Transcriptional hierarchy of Aeromonas hydrophila polar-flagellum genes

RpoN is required for the motility and contributes to the killing ability of Plesiomonas shigelloides

BACKGROUND

RpoN, also known as σ⁵⁴, first reported in Escherichia coli, is a subunit of RNA polymerase that strictly controls the expression of different genes by identifying specific promoter elements. RpoN has an important regulatory function in carbon and nitrogen metabolism and participates in the regulation of flagellar synthesis, bacterial motility and virulence. However, little is known about the effect of RpoN in Plesiomonas shigelloides.

RESULTS

To identify pathways controlled by RpoN, RNA sequencing (RNA-Seq) of the WT and the rpoN deletion strain was carried out for comparison. The RNA-seq results showed that RpoN regulates ~ 13.2% of the P. shigelloides transcriptome, involves amino acid transport and metabolism, glycerophospholipid metabolism, pantothenate and CoA biosynthesis, ribosome biosynthesis, flagellar assembly and bacterial secretion system. Furthermore, we verified the results of RNA-seq using quantitative real-time reverse transcription PCR, which indicated that the absence of rpoN caused downregulation of more than half of the polar and lateral flagella genes in P. shigelloides, and the ΔrpoN mutant was also non-motile and lacked flagella. In the present study, the ability of the ΔrpoN mutant to kill E. coli MG1655 was reduced by 54.6% compared with that of the WT, which was consistent with results in RNA-seq, which showed that the type II secretion system (T2SS-2) genes and the type VI secretion system (T6SS) genes were repressed. By contrast, the expression of type III secretion system genes was largely unchanged in the ΔrpoN mutant transcriptome and the ability of the ΔrpoN mutant to infect Caco-2 cells was also not significantly different compared with the WT.

CONCLUSIONS

We showed that RpoN is required for the motility and contributes to the killing ability of P. shigelloides and positively regulates the T6SS and T2SS-2 genes.

Transcriptional organization and regulation of the Pseudomonas putida flagellar system

A single region of the Pseudomonas putida genome, designated the flagellar cluster, includes 59 genes potentially involved in the biogenesis and function of the flagellar system. Here, we combine bioinformatics and in vivo gene expression analyses to clarify the transcriptional organization and regulation of the flagellar genes in the cluster. We have identified 11 flagellar operons and characterized 22 primary and internal promoter regions. Our results indicate that synthesis of the flagellar apparatus and core chemotaxis machinery is regulated by a three-tier cascade in which fleQ is a Class I gene, standing at the top of the transcriptional hierarchy. FleQ- and σ⁵⁴ -dependent Class II genes encode most components of the flagellar structure, part of the chemotaxis machinery and multiple regulatory elements, including the flagellar σ factor FliA. FliA activation of Class III genes enables synthesis of the filament, one stator complex and completion of the chemotaxis apparatus. Accessory regulatory proteins and an intricate operon architecture add complexity to the regulation by providing feedback and feed-forward loops to the main circuit. Because of the high conservation of the gene arrangement and promoter motifs, we believe that the regulatory circuit presented here may also apply to other environmental pseudomonads.

Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure

The bacterial flagellum is a complex and dynamic nanomachine that propels bacteria through liquids. It consists of a basal body, a hook, and a long filament. The flagellar filament is composed of thousands of copies of the protein flagellin (FliC) arranged helically and ending with a filament cap composed of an oligomer of the protein FliD. The overall structure of the filament core is preserved across bacterial species, while the outer domains exhibit high variability, and in some cases are even completely absent. Flagellar assembly is a complex and energetically costly process triggered by environmental stimuli and, accordingly, highly regulated on transcriptional, translational and post-translational levels. Apart from its role in locomotion, the filament is critically important in several other aspects of bacterial survival, reproduction and pathogenicity, such as adhesion to surfaces, secretion of virulence factors and formation of biofilms. Additionally, due to its ability to provoke potent immune responses, flagellins have a role as adjuvants in vaccine development. In this review, we summarize the latest knowledge on the structure of flagellins, capping proteins and filaments, as well as their regulation and role during the colonization and infection of the host.

