Catecholamine-Stimulated Growth of Aeromonas hydrophila Requires the TonB2 Energy Transduction System but Is Independent of the Amonabactin Siderophore

The dual role of TonB genes in turnerbactin uptake and carbohydrate utilization in the shipworm symbiont Teredinibacter turnerae

IMPORTANCE

This study highlights diversity in iron acquisition and regulation in bacteria. The mechanisms of iron acquisition and its regulation in Teredinibacter turnerae, as well as its connection to cellulose utilization, a hallmark phenotype of T. turnerae, expand the paradigm of bacterial iron acquisition. Two of the four TonB genes identified in T. turnerae exhibit functional redundancy and play a crucial role in siderophore-mediated iron transport. Unlike typical TonB genes in bacteria, none of the TonB genes in T. turnerae are clearly iron regulated. This unusual regulation could be explained by another important finding in this study, namely, that the two TonB genes involved in iron transport are also essential for cellulose utilization as a carbon source, leading to the expression of TonB genes even under iron-rich conditions.

In Vitro Antibiofilm Activity of Resveratrol against Aeromonas hydrophila

Aeromonas hydrophila is a Gram-negative bacterium that widely exists in various aquatic environments and causes septicemia in fish and humans. Resveratrol, a natural polyterpenoid product, has potential chemo-preventive and antibacterial properties. In this study, we investigated the effect of resveratrol on A. hydrophila biofilm formation and motility. The results demonstrated that resveratrol, at sub-MIC levels, can significantly inhibit the biofilm formation of A. hydrophila, and the biofilm was decreased with increasing concentrations. The motility assay showed that resveratrol could diminish the swimming and swarming motility of A. hydrophila. Transcriptome analyses (RNA-seq) showed that A. hydrophila treated with 50 and 100 μg/mL resveratrol, respectively, presented 230 and 308 differentially expressed genes (DEGs), including 90 or 130 upregulated genes and 130 or 178 downregulated genes. Among them, genes related to flagellar, type IV pilus and chemotaxis were significantly repressed. In addition, mRNA of virulence factors OmpA, extracellular proteases, lipases and T6SS were dramatically suppressed. Further analysis revealed that the major DEGs involved in flagellar assembly and bacterial chemotaxis pathways could be regulated by cyclic-di-guanosine monophosphate (c-di-GMP)- and LysR-Type transcriptional regulator (LTTR)-dependent quorum sensing (QS) systems. Overall, our results indicate that resveratrol can inhibit A. hydrophila biofilm formation by disturbing motility and QS systems, and can be used as a promising candidate drug against motile Aeromonad septicemia.

The dual role of TonB genes in turnerbactin uptake and carbohydrate utilization in the shipworm symbiont Teredinibacter turnerae

Teredinibacter turnerae is an intracellular bacterial symbiont that resides in the gills of shipworms, wood-eating bivalve mollusks. This bacterium produces a catechol siderophore, turnerbactin, required for the survival of this bacterium under iron limiting conditions. The turnerbactin biosynthetic genes are contained in one of the secondary metabolite clusters conserved among T. turnerae strains. However, Fe(III)-turnerbactin uptake mechanisms are largely unknown. Here, we show that the first gene of the cluster, fttA a homologue of Fe(III)-siderophore TonB-dependent outer membrane receptor (TBDR) genes is indispensable for iron uptake via the endogenous siderophore, turnerbactin, as well as by an exogenous siderophore, amphi-enterobactin, ubiquitously produced by marine vibrios. Furthermore, three TonB clusters containing four tonB genes were identified, and two of these genes, tonB1b and tonB2, functioned not only for iron transport but also for carbohydrate utilization when cellulose was a sole carbon source. Gene expression analysis revealed that none of the tonB genes and other genes in those clusters were clearly regulated by iron concentration while turnerbactin biosynthesis and uptake genes were up-regulated under iron limiting conditions, highlighting the importance of tonB genes even in iron rich conditions, possibly for utilization of carbohydrates derived from cellulose.

