The attenuated phenotype of a Salmonella typhimurium flgM mutant is related to expression of FliC flagellin

Strategies to Block Bacterial Pathogenesis by Interference with Motility and Chemotaxis

Infections by motile, pathogenic bacteria, such as Campylobacter species, Clostridium species, Escherichia coli, Helicobacter pylori, Listeria monocytogenes, Neisseria gonorrhoeae, Pseudomonas aeruginosa, Salmonella species, Vibrio cholerae, and Yersinia species, represent a severe economic and health problem worldwide. Of special importance in this context is the increasing emergence and spread of multidrug-resistant bacteria. Due to the shortage of effective antibiotics for the treatment of infections caused by multidrug-resistant, pathogenic bacteria, the targeting of novel, virulence-relevant factors constitutes a promising, alternative approach. Bacteria have evolved distinct motility structures for movement across surfaces and in aqueous environments. In this review, I will focus on the bacterial flagellum, the associated chemosensory system, and the type-IV pilus as motility devices, which are crucial for bacterial pathogens to reach a preferred site of infection, facilitate biofilm formation, and adhere to surfaces or host cells. Thus, those nanomachines constitute potential targets for the development of novel anti-infectives that are urgently needed at a time of spreading antibiotic resistance. Both bacterial flagella and type-IV pili (T4P) are intricate macromolecular complexes made of dozens of different proteins and their motility function relies on the correct spatial and temporal assembly of various substructures. Specific type-III and type-IV secretion systems power the export of substrate proteins of the bacterial flagellum and type-IV pilus, respectively, and are homologous to virulence-associated type-III and type-II secretion systems. Accordingly, bacterial flagella and T4P represent attractive targets for novel antivirulence drugs interfering with synthesis, assembly, and function of these motility structures.

Genetically programmable pathogen sense and destroy

Pseudomonas aeruginosa (P. aeruginosa) is a major cause of urinary tract and nosocomial infections. Here, we propose and demonstrate proof-of-principle for a potential cell therapy approach against P. aeruginosa. Using principles of synthetic biology, we genetically modified E. coli to specifically detect wild type P. aeruginosa (PAO1) via its quorum sensing (QS) molecule, 3OC 12 HSL. Engineered E. coli sentinels respond to the presence of 3OC 12 HSL by secreting CoPy, a novel pathogen-specific engineered chimeric bacteriocin, into the extracellular medium using the flagellar secretion tag FlgM. Extracellular FlgM-CoPy is designed to kill PAO1 specifically. CoPy was constructed by replacing the receptor and translocase domain of Colicin E3 with that of Pyocin S3. We show that CoPy toxicity is PAO1 specific, not affecting sentinel E. coli or the other bacterial strains tested. In order to define the system's basic requirements and PAO1-killing capabilities, we further determined the growth rates of PAO1 under different conditions and concentrations of purified and secreted FlgM-CoPy. The integrated system was tested by co-culturing PAO1 cells, on semisolid agar plates, together with engineered sentinel E. coli, capable of secreting FlgM-CoPy when induced by 3OC 12 HSL. Optical microscopy results show that the engineered E. coli sentinels successfully inhibit PAO1 growth.

Innate immune detection of flagellin positively and negatively regulates salmonella infection

Salmonella enterica serovar Typhimurium is a flagellated bacterium and one of the leading causes of gastroenteritis in humans. Bacterial flagellin is required for motility and also a prime target of the innate immune system. Innate immune recognition of flagellin is mediated by at least two independent pathways, TLR5 and Naip5-Naip6/NlrC4/Caspase-1. The functional significance of each of the two independent flagellin recognition systems for host defense against wild type Salmonella infection is complex, and innate immune detection of flagellin contributes to both protection and susceptibility. We hypothesized that efficient modulation of flagellin expression in vivo permits Salmonella to evade innate immune detection and limit the functional role of flagellin-specific host innate defenses. To test this hypothesis, we used Salmonella deficient in the anti-sigma factor flgM, which overproduce flagella and are attenuated in vivo. In this study we demonstrate that flagellin recognition by the innate immune system is responsible for the attenuation of flgM⁻ S. Typhimurium, and dissect the contribution of each flagellin recognition pathway to bacterial clearance and inflammation. We demonstrate that caspase-1 controls mucosal and systemic infection of flgM⁻ S. Typhimurium, and also limits intestinal inflammation and injury. In contrast, TLR5 paradoxically promotes bacterial colonization in the cecum and systemic infection, but attenuates intestinal inflammation. Our results indicate that Salmonella evasion of caspase-1 dependent flagellin recognition is critical for establishing infection and that evasion of TLR5 and caspase-1 dependent flagellin recognition helps Salmonella induce intestinal inflammation and establish a niche in the inflamed gut.

Flagella and bacterial pathogenicity

As locomotive organelles, flagella allow bacteria to move toward favorable environments. A flagellum consists of three parts: the basal structure (rotary motor), the hook (universal joint), and the filament (helical propeller). For ages, flagella have been generally regarded as important virulence factors, mainly because of their motility property. However, flagella are getting recognized to play multiple roles with more functions besides motility and chemotaxis. Recent evidence has pinpointed that the bacterial flagella participate in many additional processes including adhesion, biofilm formation, virulence factor secretion, and modulation of the immune system of eukaryotic cells. This mini-review summarizes data from recent studies that elucidated how flagella, as a virulence factor, contribute to bacterial pathogenicity.