Characterization of the relationship between polar and lateral flagellar genes in clinical Aeromonas dhakensis: phenotypic, genetic and biochemical analyses

Flagellar-mediated motility is a crucial virulence factor in many bacterial species. A dual flagellar system has been described in aeromonads; however, there is no flagella-related study in the emergent human pathogen Aeromonas dhakensis. Using 46 clinical A. dhakensis, phenotypic motility, genotypic characteristics (flagellar genes and sequence types), biochemical properties and their relationship were investigated in this study. All 46 strains showed swimming motility at 30 °C in 0.3% Bacto agar and carried the most prevalent 6 polar flagellar genes cheA, flgE, flgG, flgH, flgL, and flgN. On the contrary, only 18 strains (39%) demonstrated swarming motility on 0.5% Eiken agar at 30 °C and they harbored 11 lateral flagellar genes lafB, lafK, lafS, lafT, lafU, flgCL, flgGL, flgNL, fliEL, fliFL, and fliGL. No association was found between biochemical properties and motility phenotypes. Interestingly, a significant association between swarming and strains isolated from pus was observed (p = 0.0171). Three strains 187, 277, and 289 isolated from pus belonged to novel sequence types (ST522 and ST524) exhibited fast swimming and swarming profiles, and they harbored > 90% of the flagellar genes tested. Our findings provide a fundamental understanding of flagellar-mediated motility in A. dhakensis.

Polar Flagella Glycosylation in Aeromonas: Genomic Characterization and Involvement of a Specific Glycosyltransferase (Fgi-1) in Heterogeneous Flagella Glycosylation

Polar flagella from mesophilic Aeromonas strains have previously been shown to be modified with a range of glycans. Mass spectrometry studies of purified polar flagellins suggested the glycan typically includes a putative pseudaminic acid like derivative; while some strains are modified with this single monosaccharide, others modified with a heterologous glycan. In the current study, we demonstrate that genes involved in polar flagella glycosylation are clustered in highly polymorphic genomic islands flanked by pseudaminic acid biosynthetic genes (pse). Bioinformatic analysis of mesophilic Aeromonas genomes identified three types of polar flagella glycosylation islands (FGIs), denoted Group I, II and III. FGI Groups I and III are small genomic islands present in Aeromonas strains with flagellins modified with a single monosaccharide pseudaminic acid derivative. Group II were large genomic islands, present in strains found to modify polar flagellins with heterogeneous glycan moieties. Group II, in addition to pse genes, contained numerous glycosyltransferases and other biosynthetic enzymes. All Group II strains shared a common glycosyltransferase downstream of luxC that we named flagella glycosylation island 1, fgi-1, in A. piscicola AH-3. We demonstrate that Fgi-1 transfers the first sugar of the heterogeneous glycan to the pseudaminic acid derivative linked to polar flagellins and could be used as marker for polysaccharidic glycosylation of Aeromonas polar flagella.

BB0326 is responsible for the formation of periplasmic flagellar collar and assembly of the stator complex in Borrelia burgdorferi

Borrelia burgdorferi is a highly motile spirochete due to its periplasmic flagella. Unlike flagella of other bacteria, spirochetes' periplasmic flagella possess a complex structure called the collar, about which little is known in terms of function and composition. Using various approaches, we have identified a novel protein, BB0326, as a key component of the collar. We show that a peripheral portion of the collar is diminished in the Δbb0326 mutant and restored in the complemented bb0326+ cells, leading us to rename BB0326 as periplasmic flagellar collar protein A or FlcA. The ΔflcA mutant cells produced fewer, abnormally tilted and shorter flagella, as well as diminished stators, suggesting that FlcA is crucial for flagellar and stator assemblies. We provide further evidence that FlcA interacts with the stator and that this collar-stator interaction is essential for the high torque needed to power the spirochete's periplasmic flagellar motors. These observations suggest that the collar provides various important functions to the spirochete's periplasmic flagellar assembly and rotation.

Vibrio Flagellar Synthesis

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

Molecular characterization and interactome analysis of aerolysin (aer) gene from fish pathogen Aeromonas veronii: The pathogenicity inferred from sequence divergence and linked to histidine kinase (cheA)

Aerolysin (aer) is one of the most important and abundant virulence factors in the infection of fish by Aeromonas veronii. A comprehensive study on the molecular characterization and pathogenicity of the aer gene from 34 A. veronii isolates from diseased carp and catfish was carried out and its interactome was analysed to observe the functional correlations between aer and other proteins within the A. veronii network. The PCR-based amplification of aer from the 34 isolates of A. veronii showed more aer-positive isolates from catfish with a high pathogenic potential in the in vivo challenge test than the carp fish. The analysis of aer gene sequence from challenged fish revealed significant sequence divergence according to the types and geographical distribution of the fish. The networking analysis of aer from the model A. veronii B565 revealed histidine kinase (cheA) as the most functional interacting partner. The study of the interaction between aer from the experimental A. veronii and cheA demonstrated that the A chain of cheA plays a more important role than the corresponding B chain during contact, and a linker sequence of 15 residues controlled the entire interaction process. Therefore, cheA could be an excellent drug target for controlling A. veronii infection of fish.