QseBC regulates in vitro and in vivo virulence of Aeromonas hydrophila in response to norepinephrine

The inter-kingdom communication between host and pathogenic bacteria mediated by the host hormones epinephrine (Epi)/norepinephrine (NE)/autoinducer-3 (AI-3) and transduced by the bacterial two-component signal transduction system QseBC has been well demonstrated in mammalian pathogens. Aeromonas hydrophila, a common opportunistic pathogen in freshwater aquaculture, responds to NE by increased bacterial growth and enhanced virulence. However, the underlying mechanisms remain poorly understood. Our study demonstrated that deletion of qseB and qseC significantly inhibited NE-promoted growth, biofilm formation, and hemolytic activity of A. hydrophila. The adhesion ability of ΔqseB and ΔqseC to J774a.1 cells was significantly decreased compared with the wild-type strain in the presence and absence of NE, whereas NE still enhanced the adhesion ability of the mutant and wild-type strains with a similar effect, suggesting that NE-enhanced cell adhesion was independent of QseBC. Moreover, QseBC did not affect the swimming and swarming motility of A. hydrophila with or without NE. Quantitative real-time PCR analyses revealed the down-regulated expression of some virulence-related genes (hly, ast, act, aerA) in each mutant compared with the wild-type strain in the presence of NE. Tilapia infection experiments indicated that deletion of qseB or qseC weakened NE-promoted virulence of A. hydrophila. In conclusion, our study suggests that NE stimulates the growth, biofilm formation, and hemolytic activity of A. hydrophila and enhances the virulence of the pathogen in fish via the QseBC system.

TonB systems are required for Aeromonas hydrophila motility by controlling the secretion of flagellin

The TonB system is required for the active transport of iron compounds across the outer membrane in Gram-negative bacteria. Our previous data indicated that three TonB systems act coordinately to contribute to the motility of Aeromonas hydrophila NJ-35. In this study, we found that flagellum biogenesis was defective in the ΔtonB123 mutant. Subcellular localization indicated that the flagellin subunits FlaA and FlaB were trapped in the cytoplasm of ΔtonB123 mutant with reduced molecular mass. Overexpression of FlaA or FlaB in the ΔtonB123 mutant was unable to restore the secretion of flagellin subunits. Further investigation demonstrated that flagellins in the ΔtonB123 mutant showed a weak affinity for the flagellin-specific chaperone FliS, which is necessary for the export of flagellins. Deglycosylation analysis indicated that flagellins in the cytoplasm of the ΔtonB123 mutant were almost nonglycosylated. Our data suggested that disruption of tonB123 impairs the formation of flagella by inhibiting flagellin glycosylation and decreasing the binding affinity of flagellin for the chaperone FliS. Taken together, our findings indicate a new role of the TonB system in flagellar biogenesis in A. hydrophila.

Norepinephrine and Serotonin Can Modulate the Behavior of the Probiotic Enterococcus faecium NCIMB10415 towards the Host: Is a Putative Surface Sensor Involved?

The human gut microbiota has co-evolved with humans by exchanging bidirectional signals. This study aims at deepening the knowledge of this crucial relationship by analyzing phenotypic and interactive responses of the probiotic Enterococcus faecium NCIMB10415 (E. faecium SF68) to the top-down signals norepinephrine (NE) and serotonin (5HT), two neuroactive molecules abundant in the gut. We treated E. faecium NCIMB10415 with 100 µM NE and 50 µM 5HT and tested its ability to form static biofilm (Confocal Laser Scanning Microscopy), adhere to the Caco-2/TC7 monolayer, affect the epithelial barrier function (Transepithelial Electrical Resistance) and human dendritic cells (DC) maturation, differentiation, and cytokines production. Finally, we evaluated the presence of a putative hormone sensor through in silico (whole genome sequence and protein modelling) and in vitro (Micro-Scale Thermophoresis) analyses. The hormone treatments increase biofilm formation and adhesion on Caco-2/TC7, as well as the epithelial barrier function. No differences concerning DC differentiation and maturation between stimulated and control bacteria were detected, while an enhanced TNF-α production was observed in NE-treated bacteria. Investigations on the sensor support the hypothesis that a two-component system on the bacterial surface can sense 5HT and NE. Overall, the data demonstrate that E. faecium NCIMB10415 can sense both NE and 5HT and respond accordingly.