Independent emergence of Yersinia ruckeri biotype 2 in the United States and Europe

Biotype 2 (BT2) variants of the bacterium Yersinia ruckeri are an increasing disease problem in U.S. and European aquaculture and have been characterized as serovar 1 isolates that lack both peritrichous flagella and secreted phospholipase activity. The emergence of this biotype has been associated with an increased frequency of enteric redmouth disease (ERM) outbreaks in previously vaccinated salmonid fish. In this study, four independent specific natural mutations that cause the loss of both motility and secreted lipase activity were identified in BT2 strains from the United States, United Kingdom, and mainland Europe. Each of these was a unique mutation in either fliR, flhA, or flhB, all of which are genes predicted to encode essential components of the flagellar secretion apparatus. Our results demonstrate the existence of independent mutations leading to the BT2 phenotype; thus, this phenotype has emerged separately at least four times. In addition, BT2 strains from the United Kingdom were shown to have the same mutant allele found in U.S. BT2 strains, suggesting a common origin of this BT2 lineage. This differentiation of distinct BT2 lineages is of critical importance for the development and validation of alternative vaccines or other treatment strategies intended for the control of BT2 strains.

Immunogenic properties of a recombinant fusion protein containing the C-terminal 19 kDa of Plasmodium falciparum merozoite surface protein-1 and the innate immunity agonist FliC flagellin of Salmonella typhimurium

In a recent study, we demonstrated the immunogenic properties of a new malaria vaccine polypeptide based on a 19 kDa C-terminal fragment of the merozoite surface protein-1 (MSP1(19)) from Plasmodium vivax and an innate immunity agonist, the Salmonella enterica serovar Typhimurium flagellin (FliC). Herein, we tested whether the same strategy, based on the MSP1(19) component of the deadly malaria parasite Plasmodium falciparum, could also generate a fusion polypeptide with enhanced immunogenicity. The His(6)FliC-MSP1(19) fusion protein was expressed from a recombinant Escherichia coli and showed preserved in vitro TLR5-binding activity. In contrast to animals injected with His(6)MSP1(19), mice subcutaneously immunised with the recombinant His(6)FliC-MSP1(19) developed strong MSP1(19)-specific systemic antibody responses with a prevailing IgG1 subclass. Incorporation of other adjuvants, such as CpG ODN 1826, complete and incomplete Freund's adjuvants or Quil-A, improved the IgG responses after the second, but not the third, immunising dose. It also resulted in a more balanced IgG subclass response, as evaluated by the IgG1/IgG2c ratio, and higher cell-mediated immune response, as determined by the detection of antigen-specific interferon-gamma secretion by immune spleen cells. MSP1(19)-specific antibodies recognised not only the recombinant protein, but also the native protein expressed on the surface of P. falciparum parasites. Finally, sera from rabbits immunised with the fusion protein alone inhibited the in vitro growth of three different P. falciparum strains. In summary, these results extend our previous observations and further demonstrate that fusion of the innate immunity agonist FliC to Plasmodium antigens is a promising alternative to improve their immunogenicity.

Toll-like receptor 5-dependent regulation of inflammation in systemic Salmonella enterica Serovar typhimurium infection

Salmonella enterica, a gram-negative pathogen, causes a spectrum of human infections including enterocolitis and typhoid fever. We previously showed that Salmonella flagellin played a role in suppressing intestinal mucosal inflammation in a murine model of acute enterocolitis. In this study, we examined the role of flagellin in the typhoid-like systemic murine Salmonella infection by measuring bacterial proliferation, inflammation, leukocyte recruitment, and cellular apoptosis in Peyer's patches (PPs), mesenteric lymph node (MLN), and spleen. We found that relative to an isogenic wild-type (WT) strain, aflagellate Salmonella exhibited increased proliferation at 4 days postinfection in PPs and MLN but not spleen. The aflagellate mutant also elicited increased local and systemic secretion of inflammatory cytokines such as interleukin-1beta, gamma interferon, and tumor necrosis factor alpha and enhanced surface expression of ICAM-1 on macrophages and dendritic cells (DCs). Furthermore, the recruitment of macrophages and DCs in PPs and MLN, but not spleen, was enhanced upon infection with aflagellate Salmonella. The relative differences between WT and aflagellate Salmonella were highly attenuated in Toll-like receptor 5-deficient (TLR5(-/-)) mice, indicating involvement of TLR5-dependent signaling. Interestingly, infection with the aflagellate mutant also resulted in decreased levels of T-cell apoptosis in PPs relative to infection with WT Salmonella. We postulate that the initial lack of detection of the aflagellate mutant in the mucosa permits increased proliferation within the host and enhances inflammatory signaling in nonepithelial cell types, which subsequently promotes leukocyte recruitment. In contrast, lack of difference in any disease parameter measured in the spleen likely reflects that Salmonella expression of flagellin is downregulated in this organ. Thus, the characteristic inflammatory pathology of Salmonella infection occurs only in PPs and to a lesser extent in MLN during the initial phases of infection and these early responses are dependent on TLR5.

The CorA Mg2+ channel is required for the virulence of Salmonella enterica serovar typhimurium

CorA is the primary Mg(2+) channel in Salmonella enterica serovar Typhimurium. A corA mutant is attenuated in mice and defective for invasion of and replication within epithelial cells. Microarray studies show that several virulence effectors are repressed in a corA mutant strain, which ultimately manifests itself as a decrease in virulence.

Cytotoxic T cell adjuvant effects of three Salmonella enterica flagellins

Bacterial flagellins are important virulence-associated factors and strong inducers of inflammatory responses in mammalian hosts. Flagellins have also been investigated as potential vaccine adjuvants, either for induction of humoral or cellular immune responses, to different target antigens. In this study we investigated the adjuvant properties of three Salmonella enterica flagellins types (FliCd, FliCi and FljB) to an ovalbumin-derived CD8⁺ T cell-restricted epitope (OVA257–264). Although mice immunized with the three tested flagellins elicited antigen-specific activated CD8⁺ T cells, only animals immunized with FliCi and FliCd flagellins admixed with ovalbumin mounted specific in vivo cytotoxic responses to peptide-pulsed target cells. The present results indicate that Salmonella flagellins are endowed with type-specific adjuvant effects toward murine CD8⁺ T cells, a feature that may impact their use as adjuvants for prophylatic or therapeutic vaccines.