Characterization of the relationship between polar and lateral flagellar structural genes in the deep-sea bacterium Shewanella piezotolerans WP3

Bacteria with a dual flagellar system, which consists of a polar flagellum (PF) and several lateral flagella (LF), have been identified in diverse environments. Nevertheless, whether and how these two flagellar systems interact with each other is largely unknown. In the present study, the relationship between the structural genes for the PF and LF of the deep-sea bacterium Shewanella piezotolerans WP3 was investigated by genetic, phenotypic and phylogenetic analyses. The mutation of PF genes induced the expression of LF genes and the production of LF in liquid medium, while the defective LF genes led to a decrease in PF gene transcription. However, the level of PF flagellin remained unchanged in LF gene mutants. Further investigation showed that the flgH2 gene (encoding LF L-ring protein) can compensate for mutations of the flgH1 gene (encoding PF L-ring protein), but this compensation does not occur between the flagellar hook-filament junction proteins (FlgL1, FlgL2). Swarming motility was shown to specifically require LF genes, and PF genes cannot substitute for the LF genes in the lateral flagella synthesis. Considering the importance of flagella-dependent motility for bacterial survival in the abyssal sediment, our study thus provided a better understanding of the adaptation strategy of benthic bacteria.

Virulence Factors of Aeromonas hydrophila: In the Wake of Reclassification

The ubiquitous "jack-of-all-trades," Aeromonas hydrophila, is a freshwater, Gram-negative bacterial pathogen under revision in regard to its phylogenetic and functional affiliation with other aeromonads. While virulence factors are expectedly diverse across A. hydrophila strains and closely related species, our mechanistic knowledge of the vast majority of these factors is based on the molecular characterization of the strains A. hydrophila AH-3 and SSU, which were reclassified as A. piscicola AH-3 in 2009 and A. dhakensis SSU in 2013. Individually, these reclassifications raise important questions involving the applicability of previous research on A. hydrophila virulence mechanisms; however, this issue is exacerbated by a lack of genomic data on other research strains. Collectively, these changes represent a fundamental gap in the literature on A. hydrophila and confirm the necessity of biochemical, molecular, and morphological techniques in the classification of research strains that are used as a foundation for future research. This review revisits what is known about virulence in A. hydrophila and the feasibility of using comparative genomics in light of this phylogenetic revision. Conflicting data between virulence factors, secretion systems, quorum sensing, and their effect on A. hydrophila pathogenicity appears to be an artifact of inappropriate taxonomic comparisons and/or be due to the fact that these properties are strain-specific. This review audits emerging data on dominant virulence factors that are present in both A. dhakensis and A. hydrophila in order to synthesize existing data with the aim of locating where future research is needed.

flrA, flrB and flrC regulate adhesion by controlling the expression of critical virulence genes in Vibrio alginolyticus

Adhesion is an important virulence trait of Vibrio alginolyticus. Bacterial adhesion is influenced by environmental conditions; however, the molecular mechanism underlying this effect remains unknown. The expression levels of flrA, flrB and flrC were significantly downregulated in adhesion-deficient V. alginolyticus strains cultured under Cu²⁺, Pb²⁺, Hg²⁺ and low-pH stresses. Silencing these genes led to deficiencies in adhesion, motility, flagellar assembly, biofilm formation and exopolysaccharide (EPS) production. The expression levels of fliA, flgH, fliS, fliD, cheR, cheV and V12G01_22158 (Gene ID) were significantly downregulated in all of the RNAi groups, whereas the expression levels of toxT, ctxB, acfA, hlyA and tlh were upregulated in flrA- and flrC-silenced groups. These genes play a key role in the virulence mechanisms of most pathogenic Vibrio species. Furthermore, the expression of flrA, flrB and flrC was significantly influenced by temperature, salinity, starvation and pH. These results indicate that (1) flrA, flrB and flrC are important for V. alginolyticus adhesion; (2) flrA, flrB and flrC significantly influence bacterial adhesion, motility, biofilm formation and EPS production by controlling expression of key genes involved in those phenotypes; and (3) flrA, flrB and flrC regulate adhesion in the natural environment with different temperatures, pH levels, salinities and starvation time.

