The role of motility as a virulence factor in bacteria

Analysis of the HD-GYP domain cyclic dimeric GMP phosphodiesterase reveals a role in motility and the enzootic life cycle of Borrelia burgdorferi

HD-GYP domain cyclic dimeric GMP (c-di-GMP) phosphodiesterases are implicated in motility and virulence in bacteria. Borrelia burgdorferi possesses a single set of c-di-GMP-metabolizing enzymes, including a putative HD-GYP domain protein, BB0374. Recently, we characterized the EAL domain phosphodiesterase PdeA. A mutation in pdeA resulted in cells that were defective in motility and virulence. Here we demonstrate that BB0374/PdeB specifically hydrolyzed c-di-GMP with a K(m) of 2.9 nM, confirming that it is a functional phosphodiesterase. Furthermore, by measuring phosphodiesterase enzyme activity in extracts from cells containing the pdeA pdeB double mutant, we demonstrate that no additional phosphodiesterases are present in B. burgdorferi. pdeB single mutant cells exhibit significantly increased flexing, indicating a role for c-di-GMP in motility. Constructing and analyzing a pilZ pdeB double mutant suggests that PilZ likely interacts with chemotaxis signaling. While virulence in needle-inoculated C3H/HeN mice did not appear to be altered significantly in pdeB mutant cells, these cells exhibited a reduced ability to survive in Ixodes scapularis ticks. Consequently, those ticks were unable to transmit the infection to naïve mice. All of these phenotypes were restored when the mutant was complemented. Identification of this role of pdeB increases our understanding of the c-di-GMP signaling network in motility regulation and the life cycle of B. burgdorferi.

The Pseudomonas quinolone signal (PQS), and its precursor HHQ, modulate interspecies and interkingdom behaviour

The Pseudomonas quinolone signal (PQS), and its precursor 2-heptyl-4-quinolone (HHQ), play a key role in coordinating virulence in the important cystic fibrosis pathogen Pseudomonas aeruginosa. The discovery of HHQ analogues in Burkholderia and other microorganisms led us to investigate the possibility that these compounds can influence interspecies behaviour. We found that surface-associated phenotypes were repressed in Gram-positive and Gram-negative bacteria as well as in pathogenic yeast in response to PQS and HHQ. Motility was repressed in a broad range of bacteria, while biofilm formation in Bacillus subtilis and Candida albicans was repressed in the presence of HHQ, though initial adhesion was unaffected. Furthermore, HHQ exhibited potent bacteriostatic activity against several Gram-negative bacteria, including pathogenic Vibrio vulnificus. Structure-function analysis using synthetic analogues provided an insight into the molecular properties that underpin the ability of these compounds to influence microbial behaviour, revealing the alkyl chain to be fundamental. Defining the influence of these molecules on microbial-eukaryotic-host interactions will facilitate future therapeutic strategies which seek to combat microorganisms that are recalcitrant to conventional antimicrobial agents.

Swarming motility by Photorhabdus temperata is influenced by environmental conditions and uses the same flagella as that used in swimming motility

Photorhabdus temperata, an insect pathogen and nematode symbiont, is motile in liquid medium by swimming. We found that P. temperata was capable of surface movement, termed swarming behavior. Several lines of evidence indicate that P. temperata use the same flagella for both swimming and swarming motility. Both motility types required additional NaCl or KCl in the medium and had peritrichous flagella, which were composed of the same flagellin as detected by immunoblotting experiments. Mutants defective in flagellar structural proteins were nonmotile for both motility types. Unlike swimming, we observed swarming behavior to be a social form of movement in which the cells coordinately formed intricate channels covering a surface. The constituents of the swarm media affected motility. Swarming was optimal on low agar concentrations; as agar concentrations increased, swarm ring diameters decreased.

Digestion of extracellular DNA is required for giant colony formation of Staphylococcus aureus

Staphylococcus aureus spreads on soft agar surfaces and forms giant colonies. Here, we examined the inhibitory role of extracellular DNA on the colony spreading activity. The double-deletion mutation of nuc1 and nuc2, which encode secretory nucleases, increased extracellular DNA and showed a decreased ability to form giant colonies. The addition of DNase I or micrococcal nuclease to the soft agar restored the ability of the nuc1-nuc2 double mutant to form giant colonies. In addition, the promoter activities of nuc1 and nuc2 in the wild-type strain were elevated in the peripheral region of the giant colony. These findings suggest that the digestion of extracellular DNA by secretory nucleases is required for the colony spreading activity of S. aureus.

Using experimental evolution to explore natural patterns between bacterial motility and resistance to bacteriophages

Resistance of bacteria to phages may be gained by alteration of surface proteins to which phages bind, a mechanism that is likely to be costly as these molecules typically have critical functions such as movement or nutrient uptake. To address this potential trade-off, we combine a systematic study of natural bacteria and phage populations with an experimental evolution approach. We compare motility, growth rate and susceptibility to local phages for 80 bacteria isolated from horse chestnut leaves and, contrary to expectation, find no negative association between resistance to phages and bacterial motility or growth rate. However, because correlational patterns (and their absence) are open to numerous interpretations, we test for any causal association between resistance to phages and bacterial motility using experimental evolution of a subset of bacteria in both the presence and absence of naturally associated phages. Again, we find no clear link between the acquisition of resistance and bacterial motility, suggesting that for these natural bacterial populations, phage-mediated selection is unlikely to shape bacterial motility, a key fitness trait for many bacteria in the phyllosphere. The agreement between the observed natural pattern and the experimental evolution results presented here demonstrates the power of this combined approach for testing evolutionary trade-offs.

Sugar-responsive gene expression and the agr system are required for colony spreading in Staphylococcus aureus

Staphylococcus aureus spreads on soft agar surfaces, which is called "colony spreading". Here, we report that the colony spreading in S. aureus was promoted by the addition of glucose to soft agar plates. Disruption of ccpA and hprK, which are involved in catabolite repression, decreased the colony spreading ability promoted by glucose. Deletion of the agr locus, a virulence regulatory element whose expression is activated by glucose in a ccpA-dependent manner, abolished the colony spreading promoted by glucose. Disruption of clpP and arlRS, which contributes to agr expression, also decreased glucose-promoted colony spreading. These findings suggest that S. aureus colony spreading requires the expression of agr, which is positively regulated by environmental carbon sources, and that virulence gene expression and colony spreading induced by agr are simultaneously activated in the S. aureus infectious process.