Genetic mechanisms involved in the repression of flagellar assembly by Pseudomonas aeruginosa in human mucus

Pseudomonas aeruginosa downregulates flagellin transcription when it is grown in purulent mucus from patients with cystic fibrosis (CF) and non-CF bronchiectasis. This response possibly abrogates the potent inflammatory response mediated by the interaction of flagellin with Toll-like receptor 5. The molecular mechanisms involved are thus far unknown. Known flagellar transcriptional regulators were not involved, thus Tn5 mutagenesis was used to ascertain whether novel regulators existed. Five clones with independent Tn5 insertions in flgM showed derepression of flagellin synthesis, suggesting that FlgM was involved in this phenomenon. Furthermore, examination of mucus-grown bacteria showed FlgM accumulation and overexpression of fliA in mucus-grown bacteria reversed the repression of flagellin synthesis. A related study from our laboratory had identified neutrophil elastase in mucus as the molecule responsible for fliC repression, therefore we examined whether loss of the flagellar hook (FlgE), by proteolysis was involved, because the flagellar hook is required for FlgM export. Western immunoblot of membranes from mucus-grown bacteria showed the absence of FlgE, despite the fact that the protein is made and the operon encoding FlgE is upregulated in mucus. A model is proposed wherein neutrophil elastase in mucus proteolytically cleaves the flagellar hook, thus completion of the hook basal body is never sensed, resulting in FlgM accumulation within the cell, causing repression of flagellin synthesis. We speculate that the cyclical bouts of inflammation observed in CF patients may result from flagellin synthesis and its repression, caused by presence of neutrophils at the site of infection.

In vivo, fliC expression by Salmonella enterica serovar Typhimurium is heterogeneous, regulated by ClpX, and anatomically restricted

FliC is a natural antigen recognized by the innate and adaptive immune systems during Salmonella infection in mice and humans; however, the regulatory mechanisms governing its expression in vivo are incompletely understood. Here, we use flow cytometry to quantify fliC gene expression in single bacteria. In vitro, fliC transcription was not uniformly positive; a viable fliC-negative subpopulation was also identified. Intracellular Salmonella repressed transcription of fliC and its positive regulator fliA, but constitutively transcribed the master regulator flhD; fliC repression required ClpXP protease, known to degrade FlhD. In orally infected mice, fliC transcription was anatomically restricted: Salmonella transcribed fliC in the Peyer's Patches (PP) but not in the mesenteric lymph nodes and spleen. The intracellularly transcribed pagC promoter was upregulated by Salmonella in all tissues, defining the infected PP as a unique environment that initiates expression of intracellularly induced genes and yet permits transcription of fliC. Because a single bacterium can escape the GI tract to colonize deeper tissues, heterogeneous gene expression may have important implications for Salmonella pathogenesis: FliC-positive bacteria in the PP could stimulate inflammation and facilitate the priming of FliC-specific immune responses, while FliC-negative bacteria escape host detection in the gut and spread to systemic sites of replication.

The Escherichia coli O157 flagellar regulatory gene flhC and not the flagellin gene fliC impacts colonization of cattle

A virulent European Escherichia coli O157:H- isolate is nonmotile due to a 12-bp deletion in the flagellar regulatory gene flhC. To investigate the contribution of flhC in the relationship between E. coli O157:H7 and cattle, we constructed a similar flhC regulatory mutant in the well-characterized strain ATCC 43894. There was no difference in the growth rate between the wild type and this regulatory mutant, but phenotypic arrays showed substrate utilization differences. Survival in the bovine gastrointestinal tract and colonization of the rectoanal junction mucosa were assessed. Mixtures of both strains were given orally or rectally to steers or administered into the rumen of cattle dually cannulated at the rumen and duodenum. One day post-oral dose, most rectal/fecal isolates (74%) were the regulatory mutant, but by 3 days post-oral dose and throughout the 42-day experiment, > or = 80% of the isolates were wild type. Among steers given a rectal application of both strains, wild-type isolates were the majority of isolates recovered on all days. The regulatory mutant survived better than the wild type in both the rumen and duodenum. To test the role of motility, a filament mutant (delta fliC) was constructed and similar cattle experiments were performed. On all days post-oral dose, the majority of isolates (64% to 98%) were the filament mutant. In contrast, both strains were recovered equally post-rectal application. Thus, the regulatory mutant survived passage through the bovine gastrointestinal tract better than the wild type but failed to efficiently colonize cattle, and the requirement of flhC for colonization was not dependent on a functional flagellum.

Two DNA invertases contribute to flagellar phase variation in Salmonella enterica serovar Typhimurium strain LT2

Salmonella enterica serovar Typhimurium strain LT2 possesses two nonallelic structural genes, fliC and fljB, for flagellin, the component protein of flagellar filaments. Flagellar phase variation occurs by alternative expression of these two genes. This is controlled by the inversion of a DNA segment, called the H segment, containing the fljB promoter. H inversion occurs by site-specific recombination between inverted repetitious sequences flanking the H segment. This recombination has been shown in vivo and in vitro to be mediated by a DNA invertase, Hin, whose gene is located within the H segment. However, a search of the complete genomic sequence revealed that LT2 possesses another DNA invertase gene that is located adjacent to another invertible DNA segment within a resident prophage, Fels-2. Here, we named this gene fin. We constructed hin and fin disruption mutants from LT2 and examined their phase variation abilities. The hin disruption mutant could still undergo flagellar phase variation, indicating that Hin is not the sole DNA invertase responsible for phase variation. Although the fin disruption mutant could undergo phase variation, fin hin double mutants could not. These results clearly indicate that both Hin and Fin contribute to flagellar phase variation in LT2. We further showed that a phase-stable serovar, serovar Abortusequi, which is known to possess a naturally occurring hin mutation, lacks Fels-2, which ensures the phase stability in this serovar.