The FlgT Protein Is Involved in Aeromonas hydrophila Polar Flagella Stability and Not Affects Anchorage of Lateral Flagella

Aeromonas hydrophila sodium-driven polar flagellum has a complex stator-motor. Consist of two sets of redundant and non-exchangeable proteins (PomA/PomB and PomA₂/PomB₂), which are homologs to other sodium-conducting polar flagellum stator motors; and also two essential proteins (MotX and MotY), that they interact with one of those two redundant pairs of proteins and form the T-ring. In this work, we described an essential protein for polar flagellum stability and rotation which is orthologs to Vibrio spp. FlgT and it is encoded outside of the A. hydrophila polar flagellum regions. The flgT was present in all mesophilic Aeromonas strains tested and also in the non-motile Aeromonas salmonicida. The A. hydrophila ΔflgT mutant is able to assemble the polar flagellum but is more unstable and released into the culture supernatant from the cell upon completion assembly. Presence of FlgT in purified polar hook-basal bodies (HBB) of wild-type strain was confirmed by Western blotting and electron microscopy observations showed an outer ring of the T-ring (H-ring) which is not present in the ΔflgT mutant. Anchoring and motility of proton-driven lateral flagella was not affected in the ΔflgT mutant and specific antibodies did not detect FlgT in purified lateral HBB of wild type strain.

How bacteria maintain location and number of flagella?

Bacteria differ in number and location of their flagella that appear in regular patterns at the cell surface (flagellation pattern). Despite the plethora of bacterial species, only a handful of these patterns exist. The correct flagellation pattern is a prerequisite for motility, but also relates to biofilm formation and the pathogenicity of disease-causing flagellated bacteria. However, the mechanisms that maintain location and number of flagella are far from being understood. Here, we review our knowledge on mechanisms that enable bacteria to maintain their appropriate flagellation pattern. While some peritrichous flagellation patterns might occur by rather simple stochastic processes, other bacterial species appear to rely on landmark systems to define the designated flagellar position. Such landmarks are the Tip system of Caulobacter crescentus or the signal recognition particle (SRP)-GTPase FlhF and the MinD/ParA-type ATPase FlhG (synonyms: FleN, YlxH and MinD2). The latter two proteins constitute a regulatory circuit essential for diverse flagellation patterns in many Gram-positive and negative species. The interactome of FlhF/G (e.g. C-ring proteins FliM, FliN, FliY or the transcriptional regulator FleQ/FlrA) seems evolutionary adapted to meet the specific needs for a respective pattern. This variability highlights the importance of the correct flagellation pattern for motile species.

The polar and lateral flagella from Plesiomonas shigelloides are glycosylated with legionaminic acid

Plesiomonas shigelloides is the unique member of the Enterobacteriaceae family able to produce polar flagella when grow in liquid medium and lateral flagella when grown in solid or semisolid media. In this study on P. shigelloides 302-73 strain, we found two different gene clusters, one exclusively for the lateral flagella biosynthesis and the other one containing the biosynthetic polar flagella genes with additional putative glycosylation genes. P. shigelloides is the first Enterobacteriaceae were a complete lateral flagella cluster leading to a lateral flagella production is described. We also show that both flagella in P. shigelloides 302-73 strain are glycosylated by a derivative of legionaminic acid (Leg), which explains the presence of Leg pathway genes between the two polar flagella regions in their biosynthetic gene cluster. It is the first bacterium reported with O-glycosylated Leg in both polar and lateral flagella. The flagella O-glycosylation is essential for bacterial flagella formation, either polar or lateral, because gene mutants on the biosynthesis of Leg are non-flagellated. Furthermore, the presence of the lateral flagella cluster and Leg O-flagella glycosylation genes are widely spread features among the P. shigelloides strains tested.

Aeromonas flagella and colonisation mechanisms

Aeromonas species are inhabitants of aquatic environments and are able to cause disease in humans and fish among other animals. In aquaculture, they are responsible for the economically important diseases of furunculosis and motile Aeromonas septicaemia (MAS). Whereas gastroenteritis and wound infections are the major human diseases associated with the genus. As they inhabit and survive in diverse environments, aeromonads possess a wide range of colonisation factors. The motile species are able to swim in liquid environments through the action of a single polar flagellum, the flagellin subunits of which are glycosylated; although essential for function the biological role of glycan addition is yet to be determined. Approximately 60% of aeromonads possess a second lateral flagella system that is expressed in viscous environments for swarming over surfaces; both flagellar systems have been shown to be important in the initial colonisation of surfaces. Subsequently, other non-flagellar colonisation factors are employed; these can be both filamentous and non-filamentous. The aeromonads possess a number of fimbrial systems with the bundle-forming MSHA type IV pilus system, having a major role in human cell adherence. Furthermore, a series of outer-membrane proteins have also been implicated in the aeromonad adhesion process. A number of strains are also capable of cell invasion and that maybe linked with the more invasive diseases of bacteraemia or wound infections. These strains employ cell surface factors that allow the colonisation of these niches that protect them from the host's immune system such as S-layers, capsules or particular lipopolysaccharides.