A MotN mutant of Ralstonia solanacearum is hypermotile and has reduced virulence

Ralstonia solanacearum is a soil-borne plant pathogen that causes bacterial wilt disease on many plant species. We previously showed that swimming motility contributes to virulence of this bacterium in the early stages of host invasion and colonization. In this study we identified a new negative regulator of motility, named motN, that is located in a cluster of motility-related genes. A motN mutant was hypermotile both on 0.3% agar motility plates and in rich and minimal medium broth. However, like its wild-type parent, it was largely nonmotile inside plants. The motN mutant cells appeared hyperflagellated, and sheared cell protein preparations from motN contained more flagellin than preparations from wild-type cells. The motN strain was significantly reduced in virulence in a naturalistic soil soak assay on tomato plants. However, the motN mutant had wild-type virulence when it was inoculated directly into the plant vascular system. This suggests that motN makes its contribution to virulence early in disease development. The motN mutant formed weaker biofilms than the wild type, but it attached normally to tomato roots and colonized tomato stems as well as its wild-type parent. Phenotypic analysis and gene expression studies indicated that MotN directly or indirectly represses transcription of the major motility regulator FlhDC. MotN was also connected with other known motility and virulence regulators, PehSR, VsrBC, and VsrAD, via uncertain mechanisms. Together, these results demonstrate the importance of precise regulation of flagellum-mediated motility in R. solanacearum.

High temperature and bacteriophages can indirectly select for bacterial pathogenicity in environmental reservoirs

The coincidental evolution hypothesis predicts that traits connected to bacterial pathogenicity could be indirectly selected outside the host as a correlated response to abiotic environmental conditions or different biotic species interactions. To investigate this, an opportunistic bacterial pathogen, Serratia marcescens, was cultured in the absence and presence of the lytic bacteriophage PPV (Podoviridae) at 25°C and 37°C for four weeks (N = 5). At the end, we measured changes in bacterial phage-resistance and potential virulence traits, and determined the pathogenicity of all bacterial selection lines in the Parasemia plantaginis insect model in vivo. Selection at 37°C increased bacterial motility and pathogenicity but only in the absence of phages. Exposure to phages increased the phage-resistance of bacteria, and this was costly in terms of decreased maximum population size in the absence of phages. However, this small-magnitude growth cost was not greater with bacteria that had evolved in high temperature regime, and no trade-off was found between phage-resistance and growth rate. As a result, phages constrained the evolution of a temperature-mediated increase in bacterial pathogenicity presumably by preferably infecting the highly motile and virulent bacteria. In more general perspective, our results suggest that the traits connected to bacterial pathogenicity could be indirectly selected as a correlated response by abiotic and biotic factors in environmental reservoirs.

Identification of a luxO-regulated extracellular protein Pep and its roles in motility in Vibrio alginolyticus

The fish pathogen Vibrio alginolyticus virulence factors and regulation mechanism are barely known except the quorum sensing regulated extracellular products. In the present study, the cleavable isotope-coded affinity tags (cICATs) were used to analyze the differentially expressed extracellular proteins of V. alginolyticus wild-type and a ΔluxO mutant, and a putative peptidase Pep was identified. Quantitative real-time reverse transcription PCR confirmed that the transcription of pep was down-regulated by LuxO. Furthermore, western blotting demonstrated that the expression of Pep was cell-density dependent and quorum sensing regulated in V. alginolyticus. The pep in-frame mutant abolished the swimming and swarming motility, suggesting that it was involved in motility regulation in V. alginolyticus. In liquid medium, the polar flagellum was observed intact in ∆pep mutant while the chemotaxis related gene cheY, which is a response regulator causing change in direction of flagellar rotation, was down-regulated in ∆pep mutant. When ∆pep mutant was grown on hard agar plate, the lateral flagella disappeared and the transcriptions of flagellar biogenesis genes flaK, fliS, lafK, and lafA were down-regulated in ∆pep mutant. These results suggested that Pep was a luxO-regulated extracellular protein and involved in the motility of V. alginolyticus.

Activation of motility by sensing short-chain fatty acids via two steps in a flagellar gene regulatory cascade in enterohemorrhagic Escherichia coli

The regulated expression of virulence genes is critical for successful infection by an intestinal pathogen. Bacteria rely on sensing environmental signals to find preferable niches and reach the infectious state. Orally ingested enterohemorrhagic Escherichia coli (EHEC) travels through the gastrointestinal tract and encounters a variety of environmental factors, some of which act as triggering signals for the induction of virulence genes. Butyrate, one of the main short-chain fatty acids (SCFAs), is such a signal, enhancing the expression of genes for intimate attachment and type III secretion. We further explored the role of SCFAs and found a positive effect of SCFAs on flagellar expression. Although EHEC did not produce flagella when grown in Dulbecco's modified Eagle's medium (DMEM), a tissue culture medium that enhances virulence gene expression, the addition of SCFAs to the medium induced the production of flagella, and the EHEC bacteria became motile. Among SCFAs, butyrate simultaneously activates both virulence and flagellar genes. Flagella did not affect initial adherence, and they were not expressed in adherent bacteria during microcolony formation. SCFAs activated flagellar genes via two regulatory steps. Butyrate activated the flhDC regulatory genes through leucine-responsive regulatory protein (Lrp), which is also a regulator of virulence genes. However, butyrate, acetate, and propionate also activated downstream genes independently of flhDC activation. Consequently, when encountering increased concentrations of SCFAs, which are abundant in acetate, in the intestine, EHEC first activates flagellar production and motility, followed by genes involved in adherence and type III secretion, which leads to efficient adherence in a preferable niche.

Bacillus cereus cytotoxins Hbl, Nhe and CytK are secreted via the Sec translocation pathway

BACKGROUND

Bacillus cereus and the closely related Bacillus thuringiensis are Gram positive opportunistic pathogens that may cause food poisoning, and the three secreted pore-forming cytotoxins Hbl, Nhe and CytK have been implicated as the causative agents of diarrhoeal disease. It has been proposed that the Hbl toxin is secreted using the flagellar export apparatus (FEA) despite the presence of Sec-type signal peptides. As protein secretion is of key importance in virulence of a microorganism, the mechanisms by which these toxins are secreted were further investigated.

RESULTS

Sec-type signal peptides were identified in all toxin components, and secretion of Hbl component B was shown to be dependent on an intact Sec-type signal peptide sequence. Further indication that secretion of Hbl, Nhe and CytK is dependent on the Sec translocation pathway, the main pathway on which bacterial secretion relies, was suggested by the observed intracellular accumulation and reduced secretion of the toxins in cultures supplemented with the SecA inhibitor sodium azide. Although a FEA deficient strain (a flhA mutant) showed reduced toxin expression and reduced cytotoxicity, it readily secreted overexpressed Hbl B, showing that the FEA is not required for Hbl secretion. Thus, the concurrent lack of flagella and reduced toxin secretion in the FEA deficient strain may point towards the presence of a regulatory link between motility and virulence genes, rather than FEA-dependent toxin secretion.

CONCLUSIONS

The Hbl, Nhe and CytK toxins appear to be secreted using the Sec pathway, and the reduced Hbl expression of a FEA deficient strain was shown not to be due to a secretion defect.