FljA-mediated posttranscriptional control of phase 1 flagellin expression in flagellar phase variation of Salmonella enterica serovar Typhimurium

Flagellar phase variation of Salmonella is a phenomenon where two flagellin genes, fliC (phase 1) and fljB (phase 2), are expressed alternately. This is controlled by the inversion of a DNA segment containing the promoter for the fljB gene. The fljB gene constitutes an operon with the fljA gene, which encodes a negative regulator for fliC expression. Previous biochemical analysis suggested that phase variation might depend on alternative synthesis of phase-specific flagellin mRNA (H. Suzuki and T. Iino, J. Mol. Biol. 81:57-70, 1973). However, recently reported results suggested that FljA-dependent inhibition might be mediated by a posttranscriptional control mechanism (H. R. Bonifield and K. T. Hughes, J. Bacteriol. 185:3567-3574, 2003). In this study, we reexamined the mechanism of FljA-mediated inhibition of fliC expression more carefully. Northern blotting analysis revealed that no fliC mRNA was detected in phase 2 cells. However, only a moderate decrease in beta-galactosidase activity was observed from the fliC-lacZ transcriptional fusion gene in phase 2 cells compared with that in phase 1 cells. In contrast, the expression of the fliC-lacZ translational fusion gene was severely impaired in phase 2 cells. The half-life of fliC mRNA was shown to be much shorter in phase 2 cells than in phase 1 cells. Purified His-tagged FljA protein was shown to bind specifically to fliC mRNA and inhibit the translation from fliC mRNA in vitro. On the basis of these results, we propose that in phase 2 cells, FljA binds to fliC mRNA and inhibits its translation, which in turn facilitates its degradation.

Role of motility in the colonization of uropathogenic Escherichia coli in the urinary tract

Uropathogenic Escherichia coli (UPEC) causes most uncomplicated urinary tract infections (UTIs) in humans. Flagellum-mediated motility and chemotaxis have been suggested to contribute to virulence by enabling UPEC to escape host immune responses and disperse to new sites within the urinary tract. To evaluate their contribution to virulence, six separate flagellar mutations were constructed in UPEC strain CFT073. The mutants constructed were shown to have four different flagellar phenotypes: fliA and fliC mutants do not produce flagella; the flgM mutant has similar levels of extracellular flagellin as the wild type but exhibits less motility than the wild type; the motAB mutant is nonmotile; and the cheW and cheY mutants are motile but nonchemotactic. Virulence was assessed by transurethral independent challenges and cochallenges of CBA mice with the wild type and each mutant. CFU/ml of urine or CFU/g bladder or kidney was determined 3 days postinoculation for the independent challenges and at 6, 16, 48, 60, and 72 h postinoculation for the cochallenges. While these mutants colonized the urinary tract during independent challenge, each of the mutants was outcompeted by the wild-type strain to various degrees at specific time points during cochallenge. Altogether, these results suggest that flagella and flagellum-mediated motility/chemotaxis may not be absolutely required for virulence but that these traits contribute to the fitness of UPEC and therefore significantly enhance the pathogenesis of UTIs caused by UPEC.

FliC-specific CD4+ T cell responses are restricted by bacterial regulation of antigen expression

Salmonella typhimurium, a facultatively intracellular pathogen, regulates expression of virulence factors in response to distinct environments encountered during the course of infection. We tested the hypothesis that the transition from extra- to intracellular environments during Salmonella infection triggers changes in Ag expression that impose both temporal and spatial limitations on the host T cell response. CD4(+) T cells recovered from Salmonella immune mice were propagated in vitro using Ag derived from bacteria grown in conditions designed to emulate extra- or intracellular environments in vivo. Extracellular phase bacteria supported a dominant T cell response to the flagellar subunit protein FliC, whereas intracellular phase bacteria were unable to support expansion of FliC-specific T cells from populations known to contain T cells with reactivity to this Ag. This result was attributed to bacterial regulation of FliC expression: transcription and protein levels were repressed in bacteria growing in the spleens of infected mice. Furthermore, Salmonella-infected splenocytes taken directly ex vivo stimulated FliC-specific T cell clones only when intracellular FliC expression was artificially up-regulated. Although it has been suggested that a microanatomical separation of immune T cells and infected APC exists in vivo, we demonstrate that intracellular Salmonella can repress FliC expression below the T cell activation threshold. This potentially provides a mechanism for intracellular Salmonella at systemic sites to avoid detection by Ag-specific T cells primed at intestinal sites early in infection.

Regulation of type III secretion in Bordetella

The BvgAS virulence control system regulates the expression of type III secretion genes in Bordetella subspecies that infect humans and other mammals. We have identified five open reading frames, btrS, btrU, btrX, btrW and btrV, that are activated by BvgAS and encode regulatory factors that control type III secretion at the levels of transcription, protein expression and secretion. The btrS gene product bears sequence similarity to ECF (extracytoplasmic function) sigma factors and is required for transcription of the bsc locus. btrU, btrW and btrV encode proteins predicted to contain PP2C-like Ser phosphatase, HPK (His protein kinase)-like Ser kinase and STAS anti-sigma factor antagonist domains, respectively, which are characteristic of Gram-positive partner switching proteins in Bacillus subtilis. BtrU and BtrW are required for secretion of proteins that are exported by the bsc type III secretion system, whereas BtrV is specifically required for protein synthesis and/or stability. Bordetella species have thus evolved a unique cascade to differentially regulate type III secretion that combines a canonical phosphorelay system with an ECF sigma factor and a set of proteins with domain signatures that define partner switchers, which were traditionally thought to function only in Gram-positive bacteria. The presence of multiple layers and mechanisms of regulation most likely reflects the need to integrate multiple signals in controlling type III secretion. The bsc and btr loci are nearly identical between broad-host-range and human-specific Bordetella. Comparative analysis of Bordetella subspecies revealed that, whereas bsc and btr loci were transcribed in all subspecies, only broad-host-range strains expressed a functional type III secretion system in vitro. The block in type III secretion is post-transcriptional in human-adapted strains, and signal recognition appears to be a point of divergence between subspecies.

