmonicida uses the blood volume for proliferation during the infection of the fish and the salmonid intestine as a reservoir that favors survival and transmission. In addition, a motility-deficient V. salmonicida strain led us to investigate the impact of motility in the CV pathogenesis by comparing the virulence properties of the mutant with the wild type LFI1238 strain in both i.p. and immersion challenge experiments. V. salmonicida was shown to be highly dependent on motility to gain access to the fish host. After invasion, motility was no longer required for virulence, but the absence of normal flagellation delayed the disease development.

Regulation of toxin production by Bacillus cereus and its food safety implications

Toxin expression is of utmost importance for the food-borne pathogen B. cereus, both in food poisoning and non-gastrointestinal host infections as well as in interbacterial competition. Therefore it is no surprise that the toxin gene expression is tightly regulated by various internal and environmental signals. An overview of the current knowledge regarding emetic and diarrheal toxin transcription and expression is presented in this review. The food safety aspects and management tools such as temperature control, food preservatives and modified atmosphere packaging are discussed specifically for B. cereus emetic and diarrheal toxin production.

Identification and cloning of immunogenic Aliivibrio salmonicida Pal-like protein present in profiled outer membrane and secreted subproteome

Aliivibrio salmonicida is the aetiological agent of cold water vibriosis affecting farmed fish species, a disease that today is fully controlled by vaccination. However, the molecular mechanisms behind the successful vaccine are largely unknown. In order to gain insight into the possible mechanisms of A. salmonicida vaccines, we report here the profiles of both the outer membrane and secreted subproteomes of A. salmonicida LFI315. The 2 subproteomes were resolved by 2-dimensional electrophoresis that identified a total of 82 protein entries. Monoclonal antibodies specific to an unidentified protein antigen were utilized in the immunoproteomic analysis of both outer membrane proteins and extracellular proteins. The immunogenic protein was located in both subproteomes and identified as a 20 kDa peptidoglycan-associated lipoprotein (Pal). The identity of the antigen was verified by heterologous expression of the cloned A. salmonicida pal gene (VSAL_I1899). It is likely that the immunogenic Pal-like protein is among the constituents that act as a protective antigen in the successful vaccine used today. In view of this, it may be considered a potentially useful component in future vaccine development and pathogenicity studies.

Morphological analysis of the sheathed flagellum of Brucella melitensis

Background

It was recently shown that B. melitensis is flagellated. However, the flagellar structure remains poorly described.

Findings

We analyzed the structure of the polar sheathed flagellum of B. melitensis by TEM analysis and demonstrated that the Ryu staining is a good method to quickly visualize the flagellum by optical microscopy. The TEM analysis demonstrated that an extension of the outer membrane surrounds a filament ending by a club-like structure. The ΔftcR, ΔfliF, ΔflgE and ΔfliC flagellar mutants still produce an empty sheath.

Conclusions

Our results demonstrate that the flagellum of B. melitensis has the characteristics of the sheathed flagella. Our results also suggest that the flagellar sheath production is not directly linked to the flagellar structure assembly and is not regulated by the FtcR master regulator.

Role of FlgT in anchoring the flagellum of Vibrio cholerae

Flagellar motility has long been regarded as an important virulence factor. In Vibrio cholerae, the single polar flagellum is essential for motility as well as for proper attachment and colonization. In this study, we demonstrate that the novel flagellar protein FlgT is involved in anchoring the flagellum to the V. cholerae cell. A screen for novel colonization factors by use of TnphoA mutagenesis identified flgT. An in-frame deletion of flgT established that FlgT is required for attachment, colonization, and motility. Transmission electron microscopy revealed that while the flgT mutant is capable of assembling a phenotypically normal flagellum, the flgT population is mostly aflagellate compared to the wild-type population. Further analyses indicated that the flagellum of the flgT mutant is released into the culture supernatant from the cell upon completion of assembly. Additionally, hook basal body complexes appear to be released along with the filament. These results indicate that FlgT functions to stabilize the flagellar apparatus at the pole of the cell.