Acute depletion of activated memory B cells involves the PD-1 pathway in rapidly progressing SIV-infected macaques

Rapid progression to AIDS is a significant problem, especially in developing countries, where the majority of HIV-infected individuals reside. As rapid disease progression is also frequently observed in SIV-infected macaques, they represent a valuable tool to investigate the pathogenesis of this condition in humans. Here, we have shown that pathogenic SIV infection in rhesus macaques resulted in a rapid depletion (as early as week 2) of activated memory B (CD21-CD27+; mBAct) cells that was strongly associated with rapid disease progression. This depletion was progressive and sustained in rapid progressors, but less severe and transient in typical progressors. Because of the rapid and sustained depletion of mBAct cells, rapid progressors failed to develop SIV-specific Ab responses, showed a decline in non-SIV-specific Ab titers, and succumbed faster to intestinal bacterial infections. Depletion of mBAct cells was strongly associated with preferential depletion of mBAct cells expressing programmed death-1 (PD-1), and in vitro blockade of PD-1 improved their survival. Furthermore, in vivo PD-1 blockade in SIV-infected macaques enhanced Ab responses to non-SIV as well as SIV Ags. Our results identify depletion of mBAct cells as a very early predictor of rapid disease progression in pathogenic SIV infection and suggest an important role for the PD-1 pathway in depletion of mBAct cells and impaired humoral immune responses in SIV-infected macaques.

Regulation of motility in Erwinia carotovora subsp. carotovora: quorum-sensing signal controls FlhDC, the global regulator of flagellar and exoprotein genes, by modulating the production of RsmA, an RNA-binding protein

Erwinia carotovora subsp. carotovora causes soft-rotting (tissue-macerating) disease in many plants and plant organs. Although pectinases are the primary determinants of virulence, several ancillary factors that augment bacterial virulence have also been identified. One such factor is bacterial motility. Flagellum formation and bacterial movement are regulated in many enterobacteria, including E. carotovora subsp. carotovora, by FlhDC, the master regulator of flagellar genes and FliA, a flagellum-specific σ factor. We document here that motility of E. carotovora subsp. carotovora is positively regulated by the quorum-sensing signal, N-acylhomoserine lactone (AHL), and negatively regulated by RsmA, a post-transcriptional regulator. RsmA, an RNA-binding protein, causes translational repression and promotes RNA decay. Our data show that RsmA negatively regulates flhDC and fliA expression. Moreover, the chemical stabilities of transcripts of these genes are greater in an RsmA- mutant than in RsmA+ bacteria. These observations contrast with positive regulation of flhDC and motility by CsrA (=RsmA) in Escherichia coli. In the absence of AHL, the AHL receptors ExpR1/ExpR2 (=AhlR) in Erwinia carotovora subsp. carotovora negatively regulate motility and expression of flhDC and fliA by activating RsmA production. In the presence of AHL, regulatory effects of ExpR1/ExpR2 are neutralized, resulting in reduced levels of rsmA expression and enhanced motility.

Assessing adhesion, biofilm formation and motility of Acidovorax citrulli using microfluidic flow chambers

Acidovorax citrulli is the causal agent of bacterial fruit blotch of cucurbits. We have shown previously that type IV pili (TFP) are required for wild-type levels of virulence of A. citrulli on melon and that this pathogen can colonize and move thorough the xylem vessels of host seedlings. Here, comparative studies between wild-type and TFP mutant strains using microfluidic flow chambers demonstrated that TFP play a critical role in both the surface attachment and the biofilm formation of A. citrulli under a medium flow. Additionally, TFP null mutants were unable to perform twitching movement against the direction of medium flow. Assays using a flagellin mutant showed that, in contrast to TFP, polar flagella do not contribute to the adhesion and biofilm formation of A. citrulli under tested conditions. Also, flagellum-mediated swimming motility of wild-type strains was not observed under medium flow. These results imply that TFP may play an important role in colonization and spread in the xylem vessels under sap flow conditions, while polar flagella could be more important for spread during periods of time when xylem flow is minimal.

Built shallow to maintain homeostasis and persistent infection: insight into the transcriptional regulatory network of the gastric human pathogen Helicobacter pylori

Transcriptional regulatory networks (TRNs) transduce environmental signals into coordinated output expression of the genome. Accordingly, they are central for the adaptation of bacteria to their living environments and in host-pathogen interactions. Few attempts have been made to describe a TRN for a human pathogen, because even in model organisms, such as Escherichia coli, the analysis is hindered by the large number of transcription factors involved. In light of the paucity of regulators, the gastric human pathogen Helicobacter pylori represents a very appealing system for understanding how bacterial TRNs are wired up to support infection in the host. Herein, we review and analyze the available molecular and "-omic" data in a coherent ensemble, including protein-DNA and protein-protein interactions relevant for transcriptional control of pathogenic responses. The analysis covers approximately 80% of the annotated H. pylori regulators, and provides to our knowledge the first in-depth description of a TRN for an important pathogen. The emerging picture indicates a shallow TRN, made of four main modules (origons) that process the physiological responses needed to colonize the gastric niche. Specific network motifs confer distinct transcriptional response dynamics to the TRN, while long regulatory cascades are absent. Rather than having a plethora of specialized regulators, the TRN of H. pylori appears to transduce separate environmental inputs by using different combinations of a small set of regulators.

Salmonella enterica serovar Enteritidis tatB and tatC mutants are impaired in Caco-2 cell invasion in vitro and show reduced systemic spread in chickens

Salmonella enterica subsp. enterica serovar Enteritidis is a leading causative agent of gastroenteritis in humans. This pathogen also colonizes the intestinal tracts of poultry and can spread systemically in chickens. Transfer to humans usually occurs through undercooked or improperly handled poultry meat or eggs. The bacterial twin-arginine transport (Tat) pathway is responsible for the translocation of folded proteins across the cytoplasmic membrane. In order to study the role of the Tat system in the infection and colonization of chickens by Salmonella Enteritidis, we constructed chromosomal deletion mutants of the tatB and tatC genes, which are essential components of the Tat translocon. We observed that the tat mutations affected bacterial cell morphology, motility, and sensitivity to albomycin, sodium dodecyl sulfate (SDS), and EDTA. In addition, the mutant strains showed reduced invasion of polarized Caco-2 cells. The wild-type phenotype was restored in all our Salmonella Enteritidis tat mutants by introducing episomal copies of the tatABC genes. When tested in chickens by use of a Salmonella Enteritidis Delta tatB strain, the Tat system inactivation did not substantially affect cecal colonization, but it delayed systemic infection. Taken together, our data demonstrated that the Tat system plays a role in Salmonella Enteritidis pathogenesis.

A mutation in tdcA attenuates the virulence of Salmonella enterica serovar Typhimurium

The Salmonella tdc operon encodes enzymes belonging to a metabolic pathway that degrades L-serine and L-threonine. The upregulation of the tdc operon and increased virulence of Salmonella grown under oxygen-limiting conditions prompted us to investigate the role of the tdc operon in the pathogenesis of Salmonella Typhimurium. A Salmonella strain carrying a null mutation in tdcA, which encodes the transcriptional activator of the tdc operon, was impaired in mice infected intraperitoneally with the bacterium. In addition, the Salmonella tdcA mutant showed reduced replication compared with the parental strain in cultured animal cells, although their growth rates were similar in various culture media. To understand the function of TdcA in pathogenesis, we performed two-dimensional gel electrophoresis and found that flagellar and PhoP-regulated proteins were affected by the tdcA mutation. The results of beta-galactosidase assays and FACS analysis showed that, among the four PhoP-dependent genes tested, the expression of ssaG, which is located in Salmonella pathogenicity island 2 (SPI2), was reduced in the tdcA mutant, especially in the intracellular environment of macrophages. Taken together, our data suggest that tdcA plays an important role in the pathogenesis of Salmonella.