Salmonella typhimurium transcytoses flagellin via an SPI2-mediated vesicular transport pathway

Apical colonization of polarized epithelia by Salmonella typhimurium results in translocation of flagellin to the basolateral membrane domain, thus enabling activation of toll-like receptor 5 (TLR5)-mediated pro-inflammatory gene expression. Such flagellin transcytosis occurred without a change in epithelial permeability to 40 kDa FITC dextran, did not require bacterial motility and was independent of transepithelial movement of intact bacteria. Flagellin transcytosis was blocked at 20 degrees C, suggesting dependence on vesicular transport consistent with results from confocal microscopy that showed flagellin independent of bacteria inside epithelial cells. Furthermore, vesicles isolated from S. typhimurium-infected epithelia were highly enriched in flagellin. Flagellin transcytosis was dependent upon genes of Salmonella pathogenicity island (SPI)-2, which alter vesicular trafficking, but independent of SPI-1 that mediates bacterial invasion. Furthermore, such SPI-2 mutants were unable to mediate the localization of flagellin into intracellular vesicles consistent with flagellin transcytosis mediated by a S. typhimurium take-over of host vesicle trafficking pathways. As a result of their inability to transcytose flagellin, apical colonization by SPI-2 mutants induced substantially less epithelial IL-8 secretion than wild-type strains suggesting that such SPI-2 mediated transcytosis of flagellin plays a role in the pathogenesis of the mucosal inflammation characteristic of human Salmonellosis.

STAT Signaling Underlies Difference between Flagellin-induced and Tumor Necrosis Factor-α-induced Epithelial Gene Expression

Both bacterial flagellin and the cytokine tumor necrosis factor-alpha (TNFalpha) are potent activators of intestinal epithelial cell pro-inflammatory gene expression in general; nonetheless, there seem to be distinct differences in the specific patterns of gene expression induced by these agonists. The goal of this study was to define one such difference and elucidate the signaling mechanism responsible for such differential gene induction by these agonists. We observed that expression of inducible nitric-oxide synthase is substantially induced by flagellin but only minimally expressed in response to TNFalpha. This difference seemed to be underlain by differential induction of signal transducers and activators of transcription (STAT) activation in that, whereas flagellin and TNFalpha seemed to be equipotent activators of p38 mitogen-activated protein kinase and nuclear factor-kappaB, flagellin induced substantially higher levels of STAT-1 and -3 tyrosine phosphorylation. Such flagellin-induced STAT activation exhibited delayed kinetics and was ablated by treatment with cycloheximide. Flagellin-induced activation of STAT-3 was abolished via neutralizing antibodies to interleukin (IL)-6, but not interferon (IFN)beta nor IFNgamma; none of these neutralizing antibodies had any effect on flagellin-induced STAT-1 tyrosine phosphorylation. Flagellin induced substantially more IL-6 expression than did TNFalpha, but neither agonist elicited detectable levels of IFN expression. Flagellin-induced expression of inducible nitric-oxide synthase but not IL-6, was abolished by blocking STAT activation with AG490, and was reduced by blocking STAT-3 activation with anti-IL-6. Together, these results indicate that epithelial cell induction of flagellin-specific gene expression is mediated, in part, by STAT activation that results from autocrine activation via IL-6.

TLR5-mediated activation of p38 MAPK regulates epithelial IL-8 expression via posttranscriptional mechanism

Toll-like receptors (TLRs) activate antimicrobial gene expression in response to detection of specific bacterial products. Relatively little is known about TLR5, the only TLR thought to be preferentially expressed by epithelial cells, beyond that it confers activation of the transcription factor NF-kappaB in a MyD-88 dependent manner in response to flagellin. Because TLRs, in general, are also thought to signal through members of the MAPK family, we examined flagellin-induced MAPK activation (via examining its phosphorylation status) and its subsequent role in expression of the chemokine IL-8 in polarized intestinal epithelia. Flagellin, like other proinflammatory stimuli (TNF-alpha, Salmonella typhimurium), activated p38 MAPK in a TLR5-dependent manner, whereas aflagellate bacteria or EGF did not activate this kinase. Although ERK1 and -2 were also observed to be activated in response to flagellin, their activation was not restricted to proinflammatory stimuli because they were also potently activated by aflagellate bacteria (S. typhimurium or Escherichia coli) and EGF (neither of which activate NF-kappaB in these cells). Pharmacological inhibition of p38 MAPK (by SB-203580) potently (IC50 = 10 nM) reduced expression of IL-8 protein (maximal inhibition, 75%) but had no effect on NF-kappaB activation, only slightly attenuated upregulation of IL-8 mRNA levels in response to flagellin, and did not effect IL-8 mRNA stability. Together, these results indicate that epithelial TLR5 mediates p38 activation and subsequently regulates flagellin-induced IL-8 expression independently of NF-kappaB, probably by influencing IL-8 mRNA translation.