The Vibrio cholerae flagellar regulatory hierarchy controls expression of virulence factors

Vibrio cholerae is a motile bacterium responsible for the disease cholera, and motility has been hypothesized to be inversely regulated with virulence. We examined the transcription profiles of V. cholerae strains containing mutations in flagellar regulatory genes (rpoN, flrA, flrC, and fliA) by utilizing whole-genome microarrays. Results revealed that flagellar transcription is organized into a four-tiered hierarchy. Additionally, genes with proven or putative roles in virulence (e.g., ctx, tcp, hemolysin, and type VI secretion genes) were upregulated in flagellar regulatory mutants, which was confirmed by quantitative reverse transcription-PCR. Flagellar regulatory mutants exhibit increased hemolysis of human erythrocytes, which was due to increased transcription of the thermolabile hemolysin (tlh). The flagellar regulatory system positively regulates transcription of a diguanylate cyclase, CdgD, which in turn regulates transcription of a novel hemagglutinin (frhA) that mediates adherence to chitin and epithelial cells and enhances biofilm formation and intestinal colonization in infant mice. Our results demonstrate that the flagellar regulatory system modulates the expression of nonflagellar genes, with induction of an adhesin that facilitates colonization within the intestine and repression of virulence factors maximally induced following colonization. These results suggest that the flagellar regulatory hierarchy facilitates correct spatiotemporal expression patterns for optimal V. cholerae colonization and disease progression.

Characterization of two outer membrane proteins, FlgO and FlgP, that influence vibrio cholerae motility

Vibrio cholerae is highly motile by the action of a single polar flagellum. The loss of motility reduces the infectivity of V. cholerae, demonstrating that motility is an important virulence factor. FlrC is the sigma-54-dependent positive regulator of flagellar genes. Recently, the genes VC2206 (flgP) and VC2207 (flgO) were identified as being regulated by FlrC via a microarray analysis of an flrC mutant (D. C. Morris, F. Peng, J. R. Barker, and K. E. Klose, J. Bacteriol. 190:231-239, 2008). FlgP is reported to be an outer membrane lipoprotein required for motility that functions as a colonization factor. The study reported here focuses on the characterization of flgO, the first gene in the flgOP operon. We show that FlgO and FlgP are important for motility, as strains with mutations in the flgOP genes have reduced motility phenotypes. The flgO and flgP mutant populations display fewer motile cells as well as reduced numbers of flagellated cells. The flagella produced by the flgO and flgP mutant strains are shorter in length than the wild-type flagella, which can be restored by inhibiting rotation of the flagellum. FlgO is an outer membrane protein that localizes throughout the membrane and not at the flagellar pole. Although FlgO and FlgP do not specifically localize to the flagellum, they are required for flagellar stability. Due to the nature of these motility defects, we established that the flagellum is not sufficient for adherence; rather, motility is the essential factor required for attachment and thus colonization by V. cholerae O1 of the classical biotype. This study reveals a novel mechanism for which the outer membrane proteins FlgO and FlgP function in motility to mediate flagellar stability and influence attachment and colonization.

Functional characterization of FlgM in the regulation of flagellar synthesis and motility in Yersinia pseudotuberculosis

We describe here the functional characterization of the flgM gene in Yersinia pseudotuberculosis. Direct interaction of FlgM with the alternative sigma factor σ 28 (FliA) was first confirmed. A conserved region in the C-terminus of FlgM was found which included the σ 28 binding domain. By site-directed mutagenesis, bacterial two-hybrid analysis and Western blotting, the primary FlgM binding sites with σ 28 were shown to be Ile85, Ala86 and Leu89. A role for FlgM in swimming motility was demonstrated by inactivation of flgM and subsequent complementation in trans. Transcriptional fusion analyses showed differential gene expression of flhDC, fliA, flgM and fliC in the fliA and flgM mutants compared with the wild-type. flhDC expression was not influenced by σ 28 or FlgM while fliA expression was abolished in the fliA mutant and considerably reduced in the flgM mutant when compared to the wild-type, indicating that both FliA and FlgM can activate fliA transcription. Conversely, flgM transcription was higher in the fliA mutant when compared to the wild-type, suggesting that flgM transcription was repressed by σ 28. Interestingly, fliC expression was markedly increased in the flgM mutant, suggesting a negative regulatory role for FlgM in fliC expression. The transcription of other σ-dependent genes (cheW, flgD, flaA, csrA and fliZ) was also examined in fliA and flgM mutant backgrounds and this revealed that other σ-factors apart from σ 28 may be involved in flagellar biogenesis in Y. pseudotuberculosis. Taking together the motility phenotypes and effects of flgM mutation on the regulation of these key motility genes, we propose that the mechanisms regulating flagellar biogenesis in Y. pseudotuberculosis may differ from those described for other bacteria.