Experimental adaptation of Burkholderia cenocepacia to onion medium reduces host range

It is unclear whether adaptation to a new host typically broadens or compromises host range, yet the answer bears on the fate of emergent pathogens and symbionts. We investigated this dynamic using a soil isolate of Burkholderia cenocepacia, a species that normally inhabits the rhizosphere, is related to the onion pathogen B. cepacia, and can infect the lungs of cystic fibrosis patients. We hypothesized that adaptation of B. cenocepacia to a novel host would compromise fitness and virulence in alternative hosts. We modeled adaptation to a specific host by experimentally evolving 12 populations of B. cenocepacia in liquid medium composed of macerated onion tissue for 1,000 generations. The mean fitness of all populations increased by 78% relative to the ancestor, but significant variation among lines was observed. Populations also varied in several phenotypes related to host association, including motility, biofilm formation, and quorum-sensing function. Together, these results suggest that each population adapted by fixing different sets of adaptive mutations. However, this adaptation was consistently accompanied by a loss of pathogenicity to the nematode Caenorhabditis elegans; by 500 generations most populations became unable to kill nematodes. In conclusion, we observed a narrowing of host range as a consequence of prolonged adaptation to an environment simulating a specific host, and we suggest that emergent pathogens may face similar consequences if they become host-restricted.

Identification and characterization of the aspartate chemosensory receptor of Campylobacter jejuni

Campylobacter jejuni is a highly motile bacterium that responds via chemotaxis to environmental stimuli to migrate towards favourable conditions. Previous in silico analysis of the C. jejuni strain NCTC11168 genome sequence identified 10 open reading frames, tlp1-10, that encode putative chemosensory receptors. We describe the characterization of the role and specificity of the Tlp1 chemoreceptor (Cj1506c). In vitro and in vivo models were used to determine if Tlp1 had a role in host colonization. The tlp1(-) isogenic mutant was more adherent in cell culture, however, showed reduced colonization ability in chickens. Specific interactions between the purified sensory domain of Tlp1 and l-aspartate were identified using an amino acid array and saturation transfer difference nuclear magnetic resonance spectroscopy. Chemotaxis assays showed differences between migration of wild-type C. jejuni cells and that of a tlp1(-) isogenic mutant, specifically towards aspartate. Furthermore, using yeast two-hybrid and three-hybrid systems for analysis of protein-protein interactions, the cytoplasmic signalling domain of Tlp1 was found to preferentially interact with CheV, rather than the CheW homologue of the chemotaxis signalling pathway; this interaction was confirmed using immune precipitation assays. This is the first identification of an aspartate receptor in bacteria other than Escherichia coli and Salmonella enterica serovar Typhimurium.

A novel system of cytoskeletal elements in the human pathogen Helicobacter pylori

Pathogenicity of the human pathogen Helicobacter pylori relies upon its capacity to adapt to a hostile environment and to escape from the host response. Therefore, cell shape, motility, and pH homeostasis of these bacteria are specifically adapted to the gastric mucus. We have found that the helical shape of H. pylori depends on coiled coil rich proteins (Ccrp), which form extended filamentous structures in vitro and in vivo, and are differentially required for the maintenance of cell morphology. We have developed an in vivo localization system for this pathogen. Consistent with a cytoskeleton-like structure, Ccrp proteins localized in a regular punctuate and static pattern within H. pylori cells. Ccrp genes show a high degree of sequence variation, which could be the reason for the morphological diversity between H. pylori strains. In contrast to other bacteria, the actin-like MreB protein is dispensable for viability in H. pylori, and does not affect cell shape, but cell length and chromosome segregation. In addition, mreB mutant cells displayed significantly reduced urease activity, and thus compromise a major pathogenicity factor of H. pylori. Our findings reveal that Ccrp proteins, but not MreB, affect cell morphology, while both cytoskeletal components affect the development of pathogenicity factors and/or cell cycle progression.

Positive regulation of flhDC expression by OmpR in Yersinia pseudotuberculosis

OmpR has been demonstrated to negatively regulate the expression of the flagellar master operon flhDC in a wide variety of bacterial species. Here we report the positive regulation of flhDC expression by OmpR in Yersinia pseudotuberculosis. A σ 70-dependent promoter was identified by primer extension analysis and an active region with two conserved OmpR-binding sites around the flhDC promoter was confirmed. To confirm the regulation of flhDC expression by OmpR, flhDC as well as the downstream flagellar genes fliA, flgD, flgA, flgM, fliC and flaA were fused to lacZ, and decreased expression of all these genes in an ompR mutant (ΔompR) was detected. Furthermore, ΔompR was defective in bacterial motility and flagella synthesis. This defect was due to the low level of expression of flhDC in ΔompR since overproduction of FlhDC in ΔompR restored bacterial motility. The importance of two conserved OmpR-binding sites around the flhDC promoter region in the regulation of flhDC expression by OmpR was demonstrated by the fact that mutation of either one or both sites significantly decreased the promoter activity in the wild-type but not in ΔompR. The binding of OmpR to these two sites was also demonstrated by DNA mobility shift assay. The possible mechanism underlying this positive regulation in Y. pseudotuberculosis is discussed. To our knowledge, this is the first report to demonstrate that OmpR positively regulates flhDC expression.

A tdcA mutation reduces the invasive ability of Salmonella enterica serovar typhimurium

We previously observed that the transcription of some flagellar genes decreased in Salmonella Typhimurium tdcA mutant, which is a gene encoding the transcriptional activator of the tdc operon. Since flagella-mediated bacterial motility accelerates the invasion of Salmonella, we have examined the effect of tdcA mutation on the invasive ability as well as the flagellar biosynthesis in S. Typhimurium. A tdcA mutation caused defects in motility and formation of flagellin protein, FliC in S. Typhimurium. Invasion assays in the presence of a centrifugal force confirmed that the defect of flagellum synthesis decreases the ability of Salmonella to invade into cultured epithelial cells. In addition, we also found that the expression of Salmonella pathogenicity island 1 (SPI1) genes required for Salmonella invasion was down-regulated in the tdcA mutant because of the decreased expression of fliZ, a positive regulator of SPI1 transcriptional activator, hilA. Finally, the virulence of a S. Typhimurium tdcA mutant was attenuated compared to a wild type when administered orally. This study implies the role of tdcA in the invasion process of S. Typhimurium.

Amino acid substitutions and intragenic duplications of Bacillus sp. PS3 flagellin cause complementation of the Bacillus subtilis flagellin deletion mutant

Bacillus sp. PS3 produces a glycosylated flagellin. In this study, a number of the glycosylated residues of the flagellin protein were found to be located in the central variable region of this protein. We also report that the motility defect of the Bacillus subtilis flagellin mutant was complemented by Bacillus sp. PS3 flagellin variants without glycosylation, which contained amino acid substitutions and intragenic duplications in the variable region of flagellin.