Global regulation by CsrA in Salmonella typhimurium

CsrA is a regulator of invasion genes in Salmonella enterica serovar Typhimurium. To investigate the wider role of CsrA in gene regulation, we compared the expression of Salmonella genes in a csrA mutant with those in the wild type using a DNA microarray. As expected, we found that expression of Salmonella pathogenicity island 1 (SPI-1) invasion genes was greatly reduced in the csrA mutant, as were genes outside the island that encode proteins translocated into eukaryotic cells by the SPI-1 type III secretion apparatus. The flagellar synthesis operons, flg and fli, were also poorly expressed, and the csrA mutant was aflagellate and non-motile. The genes of two metabolic pathways likely to be used by Salmonella in the intestinal milieu also showed reduced expression: the pdu operon for utilization of 1,2-propanediol and the eut operon for ethanolamine catabolism. Reduced expression of reporter fusions in these two operons confirmed the microarray data. Moreover, csrA was found to regulate co-ordinately the cob operon for synthesis of vitamin B12, required for the metabolism of either 1,2-propanediol or ethanolamine. Additionally, the csrA mutant poorly expressed the genes of the mal operon, required for transport and use of maltose and maltodextrins, and had reduced amounts of maltoporin, normally a dominant protein of the outer membrane. These results show that csrA controls a number of gene classes in addition to those required for invasion, some of them unique to Salmonella, and suggests a co-ordinated bacterial response to conditions that exist at the site of bacterial invasion, the intestinal tract of a host animal.

The role of motility as a virulence factor in bacteria

Many bacteria that cause diseases of humans, animals and plants use flagella to move. This review summarises recent studies that have analysed the role of motility and chemotaxis in the host-parasite relationship of pathogenic bacteria. These studies have shown that for many pathogens, motility is essential in some phases of their life cycle and that virulence and motility are often intimately linked by complex regulatory networks. Possibilities to exploit bacterial motility as a specific therapeutic antibacterial target to cure or prevent disease are discussed.

Absence of all components of the flagellar export and synthesis machinery differentially alters virulence of Salmonella enterica serovar Typhimurium in models of typhoid fever, survival in macrophages, tissue culture invasiveness, and calf enterocolitis

In this study, we constructed an flhD (the master flagellar regulator gene) mutant of Salmonella enterica serovar Typhimurium and compared the virulence of the strain to that of the wild-type strain in a series of assays that included the mouse model of typhoid fever, the mouse macrophage survival assay, an intestinal epithelial cell adherence and invasion assay, and the calf model of enterocolitis. We found that the flhD mutant was more virulent than its parent in the mouse and displayed slightly faster net growth between 4 and 24 h of infection in mouse macrophages. Conversely, the flhD mutant exhibited diminished invasiveness for human and mouse intestinal epithelial cells, as well as a reduced capacity to induce fluid secretion and evoke a polymorphonuclear leukocyte response in the calf ligated-loop assay. These findings, taken with the results from virulence assessment assays done on an fljB fliC mutant of serovar Typhimurium that does not produce flagellin but does synthesize the flagellar secretory apparatus, indicate that neither the presence of flagella (as previously reported) nor the synthesis of the flagellar export machinery are necessary for pathogenicity of the organism in the mouse. Conversely, the presence of flagella is required for the full invasive potential of the bacterium in tissue culture and for the influx of polymorphonuclear leukocytes in the calf intestine, while the flagellar secretory components are also necessary for the induction of maximum fluid secretion in that enterocolitis model. A corollary to this conclusion is that, as has previously been surmised but not demonstrated in a comparative investigation of the same mutant strains, the mouse systemic infection and macrophage assays measure aspects of virulence different from those of the tissue culture invasion assay, and the latter is more predictive of findings in the calf enterocolitis model.

Flagellar phase variation of Salmonella enterica serovar Typhimurium contributes to virulence in the murine typhoid infection model but does not influence Salmonella-induced enteropathogenesis

Although Salmonella enterica serovar Typhimurium can undergo phase variation to alternately express two different types of flagellin subunit proteins, FljB or FliC, no biological function for this phenomenon has been described. In this investigation, we constructed phase-locked derivatives of S. enterica serovar Typhimurium that expressed only FljB (termed locked-ON) or FliC (termed locked-OFF). The role of phase variation in models of enteric and systemic pathogenesis was then evaluated. There were no differences between the wild-type parent strain and the two phase-locked derivatives in adherence and invasion of mouse epithelial cells in vitro, survival in mouse peritoneal macrophages, or in a bovine model of gastroenteritis. By contrast, the locked-OFF mutant was virulent in mice following oral or intravenous (i.v.) inoculation but the locked-ON mutant was attenuated. When these phase-locked mutants were compared in studies of i.v. kinetics in mice, similar numbers of the two strains were isolated from the blood and spleens of infected animals at 6 and 24 h. However, the locked-OFF mutant was recovered from the blood and spleens in significantly greater numbers than the locked-ON strain by day 2 of infection. By 5 days postinfection, a majority of the mice infected with the locked-OFF mutant had died compared with none of the mice infected with the locked-ON mutant. These results suggest that phase variation is not involved in the intestinal stage of infection but that once S. enterica serovar Typhimurium reaches the spleens of susceptible mice those organisms in the FliC phase can grow and/or survive better than those in the FljB phase. Additional experiments with wild-type S. enterica serovar Typhimurium, fully capable of switching flagellin type, supported this hypothesis. We conclude that organisms that have switched to the FliC(+) phase have a selective advantage in the mouse model of typhoid fever but have no such advantage in invasion of epithelial cells or the induction of enteropathogenesis.