Differential modulation of NF-κB-mediated pro-inflammatory response in human intestinal epithelial cells by cheY homologues of Vibrio cholerae

Vibrio cholerae, the etiological agent of cholera, colonizes the small intestine, produces an enterotoxin and causes acute inflammatory response at intestinal epithelial surface. Chemotaxis and motility greatly influence the infectivity of V. cholerae although the role of chemotaxis genes in V. cholerae pathogenesis is less well understood. Four cheY genes are present in three clusters in the complete genome sequence of V. cholerae. A less motile and less adherent mutant was generated by inactivation of cheY-3 (O395Y3N) or cheY-4 (O395Y4N) whereas alterations in motility or adherence were not observed for cheY-1 (O395Y1N) or cheY-2 (O395Y2N) insertional mutants. In contrast to O395Y1N and O395Y2N, O395Y3N and O395Y4N showed reduced cholera toxin production compared to wild-type in vitro. Infection of the human intestinal epithelial cell line Int407 with O395Y3N and O395Y4N caused reduced secretion of interleukin (IL)-1a, IL-6, tumor necrosis factor (TNF-a) and monocyte chemotactic protein-1 (MCP-1) compared to wild-type and was associated with delayed activation of nuclear factor kappa B (NF-κB) p65 and its co-activator cAMP response element binding protein (CREB). Further, the absence of nuclear translocation of NF-κB p50 subunit upon infection with O395Y3N or O395Y4N and its reversal upon complementation indicates the involvement of cheY-3 and cheY-4 in V. cholerae-induced pro-inflammatory response in the INT407 cell line.

Role of the Helicobacter hepaticus flagellar sigma factor FliA in gene regulation and murine colonization

The enterohepatic Helicobacter species Helicobacter hepaticus colonizes the murine intestinal and hepatobiliary tract and is associated with chronic intestinal inflammation, gall stone formation, hepatitis, and hepatocellular carcinoma. Thus far, the role of H. hepaticus motility and flagella in intestinal colonization is unknown. In other, closely related bacteria, late flagellar genes are mainly regulated by the sigma factor FliA (sigma(28)). We investigated the function of the H. hepaticus FliA in gene regulation, flagellar biosynthesis, motility, and murine colonization. Competitive microarray analysis of the wild type versus an isogenic fliA mutant revealed that 11 genes were significantly more highly expressed in wild-type bacteria and 2 genes were significantly more highly expressed in the fliA mutant. Most of these were flagellar genes, but four novel FliA-regulated genes of unknown function were identified. H. hepaticus possesses two identical copies of the gene encoding the FliA-dependent major flagellin subunit FlaA (open reading frames HH1364 and HH1653). We characterized the phenotypes of mutants in which fliA or one or both copies of the flaA gene were knocked out. flaA_1 flaA_2 double mutants and fliA mutants did not synthesize detectable amounts of FlaA and possessed severely truncated flagella. Also, both mutants were nonmotile and unable to colonize mice. Mutants with either flaA gene knocked out produced flagella morphologically similar to those of wild-type bacteria and expressed FlaA and FlaB. flaA_1 mutants which had flagella but displayed reduced motility did not colonize mice, indicating that motility is required for intestinal colonization by H. hepaticus and that the presence of flagella alone is not sufficient.

Decreased potency of the Vibrio cholerae sheathed flagellum to trigger host innate immunity

Vibrio cholerae is a monoflagellated gram-negative bacterium that causes the severe diarrheal disease cholera. In contrast to Salmonella enterica serovar Typhimurium infection, which is accompanied by both acute diarrhea and high-level inflammation, V. cholerae infection is largely noninflammatory in human hosts. Bacterial flagella are composed of flagellin, a highly conserved protein that is also a target of the innate immune response. Because the V. cholerae flagellum is covered by a sheath, we hypothesized that it might be less prone to activation of the innate immune response than nonsheathed flagella, such as those produced by Salmonella serovar Typhimurium. Indeed, compared with Salmonella serovar Typhimurium flagella, V. cholerae flagella demonstrated significantly reduced NF-kappaB activation in A549 human pulmonary epithelial cells. However, V. cholerae flagellin monomers, FlaD and FlaC, were almost equally potent with purified FliC, a monomer derived from Salmonella serovar Typhimurium flagella, in NF-kappaB activation. Heat- and acid-induced dissociation assays showed that Salmonella serovar Typhimurium flagella disassembled far more readily into monomeric flagellins than V. cholerae flagella, suggesting that the differential levels of NF-kappaB activation by V. cholerae and Salmonella serovar Typhimurium flagella are likely attributable to the difference in their flagellin shedding. Our results suggest that monomer dissociation of V. cholerae flagella is suppressed likely due to the presence of the sheath and that this unique structural feature of V. cholerae flagella may have evolved as a strategy to evade flagellin-triggered host innate immune responses in various host species.