[Enterobacter sakazakii in powdered infant food formulas]

Species of the Cronobacter genus ("Enterobacter sakazakii" s. l.) are emergent food-borne pathogens that can cause rare but severe neonatal meningitis, bacteriaemia, and necrotizing enterocolitis. Preterm, low-birth-weight, and immuno-compromised infants exposed to these bacterial species are at particular risk. Over the last 50 years, the literature has reported, mainly in newborn children, more than one hundred cases of infection due to these pathogens. The objective of this review was to synthesize the recent advances in knowledge of species of the Cronobacter genus, in particular with regards to taxonomy, physiology, pathogenicity, clinical cases, the methods for detection, isolation, and characterization, and their presence in powdered formulae for infants and young children, which were identified as the main infection vector. Researchers and international public health authorities have explored the ways contamination occur to better control the risks of pathogen development. Appropriate analysis and control measures were implemented in areas processing powdered formulae for infants and young children, and caregivers and families were informed to undertake good hygienic practices.

Predation on multiple trophic levels shapes the evolution of pathogen virulence

The pathogen virulence is traditionally thought to co-evolve as a result of reciprocal selection with its host organism. In natural communities, pathogens and hosts are typically embedded within a web of interactions with other species, which could affect indirectly the pathogen virulence and host immunity through trade-offs. Here we show that selection by predation can affect both pathogen virulence and host immune defence. Exposing opportunistic bacterial pathogen Serratia marcescens to predation by protozoan Tetrahymena thermophila decreased its virulence when measured as host moth Parasemia plantaginis survival. This was probably because the bacterial anti-predatory traits were traded off with bacterial virulence factors, such as motility or resource use efficiency. However, the host survival depended also on its allocation to warning signal that is used against avian predation. When infected with most virulent ancestral bacterial strain, host larvae with a small warning signal survived better than those with an effective large signal. This suggests that larval immune defence could be traded off with effective defence against bird predators. However, the signal size had no effect on larval survival when less virulent control or evolved strains were used for infection suggesting that anti-predatory defence against avian predators, might be less constrained when the invading pathogen is rather low in virulence. Our results demonstrate that predation can be important indirect driver of the evolution of both pathogen virulence and host immunity in communities with multiple species interactions. Thus, the pathogen virulence should be viewed as a result of both past evolutionary history, and current ecological interactions.

Proteomics identification of azaspiracid toxin biomarkers in blue mussels, Mytilus edulis

Azaspiracids are a class of recently discovered algae-derived shellfish toxins. Their distribution globally is on the increase with mussels being most widely implicated in azaspiracid-related food poisoning events. Evidence that these toxins were bound to proteins in contaminated mussels has been shown recently. In the present study characterization of these proteins in blue mussels, Mytilus edulis, was achieved using a range of advanced proteomics tools. Four proteins present only in the hepatopancreas of toxin-contaminated mussels sharing identity or homology with cathepsin D, superoxide dismutase, glutathione S-transferase Pi, and a bacterial flagellar protein have been characterized. Several of the proteins are known to be involved in self-defense mechanisms against xenobiotics or up-regulated in the presence of carcinogenic agents. These findings would suggest that azaspiracids should now be considered and evaluated as potential tumorigenic compounds. The presence of a bacterial protein only in contaminated mussels was an unexpected finding and requires further investigation. The proteins identified in this study should assist with development of urgently required processes for the rapid depuration of azaspiracid-contaminated shellfish. Moreover they may serve as early warning indicators of shellfish exposed to this family of toxins.

Divergent evolution and purifying selection of the flaA gene sequences in Aeromonas

BACKGROUND

The bacterial flagellum is the most important organelle of motility in bacteria and plays a key role in many bacterial lifestyles, including virulence. The flagellum also provides a paradigm of how hierarchical gene regulation, intricate protein-protein interactions and controlled protein secretion can result in the assembly of a complex multi-protein structure tightly orchestrated in time and space. As if to stress its importance, plants and animals produce receptors specifically dedicated to the recognition of flagella. Aside from motility, the flagellum also moonlights as an adhesion and has been adapted by humans as a tool for peptide display. Flagellar sequence variation constitutes a marker with widespread potential uses for studies of population genetics and phylogeny of bacterial species.

RESULTS

We sequenced the complete flagellin gene (flaA) in 18 different species and subspecies of Aeromonas. Sequences ranged in size from 870 (A. allosaccharophila) to 921 nucleotides (A. popoffii). The multiple alignment displayed 924 sites, 66 of which presented alignment gaps. The phylogenetic tree revealed the existence of two groups of species exhibiting different FlaA flagellins (FlaA1 and FlaA2). Maximum likelihood models of codon substitution were used to analyze flaA sequences. Likelihood ratio tests suggested a low variation in selective pressure among lineages, with an omega ratio of less than 1 indicating the presence of purifying selection in almost all cases. Only one site under potential diversifying selection was identified (isoleucine in position 179). However, 17 amino acid positions were inferred as sites that are likely to be under positive selection using the branch-site model. Ancestral reconstruction revealed that these 17 amino acids were among the amino acid changes detected in the ancestral sequence.

CONCLUSION

The models applied to our set of sequences allowed us to determine the possible evolutionary pathway followed by the flaA gene in Aeromonas, suggesting that this gene have probably been evolving independently in the two groups of Aeromonas species since the divergence of a distant common ancestor after one or several episodes of positive selection.

REVIEWERS

This article was reviewed by Alexey Kondrashov, John Logsdon and Olivier Tenaillon (nominated by Laurence D Hurst).

Role of flagella in virulence of the coral pathogen Vibrio coralliilyticus

A recently available transposition system was utilized to isolate a nonmotile mutant of the coral-bleaching pathogen Vibrio coralliilyticus. The mutation was localized to the fhlA gene, and the mutant lacked flagella. The flhA mutant was unable to exhibit chemotaxis toward coral mucus or to adhere to corals and subsequently cause infection.

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.

RsmC of Erwinia carotovora subsp. carotovora negatively controls motility, extracellular protein production, and virulence by binding FlhD and modulating transcriptional activity of the master regulator, FlhDC

RsmC and FlhDC are global regulators controlling extracellular proteins/enzymes, rsmB RNA, motility, and virulence of Erwinia carotovora subsp. carotovora. FlhDC, the master regulator of flagellar genes, controls these traits by positively regulating gacA, fliA, and rsmC and negatively regulating hexA. RsmC, on the other hand, is a negative regulator of extracellular proteins/enzymes, motility, and virulence since the deficiency of RsmC in FlhDC(+) strain results in overproduction of extracellular proteins/enzymes, hypermotility, and hypervirulence. These phenotypes are abolished in an RsmC(-) FlhDC(-) double mutant. We show that RsmC interferes with FlhDC action. Indeed, the expression of all three targets (i.e., gacA, rsmC, and fliA) positively regulated in E. carotovora subsp. carotovora by FlhDC is inhibited by RsmC. RsmC also partly relieves the inhibition of hexA expression by FlhDC. The results of yeast two-hybrid analysis revealed that RsmC binds FlhD and FlhDC, but not FlhC. We propose that binding of RsmC with FlhD/FlhDC interferes with its regulatory functions and that RsmC acts as an anti-FlhD(4)FlhC(2) factor. We document here for the first time that RsmC interferes with activation of fliA and motility in several members of the Enterobacteriaceae family. The extent of E. carotovora subsp. carotovora RsmC-mediated inhibition of FlhDC-dependent expression of fliA and motility varies depending upon enterobacterial species. The data presented here support the idea that differences in structural features in enterobacterial FlhD are responsible for differential susceptibility to E. carotovora subsp. carotovora RsmC action.