A flagellar gene fliZ regulates the expression of invasion genes and virulence phenotype in Salmonella enterica serovar Typhimurium

We previously reported that the fliZ gene encodes a positive regulatory factor for the class 2 flagellar operons in Salmonella enterica serovar Typhimurium. In this study, we found that the fliZ mutation reduced not only the amounts of excreted flagellar proteins, but also those of several secreted invasion proteins encoded by the genes within Salmonella pathogenicity island 1. Using the lacZ gene fused to a subset of virulence-associated genes, we show that this downregulation was caused by a decreased transcription of the hilA gene, which encodes a positive regulator for the invasion genes. We further show that the fliZ mutation reduced invasion ability of S. enterica serovar Typhimurium to HEp-2 cells. Consistent with these results, orally challenged cells of the fliZ mutant show an attenuated virulence phenotype in a mouse typhoid model. These results indicate that the fliZ gene product positively regulates the invasion genes and is necessary for expression of full virulence.

Salmonella typhimurium translocates flagellin across intestinal epithelia, inducing a proinflammatory response

This study investigated whether soluble paracrine factors mediated Salmonella-induced IL-8 expression in polarized model intestinal epithelia. We found that the basolateral media of model epithelia that had been apically infected with Salmonella typhimurium for a short period (10 minutes) could activate IL-8 secretion in virgin model epithelia, demonstrating that a proinflammatory factor (PIF) was indeed present. Initial characterization found that PIF was a heat-stable protein with a molecular mass of about 50 kDa that acts on the basolateral, but not apical, surface of model intestinal epithelia to elicit IL-8 secretion. PIF was not present in the media of model epithelia stimulated with other inducers of IL-8 secretion (TNF-alpha or carbachol) but was present in S. typhimurium supernatants, indicating PIF is of bacterial origin. PIF was purified from bacterial culture supernatants by anion/cation exchange chromatography and SDS-PAGE and found by using microsequencing to be the protein flagellin. In support of this finding, flagellin-deficient S. typhimurium mutants did not secrete detectable levels of PIF (i.e., a bioactivity that induced IL-8 secretion when placed basolaterally on model epithelia). Furthermore, viable flagellin-deficient mutant organisms (fliC/fljB and flhD) failed to elicit IL-8 secretion when added apically to model intestinal epithelia. These findings indicate that translocation of flagellin across epithelia, subsequent to apical epithelial-S. typhimurium interaction, is likely a major means of activating a mucosal inflammatory response.

The flagellar sigma factor FliA (sigma(28)) regulates the expression of Salmonella genes associated with the centisome 63 type III secretion system

One of the essential features of all pathogenic strains of Salmonella enterica is the ability to enter into nonphagocytic cells. This pathogenic property is mediated by the Salmonella pathogenicity island 1 (SPI-1)-encoded type III secretion system. Expression of components and substrates of this system is subject to complex regulatory mechanisms. These mechanisms include a number of specific and global transcriptional regulatory proteins. In this study we have compared in S. enterica serovars Typhimurium and Typhi the effect of mutations in flagellar genes on the phenotypes associated with the SPI-1 type III protein secretion system. We found that serovar Typhi strains carrying a null mutation in either of the flagellar regulatory genes flhDC or fliA were severely deficient in entry into cultured epithelial cells and macrophage cytotoxicity. This defect could not be reversed by applying a mild centrifugal force, suggesting that the effects of the mutations were not due to the absence of motility. In contrast, the same mutations had no significant effect on the ability of serovar Typhimurium to enter into cultured Henle-407 cells or to induce macrophage cell death. Consistent with these observations, we found that the mutations in the flagellar regulatory proteins significantly reduced the expression of components of the SPI-1-encoded type III system in serovar Typhi but had a marginal effect in serovar Typhimurium. Our results therefore indicate that there is an overlap between regulatory mechanisms that control flagellar and type III secretion gene expression in Salmonella serovar Typhi.

flhDC, the flagellar master operon of Xenorhabdus nematophilus: requirement for motility, lipolysis, extracellular hemolysis, and full virulence in insects

Xenorhabdus is a major insect pathogen symbiotically associated with nematodes of the family Steinernematidae. This motile bacterium displays swarming behavior on suitable media, but a spontaneous loss of motility is observed as part of a phenomenon designated phase variation which involves the loss of stationary-phase products active as antibiotics and potential virulence factors. To investigate the role of one of the transcriptional activators of flagellar genes, FlhDC, in motility and virulence, the Xenorhabdus nematophilus flhDC locus was identified by functional complementation of an Escherichia coli flhD null mutant and DNA sequencing. Construction of X. nematophilus flhD null mutants confirmed that the flhDC operon controls flagellin expression but also revealed that lipolytic and extracellular hemolysin activity is flhDC dependent. We also showed that the flhD null mutant displayed a slightly attenuated virulence phenotype in Spodoptera littoralis compared to that of the wild-type strain. Thus, these data indicated that motility, lipase, hemolysin, or unknown functions controlled by the flhDC operon are involved in the infectious process in insects. Our investigation expands the view of the flagellar regulon as a checkpoint coupled to a major network involving bacterial physiological aspects as well as motility.

Serological and molecular size characterization of flagellins of Pseudomonas syringae pathovars and related bacteria

Flagella from a total of 118 strains representing mostly pathovars of the phytopathogenic group Pseudomonas syringae, but also P. chlororaphis, P. cichorii, P. corrugata, P. fluorescens, P. fuscovaginae, P. stutzeri, P. viridiflava, as well as related phytopathogenic genera (Burkholderia cepacia and Ralstonia solanacearum) were characterized by immuno-fluorescent staining, SDS-PAGE, and immunoblotting. Eighty-six strains of the P. syringae group pathovars, P. cichorii and P. viridiflava were shown to possess flagella of serotypes H1 or H2, composed of a unique flagellin, whose molecular size varied between 31 and 31.5 kDa. Similarities between the P. syringae flagellin and a 31 kDa surface protein involved in pathogenicity are pointed out. The distribution of H1 and H2 antigens in the nine recently described genomospecies of P. syringae-P. viridiflava group suggested that flagellin would represent a phylogenetic marker within the arginin-dihydrolase-negative fluorescent pseudomonads. The characterization of flagellin was proposed as an identification tool at a level situated between genus and species.