Aeromonas hydrophila lateral flagellar gene transcriptional hierarchy

Aeromonas hydrophila AH-3 lateral flagella are not assembled when bacteria grow in liquid media; however, lateral flagellar genes are transcribed. Our results indicate that A. hydrophila lateral flagellar genes are transcribed at three levels (class I to III genes) and share some similarities with, but have many important differences from, genes of Vibrio parahaemolyticus. A. hydrophila lateral flagellum class I gene transcription is σ(70) dependent, which is consistent with the fact that lateral flagellum is constitutively transcribed, in contrast to the characteristics of V. parahaemolyticus. The fact that multiple genes are included in class I highlights that lateral flagellar genes are less hierarchically transcribed than polar flagellum genes. The A. hydrophila lafK-fliEJL gene cluster (where the subscript L distinguishes genes for lateral flagella from those for polar flagella) is exclusively from class I and is in V. parahaemolyticus class I and II. Furthermore, the A. hydrophila flgAMNL cluster is not transcribed from the σ(54)/LafK-dependent promoter and does not contain class II genes. Here, we propose a gene transcriptional hierarchy for the A. hydrophila lateral flagella.

The GTPase activity of FlhF is dispensable for flagellar localization, but not motility, in Pseudomonas aeruginosa

The opportunistic human pathogen Pseudomonas aeruginosa uses two surface organelles, flagella and pili, for motility and adhesion in biotic and abiotic environments. Polar flagellar placement and number are influenced by FlhF, which is a signal recognition particle (SRP)-type GTPase. The FlhF proteins of Bacillus subtilis and Campylobacter jejuni were recently shown to have GTPase activity. However, the phenotypes associated with flhF deletion and/or mutation differ between these organisms and P. aeruginosa, making it difficult to generalize a role for FlhF in pseudomonads. In this study, we confirmed that FlhF of P. aeruginosa binds and hydrolyzes GTP. We mutated FlhF residues that we predicted would alter nucleotide binding and hydrolysis and determined the effects of these mutations on FlhF enzymatic activity, protein dimerization, and bacterial motility. Both hydrolytically active and inactive FlhF point mutants restored polar flagellar assembly, as seen for wild-type FlhF. However, differential effects on flagellar function were observed in single-cell assays of swimming motility and flagellar rotation. These findings indicate that FlhF function is influenced by its nucleotide binding and hydrolytic activities and demonstrate that FlhF affects P. aeruginosa flagellar function as well as assembly.

The main Aeromonas pathogenic factors

The members of the Aeromonas genus are ubiquitous, water-borne bacteria. They have been isolated from marine waters, rivers, lakes, swamps, sediments, chlorine water, water distribution systems, drinking water and residual waters; different types of food, such as meat, fish, seafood, vegetables, and processed foods. Aeromonas strains are predominantly pathogenic to poikilothermic animals, and the mesophilic strains are emerging as important pathogens in humans, causing a variety of extraintestinal and systemic infections as well as gastrointestinal infections. The most commonly described disease caused by Aeromonas is the gastroenteritis; however, no adequate animal model is available to reproduce this illness caused by Aeromonas. The main pathogenic factors associated with Aeromonas are: surface polysaccharides (capsule, lipopolysaccharide, and glucan), S-layers, iron-binding systems, exotoxins and extracellular enzymes, secretion systems, fimbriae and other nonfilamentous adhesins, motility and flagella.

Differential glycosylation of polar and lateral flagellins in Aeromonas hydrophila AH-3

Polar and lateral flagellin proteins from Aeromonas hydrophila strain AH-3 (serotype O34) were found to be glycosylated with different carbohydrate moieties. The lateral flagellin was modified at three sites in O-linkage, with a single monosaccharide of 376 Da, which we show to be a pseudaminic acid derivative. The polar flagellin was modified with a heterogeneous glycan, comprised of a heptasaccharide, linked through the same 376-Da sugar to the protein backbone, also in O-linkage. In-frame deletion mutants of pseudaminic acid biosynthetic genes pseB and pseF homologues resulted in abolition of polar and lateral flagellar formation by posttranscriptional regulation of the flagellins, which was restored by complementation with wild type pseB or F homologues or Campylobacter pseB and F.