Intact flagellar motor of Borrelia burgdorferi revealed by cryo-electron tomography: evidence for stator ring curvature and rotor/C-ring assembly flexion

The bacterial flagellar motor is a remarkable nanomachine that provides motility through flagellar rotation. Prior structural studies have revealed the stunning complexity of the purified rotor and C-ring assemblies from flagellar motors. In this study, we used high-throughput cryo-electron tomography and image analysis of intact Borrelia burgdorferi to produce a three-dimensional (3-D) model of the in situ flagellar motor without imposing rotational symmetry. Structural details of B. burgdorferi, including a layer of outer surface proteins, were clearly visible in the resulting 3-D reconstructions. By averaging the 3-D images of approximately 1,280 flagellar motors, a approximately 3.5-nm-resolution model of the stator and rotor structures was obtained. flgI transposon mutants lacked a torus-shaped structure attached to the flagellar rod, establishing the structural location of the spirochetal P ring. Treatment of intact organisms with the nonionic detergent NP-40 resulted in dissolution of the outermost portion of the motor structure and the C ring, providing insight into the in situ arrangement of the stator and rotor structures. Structural elements associated with the stator followed the curvature of the cytoplasmic membrane. The rotor and the C ring also exhibited angular flexion, resulting in a slight narrowing of both structures in the direction perpendicular to the cell axis. These results indicate an inherent flexibility in the rotor-stator interaction. The FliG switching and energizing component likely provides much of the flexibility needed to maintain the interaction between the curved stator and the relatively symmetrical rotor/C-ring assembly during flagellar rotation.

Effects of chelating reagents on colonial appearance of Paenibacillus alvei isolated from canine oral cavity

A bacterial strain isolated from the oral cavity of a healthy dog revealed an unusual colony formation in nebular appearance on agar plates. The isolated bacterial strain was Gram-positive, spore-forming rod with peritrichous flagella, and grown under aerobic conditions, but unable to grow at 45 degrees C. The strain was tentatively classified as Paenibacillus alvei according to the biochemical properties and the 16S rRNA gene sequence. The isolate exhibits collective locomotion on solid agar plates. The bacterial motility was inhibited with EDTA and was restored by adding magnesium. We concluded that magnesium ion is essential for collective locomotion of P. alvei. This suggests that EDTA is useful for inhibition of biofilm formation.

A novel gene, fudoh, in the SCCmec region suppresses the colony spreading ability and virulence of Staphylococcus aureus

Staphylococcus aureus colonies can spread on soft agar plates. We compared colony spreading of clinically isolated methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA). All MSSA strains showed colony spreading, but most MRSA strains (73%) carrying SCCmec type-II showed little colony spreading. Deletion of the entire SCCmec type-II region from these MRSA strains restored colony spreading. Introduction of a novel gene, fudoh, carried by SCCmec type-II into Newman strain suppressed colony spreading. MRSA strains with high spreading ability (27%) had no fudoh or a point-mutated fudoh that did not suppress colony spreading. The fudoh-transformed Newman strain had decreased exotoxin production and attenuated virulence in mice. Most community-acquired MRSA strains carried SCCmec type-IV, which does not include fudoh, and showed high colony spreading ability. These findings suggest that fudoh in the SCCmec type-II region suppresses colony spreading and exotoxin production, and is involved in S. aureus pathogenesis.

Identification and molecular characterization of twin-arginine translocation system (Tat) in Xanthomonas oryzae pv. oryzae strain PXO99

Xanthomonas oryzae pv. oryzae causes bacterial leaf blight, one of the most widespread and destructive bacterial diseases in rice. This study identified and characterized the contribution of the twin-arginine translocation (Tat) pathway to motility, chemotaxis, extracellular polysaccharide (EPS) production and virulence in X. oryzae pv. oryzae strain PXO99. The tatC disruption mutant (strain TCM) of strain PXO99 were generated, and confirmed both by PCR and Southern blotting. Strain PXO99 cells were highly motile in NYGB 0.3% soft agar plate. In contrast, the tatC mutation impaired motility. Furthermore, strain TCM cells lacked detectable flagella and exhibited almost no chemotaxis toward glucose under aerobic conditions, indicating that the Tat secretion pathway contributed to flagellar biogenesis and chemotactic responses. It was also observed that strain TCM exhibited a reductive production of extracellular polysaccharide (EPS) and a significant reduction of virulence on rice plants when compared with the wild type PXO99. However, the tatC mutation in strain PXO99 did not affect growth rate and the ability to induce hypersensitive response (HR) in nonhost tobacco (Nicotiana tabacum L. cv. Samsun). Our findings indicated that the Tat system of X. oryzae pv. oryzae played an important role in the pathogen's virulence.

Biomechanical ordering of dense cell populations

The structure of bacterial populations is governed by the interplay of many physical and biological factors, ranging from properties of surrounding aqueous media and substrates to cell-cell communication and gene expression in individual cells. The biomechanical interactions arising from the growth and division of individual cells in confined environments are ubiquitous, yet little work has focused on this fundamental aspect of colony formation. We analyze the spatial organization of Escherichia coli growing in a microfluidic chemostat. We find that growth and expansion of a dense colony of cells leads to a dynamical transition from an isotropic disordered phase to a nematic phase characterized by orientational alignment of rod-like cells. We develop a continuum model of collective cell dynamics based on equations for local cell density, velocity, and the tensor order parameter. We use this model and discrete element simulations to elucidate the mechanism of cell ordering and quantify the relationship between the dynamics of cell proliferation and the spatial structure of the population.

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.

Catabolite repression control of flagellum production by Serratia marcescens

Serratia marcescens is an emerging opportunistic pathogen with a remarkably broad host range. The cAMP-regulated catabolite repression system of S. marcescens has recently been identified and demonstrated to regulate biofilm formation through the production of surface adhesions. Here we report that mutations in components of the catabolite repression system (cyaA and crp) eliminate flagellum production and swimming motility. Exogenous cAMP was able to restore flagellum production to adenylate cyclase mutants, as determined by transmission electron microscopy and PAGE analysis. A transposon-generated suppressor mutation of the crp motility defect mapped to upstream of the flhDC operon. This suppressor mutation resulted in an upregulation of flhD expression and flagellum production, indicating that flhDC expression is sufficient to restore flagellum production to crp mutants. Lastly, and contrary to a previous report, we found that flhD expression is controlled by the catabolite repression system using quantitative RT-PCR. Together, these data indicate that flagellum production is regulated by the cAMP-dependent catabolite repression system. Given the role of flagella in bacterial pathogenicity, the regulatory pathway described here may assist us in better understanding the putative role of motility in dissemination and virulence of this opportunistic pathogen.