The anti-sigma factors

A mechanism for regulating gene expression at the level of transcription utilizes an antagonist of the sigma transcription factor known as the anti-sigma (anti-sigma) factor. The cytoplasmic class of anti-sigma factors has been well characterized. The class includes AsiA form bacteriophage T4, which inhibits Escherichia coli sigma 70; FlgM, present in both gram-positive and gram-negative bacteria, which inhibits the flagella sigma factor sigma 28; SpoIIAB, which inhibits the sporulation-specific sigma factor, sigma F and sigma G, of Bacillus subtilis; RbsW of B. subtilis, which inhibits stress response sigma factor sigma B; and DnaK, a general regulator of the heat shock response, which in bacteria inhibits the heat shock sigma factor sigma 32. In addition to this class of well-characterized cytoplasmic anti-sigma factors, a new class of homologous, inner-membrane-bound anti-sigma factors has recently been discovered in a variety of eubacteria. This new class of anti-sigma factors regulates the expression of so-called extracytoplasmic functions, and hence is known as the ECF subfamily of anti-sigma factors. The range of cell processes regulated by anti-sigma factors is highly varied and includes bacteriophage phage growth, sporulation, stress response, flagellar biosynthesis, pigment production, ion transport, and virulence.

Effect of cellular level of FliK on flagellar hook and filament assembly in Salmonella typhimurium

Frameshift mutations in the fliK gene of Salmonella result in abnormal elongation of the hook and the failure to assemble filament (polyhook phenotype). Second-site suppressor mutations restore filament assembly, but the cells often remain defective in hook-length control (polyhook-filament phenotype). Where the suppressor mutations are intragenic, the second mutation restores the original frame, generating a region of frameshifted sequence, but restoring the natural C terminus. Some of these frameshifted sequences contain a UGA (opal) termination codon. These cells have few flagella and swarm poorly. We suspected that readthrough of UGA by tRNATrp might be the reason for the partial function. When the UGA codon was changed to the Trp codon UGG, flagellar assembly and function were restored to wild-type levels. Conversely, underexpression of the wild-type fliK gene, achieved by changing the sole Trp codon in the sequence (Trp271) to UGA, decreased both the number of flagella and the ability to swarm. These results validate the readthrough hypothesis and indicate that low levels of FliK sustain some degree of flagellation and motility. At low levels of FliK, most flagella had polyhooks. With increasing amounts, the morphology progressively changed to polyhook-filament, and eventually to wild-type hook-filament. When FliK was overproduced, the hook length was slightly shorter (46(+/-7) nm) than that of the wild-type strain (55(+/-9) nm). FliK levels were measured by immunoblotting. Wild-type levels were about 40 to 80 molecules/cell. FliK synthesized by UGA readthrough could be detected when overproduced from plasmid fliK-W271opal, and the levels indicated a probability of readthrough of 0.002 to 0.01. This value was used to estimate the cellular level of underexpressed FliK, which could partly restore function to a fliK mutant, at about 0.07 to 0.8 molecule/cell. These results suggest that FliK does not form a large structure in the cytoplasm and may function as a regulatory protein for protein export. A model for hook-length control is presented that involves feedback from the assembly point to the export apparatus.

Cloning and characterization of the region III flagellar operons of the four Shigella subgroups: genetic defects that cause loss of flagella of Shigella boydii and Shigella sonnei

To detect genetic defects that might have caused loss of flagella in Shigella boydii and Shigella sonnei, the region III flagellar (fli) operons were cloned from certain strains and analyzed with reference to the restriction maps and genetic maps of Escherichia coli fli operons. S. boydii NCTC9733 (strain C5 in this paper) had the 988-bp internal deletion in the fliF gene that encodes a large substructural protein of the basal body. Two strains (C1 and C8) had deletions of the entire fliF operon, and the remaining three (C3, C4, and C9) differed in the size of the restriction fragments carrying the fliF and fliL operons. Loss of flagella in S. boydii appears to originate in some defect in the fliF operon. S. sonnei IID969 lacked the fliD gene and, in place of it, carried two IS600 elements as inverted repeats. Genes downstream from fliD were not detected in the cloned fragment despite its large size but did appear elsewhere in the chromosome. The fliD gene encodes a cap protein of the flagellar filament, and its deletion results in overexpression of class 3 operons by the increased amount of FliA (sigmaF) caused by the excess export of the anti-sigma factor FlgM. Three other strains also had the fliD deletion, and two of them had another deletion in the fliF-fliG-fliH region. The fliD deletion might be the primary cause of loss of flagella in S. sonnei. The lack of FliF or FliD in each subgroup is discussed in connection with the maintenance of virulence and bacterial growth. We also discuss the process of loss of flagella in relation to transposition of IS elements and alterations of the noncoding region, which were found to be common to at least three subgroups.

Induction of flagellation and a novel agar-penetrating flagellar structure in Salmonella enterica grown on solid media: possible consequences for serological identification

Salmonella enterica grown on solid medium containing iron, thiosulfate and 100 mM hexoses and amino acids underwent cell surface differentiation involving increased flagellation (electrophoretic isotypes 60, 54 and 50 kDa), conversion from rough to smooth lipopolysaccharide, and assembly of a matrix that penetrated 1.4% agar. Flagellation was also induced in the avian pathogen S. enterica var Pullorum, which is diagnostically defined as aflagellate. Induction correlated closely with a simple colonial color change when Hektoen Enteric agar was used as the basal growth medium. Group D1 egg-contaminating Salmonella grown under inducing conditions deviated from their expected H-antigen immunoreactivity, suggesting possible consequences for the interpretation of the Kauffman-White identification scheme.