Possible involvement of put A gene in Helicobacter pylori colonization in the stomach and motility

H. pylori is a gram-negative bacterium associated with gastric inflammation and peptic ulcer and considered a risk factor for gastric cancer in its natural habitat. However, the energy metabolism of H. pylori in the stomach remains to be clarified. H. pylori shows rather high respiratory activity with L-proline and significantly large amounts of L-proline are present in the gastric juice from H. pylori infected patients. We constructed a disrupted mutant of the put A gene, which encodes the proline utilization A (Put A) flavin-linked enzyme, in order to examine the role of put A in the gastric colonization of H. pylori. The put A disrupted mutant, DeltaputA, was constructed by inserting a chloramphenicol resistant gene into put A. DeltaputA did not show respiratory activity using L-proline and could not incorporate L-proline into cells. DeltaputA also did not show motility in response to amino acids and did not display the swarming activity observed with the wild-type. DeltaputA had lost its ability to colonize the stomach of nude mice, an ability possessed by the wild-type. These findings indicate that put A may play an important role in H. pylori colonization on the gastric mucus layer.

Xenorhabdus nematophila lrhA is necessary for motility, lipase activity, toxin expression, and virulence in Manduca sexta insects

The gram-negative insect pathogen Xenorhabdus nematophila possesses potential virulence factors including an assortment of toxins, degradative enzymes, and regulators of these compounds. Here, we describe the lysR-like homolog A (lrhA) gene, a gene required by X. nematophila for full virulence in Manduca sexta insects. In several other gram-negative bacteria, LrhA homologs are transcriptional regulators involved in the expression (typically repression) of virulence factors. Based on phenotypic and genetic evidence, we report that X. nematophila LrhA has a positive effect on transcription and expression of certain potential virulence factors, including a toxin subunit-encoding gene, xptD1. Furthermore, an lrhA mutant lacks in vitro lipase activity and has reduced swimming motility compared to its wild-type parent. Quantitative PCR revealed that transcript levels of flagellar genes, a lipase gene, and xptD1 were significantly lower in the lrhA mutant than in the wild type. In addition, lrhA itself is positively regulated by the global regulator Lrp. This work establishes a role for LrhA as a vital component of a regulatory hierarchy necessary for X. nematophila pathogenesis and expression of surface-localized and secreted factors. Future research is aimed at identifying and characterizing virulence factors within the LrhA regulon.

FliZ, a flagellar regulator, is at the crossroads between motility, haemolysin expression and virulence in the insect pathogenic bacterium Xenorhabdus

There is a complex interplay between the regulation of flagellar motility and the expression of virulence factors in many bacterial pathogens. We investigated the role of FliZ in the regulation of flagellar and virulence genes in Xenorhabdus nematophila, an insect pathogen. The fliZ gene is the second gene in the fliAZ operon in X. nematophila. In vivo transcription analysis revealed a positive feedback loop of fliAZ transcription in which FliZ activates flhDC, the master operon of flagellar regulon in X. nematophila, leading to an increased transcription of the FlhDC-dependent promoter of fliAZ. We also showed that fliAZ and flhDC mutants lacked motility, had no haemolysin or Tween lipase activity and displayed an attenuated virulence phenotype in insects. Lipase activity is controlled by FliA, whereas haemolysin production and full virulence phenotype have been reported to be FliZ-dependent. Transcriptional analysis revealed that FliZ directly controlled expression of the xhlBA and xaxAB operons, which encode haemolysins from the two-partner secretion system and the binary XaxAB toxin family respectively. We suggest that this regulatory pathway may also occur in other pathogenic enterobacteria with genes encoding members of these two growing families of haemolysins.

The genome of Erwinia tasmaniensis strain Et1/99, a non-pathogenic bacterium in the genus Erwinia

The complete genome of the bacterium Erwinia tasmaniensis strain Et1/99 consisting of a 3.9 Mb circular chromosome and five plasmids was sequenced. Strain Et1/99 represents an epiphytic plant bacterium related to Erwinia amylovora and E. pyrifoliae, which are responsible for the important plant diseases fire blight and Asian pear shoot blight, respectively. Strain Et1/99 is a non-pathogenic bacterium and is thought to compete with these and other bacteria when occupying the same habitat during initial colonization. Genome analysis revealed tools for colonization, cellular communication and defence modulation, as well as genes coding for the synthesis of levan and a not detected capsular exopolysaccharide. Strain Et1/99 may secrete indole-3-acetic acid to increase availability of nutrients provided on plant surfaces. These nutrients are subsequently accessed and metabolized. Secretion systems include the hypersensitive response type III pathway present in many pathogens. Differences or missing parts within the virulence-related factors distinguish strain Et1/99 from pathogens such as Pectobacterium atrosepticum and the related Erwinia spp. Strain Et1/99 completely lacks the sorbitol operon, which may also affect its inability to invade fire blight host plants. Erwinia amylovora in contrast depends for virulence on utilization of sorbitol, the dominant carbohydrate in rosaceous plants. The presence of other virulence-associated factors in strain Et1/99 indicates the ancestral genomic background of many plant-associated bacteria.

Regulatory network controlling extracellular proteins in Erwinia carotovora subsp. carotovora: FlhDC, the master regulator of flagellar genes, activates rsmB regulatory RNA production by affecting gacA and hexA (lrhA) expression

Erwinia carotovora subsp. carotovora produces an array of extracellular proteins (i.e., exoproteins), including plant cell wall-degrading enzymes and Harpin, an effector responsible for eliciting hypersensitive reaction. Exoprotein genes are coregulated by the quorum-sensing signal, N-acyl homoserine lactone, plant signals, an assortment of transcriptional factors/regulators (GacS/A, ExpR1, ExpR2, KdgR, RpoS, HexA, and RsmC) and posttranscriptional regulators (RsmA, rsmB RNA). rsmB RNA production is positively regulated by GacS/A, a two-component system, and negatively regulated by HexA (PecT in Erwinia chrysanthemi; LrhA [LysR homolog A] in Escherichia coli) and RsmC, a putative transcriptional adaptor. While free RsmA, an RNA-binding protein, promotes decay of mRNAs of exoprotein genes, binding of RsmA with rsmB RNA neutralizes the RsmA effect. In the course of studies of GacA regulation, we discovered that a locus bearing strong homology to the flhDC operon of E. coli also controls extracellular enzyme production. A transposon insertion FlhDC(-) mutant produces very low levels of pectate lyase, polygalacturonase, cellulase, protease, and E. carotovora subsp. carotovora Harpin (Harpin(Ecc)) and is severely attenuated in its plant virulence. The production of these exoproteins is restored in the mutant carrying an FlhDC(+) plasmid. Sequence analysis and transcript assays disclosed that the flhD operon of E. carotovora subsp. carotovora, like those of other enterobacteria, consists of flhD and flhC. Complementation analysis revealed that the regulatory effect requires functions of both flhD and flhC products. The data presented here show that FlhDC positively regulates gacA, rsmC, and fliA and negatively regulates hexA (lrhA). Evidence shows that FlhDC controls extracellular protein production through cumulative effects on hexA and gacA. Reduced levels of GacA and elevated levels of HexA in the FlhDC(-) mutant are responsible for the inhibition of rsmB RNA production, a condition conducive to the accumulation of free RsmA. Indeed, studies with an RsmA(-) FlhDC(-) double mutant and multiple copies of rsmB(+) DNA establish that the negative effect of FlhDC deficiency is exerted via RsmA. The FlhDC-mediated regulation of fliA has no bearing on exoprotein production in E. carotovora subsp. carotovora. Our observations for the first time establish a regulatory connection between FlhDC, HexA, GacA, and rsmB RNA in the context of the exoprotein production and virulence of E. carotovora subsp. carotovora.

Motility allows S. Typhimurium to benefit from the mucosal defence

The mammalian intestine is colonized by a dense bacterial community, called microbiota. The microbiota shields from intestinal infection (colonization resistance). Recently, we have shown that enteropathogenic Salmonella spp. can exploit inflammation to compete with the intestinal microbiota. The mechanisms explaining the enhanced pathogen growth in the inflamed intestine are elusive. Here, we analysed the function of bacterial flagella in the inflamed intestine using a mouse model for acute Salmonella Typhimurium enterocolitis. Mutations affecting flagellar assembly (Fla(-)) and chemotaxis (Che(-)) impaired the pathogen's fitness in the inflamed intestine, but not in the normal gut. This was attributable to a localized source of high-energy nutrients (e.g. galactose-containing glyco-conjugates, mucin) released as an element of the mucosal defence. Motility allows Salmonella Typhimurium to benefit from these nutrients and utilize them for enhanced growth. Thus, nutrient availability contributes to enhanced pathogen growth in the inflamed intestine. Strategies interfering with bacterial motility or nutrient availability might offer starting points for therapeutic approaches.

Mucins in the mucosal barrier to infection

The mucosal tissues of the gastrointestinal, respiratory, reproductive, and urinary tracts, and the surface of the eye present an enormous surface area to the exterior environment. All of these tissues are covered with resident microbial flora, which vary considerably in composition and complexity. Mucosal tissues represent the site of infection or route of access for the majority of viruses, bacteria, yeast, protozoa, and multicellular parasites that cause human disease. Mucin glycoproteins are secreted in large quantities by mucosal epithelia, and cell surface mucins are a prominent feature of the apical glycocalyx of all mucosal epithelia. In this review, we highlight the central role played by mucins in accommodating the resident commensal flora and limiting infectious disease, interplay between underlying innate and adaptive immunity and mucins, and the strategies used by successful mucosal pathogens to subvert or avoid the mucin barrier, with a particular focus on bacteria.

Methionine sulfoxide reductases and virulence of bacterial pathogens

Oxidation of methionine (Met) residues in proteins by reactive oxygen species and reactive nitrogen intermediates results in altered protein structures, which subsequently affect their functions. Oxidized Met (Met-O) residues are reduced to Met by the methionine sulfoxide reductase (Msr) system, which includes mainly MsrA and MsrB. MsrA and MsrB show no sequence and structural identity with each other but both reduce methionine sulfoxides. MsrA is specific to the reduction of methionine-S-sulfoxide, whereas MsrB is specific to the reduction of methionine-R-sulfoxide. Genes encoding the enzymes MsrA and MsrB exist in most living organisms including bacteria. In recent times, absence of these enzymes has been implicated in the virulence of bacterial pathogens. In particular, pathogens deficient in Msr have been reported to have reduced ability to adhere with eukaryotic cells, to survive inside hosts and to resist in vitro oxidative stress. Bacterial proteins that are susceptible to Met oxidation, in the absence of Msr, have also been identified. This review discusses the current knowledge on the role of Msr in bacterial virulence.

The DNA nucleoid-associated protein Fis co-ordinates the expression of the main virulence genes in the phytopathogenic bacterium Erwinia chrysanthemi

Erwinia chrysanthemi strain 3937 is a necrotrophic bacterial plant pathogen. Pectinolytic enzymes and, in particular, pectate lyases (Pels) play a key role in soft rot symptoms but the efficient colonization of plants by E. chrysanthemi requires additional factors. These factors include the harpin HrpN, the cellulase Cel5, proteases (Prts), flagellar proteins and the Sap system, involved in the detoxification of plant antimicrobial peptides. HrpN and flagellum are mostly involved in the early steps of infection whereas the degradative enzymes (Pels, Cel5, Prts) are mainly required in the advanced stages. Production of these virulence factors is tightly regulated by environmental conditions. This report shows that the nucleoid-associated protein Fis plays a pivotal role in the expression of the main virulence genes. Its production is regulated in a growth phase-dependent manner and is under negative autoregulation. An E. chrysanthemi fis mutant displays a reduced motility and expression of hrpN, prtC and the sap operon. In contrast, the expression of the cel5 gene is increased in this mutant. Furthermore, the induction of the Pel activity is delayed and increased during the stationary growth phase in the fis mutant. Most of these controls occur through a direct effect because purified Fis binds to the promoter regions of fis, hrpN, sapA, cel5 and fliC. Moreover, potassium permanganate footprinting and in vitro transcription assays have revealed that Fis prevents transcription initiation at the fis promoter and also transcript elongation from the cel5 promoter. Finally, the fis mutant has a decreased virulence. These results suggest a co-ordinated regulation by Fis of virulence factors involved in certain key steps of infection, early (asymptomatic) and advanced (symptomatic) phases.

Expression of flagella is coincident with uropathogenic Escherichia coli ascension to the upper urinary tract

Uropathogenic Escherichia coli (UPEC) cause most uncomplicated urinary tract infections (UTIs) in humans. Because UTIs are considered to occur in an ascending manner, flagellum-mediated motility has been suggested to contribute to virulence by enabling UPEC to disseminate to the upper urinary tract. Previous studies from our laboratory and others have demonstrated a modest yet important role for flagella during ascending UTI. To better understand the role of flagella in vivo, we used biophotonic imaging to monitor UPEC infection and temporospatial flagellin gene expression during ascending UTI. Using em7-lux (constitutive) and fliC-lux transcriptional fusions, we show that flagellin expression by UPEC coincides with ascension of the ureters and colonization of the kidney. The patterns of fliC luminescence observed in vitro and in vivo were also validated by comparative quantitative PCR. Because fliC expression appeared coincident during ascension, we reassessed the contribution of fliC to ascending UTI using a low-dose intraurethral model of ascending UTI. Although wild-type UPEC were able to establish infection in the bladder and kidneys by 6 hours postinoculation, fliC mutant bacteria were able to colonize the bladder but were significantly attenuated in the kidneys at this early time point. By 48 hours postinoculation, the fliC mutant bacteria were attenuated in the bladder and kidneys and were not detectable in the spleen. These data provide compelling evidence that wild-type UPEC express flagellin and presumably utilize flagellum-mediated motility during UTI to ascend to the upper urinary tract and disseminate within the host.