Inter-Kingdom Signaling of Stress Hormones: Sensing, Transport and Modulation of Bacterial Physiology

Prokaryotes and eukaryotes have coexisted for millions of years. The hormonal communication between microorganisms and their hosts, dubbed inter-kingdom signaling, is a recent field of research. Eukaryotic signals such as hormones, neurotransmitters or immune system molecules have been shown to modulate bacterial physiology. Among them, catecholamines hormones epinephrine/norepinephrine, released during stress and physical effort, or used therapeutically as inotropes have been described to affect bacterial behaviors (i.e., motility, biofilm formation, virulence) of various Gram-negative bacteria (e.g., Escherichia coli, Salmonella enterica serovar Typhimurium, Pseudomonas aeruginosa, Vibrio sp.). More recently, these molecules were also shown to influence the physiology of some Gram-positive bacteria like Enterococcus faecalis. In E. coli and S. enterica, the stress-associated mammalian hormones epinephrine and norepinephrine trigger a signaling cascade by interacting with the QseC histidine sensor kinase protein. No catecholamine sensors have been well described yet in other bacteria. This review aims to provide an up to date report on catecholamine sensors in eukaryotes and prokaryotes, their transport, and known effects on bacteria.

Proteome analysis of virulent Aeromonas hydrophila reveals the upregulation of iron acquisition systems in the presence of a xenosiderophore

The Gram-negative bacterium, Aeromonas hydrophila, has been responsible for extensive losses in the catfish industry for over a decade. Due to this impact, there are ongoing efforts to understand the basic mechanisms that contribute to virulent A. hydrophila (vAh) outbreaks. Recent challenge models demonstrated that vAh cultured in the presence of the iron chelating agent deferoxamine mesylate (DFO) were more virulent to channel catfish (Ictalurus punctatus). Interestingly, differential gene expression of select iron acquisition genes was unremarkable between DFO and non-DFO cultures, posing the question: why the increased virulence? The current work sought to evaluate growth characteristics and protein expression of vAh after the addition of DFO. A comparative proteome analysis revealed differentially expressed proteins among tryptic soy broth (TSB) and TSB + DFO treatments. Upregulated proteins identified among the TSB + DFO treatment were enriched for gene ontology groups including iron ion transport, siderophore transport and siderophore uptake transport, all iron acquisition pathways. Protein-protein interactions were also evaluated among the differentially expressed proteins and predicted that many of the upregulated iron acquisition proteins likely form functional physiological networks. The proteome analysis of the vAh reveals valuable information about the basic biological processes likely leading to increased virulence during iron restriction in this organism.

Identification of a new effector-immunity pair of Aeromonas hydrophila type VI secretion system

The type VI secretion system (T6SS) is a multiprotein weapon that kills eukaryotic predators or prokaryotic competitors by delivering toxic effectors. Despite the importance of T6SS in bacterial environmental adaptation, it is still challenging to systematically identify T6SS effectors because of their high diversity and lack of conserved domains. In this report, we discovered a putative effector gene, U876-17730, in the whole genome of Aeromonas hydrophila NJ-35 based on the reported conservative domain DUF4123 (domain of unknown function), with two cognate immunity proteins encoded downstream. Phylogenetic tree analysis of amino acids indicates that AH17730 belongs to the Tle1 (type VI lipase effector) family, and therefore was named Tle1^AH. The deletion of tle1^AH resulted in significantly decreased biofilm formation, antibacterial competition ability and virulence in zebrafish (Danio rerio) when compared to the wild-type strain. Only when the two immunity proteins coexist can bacteria protect themselves from the toxicity of Tle1^AH. Further study shows that Tle1^AH is a kind of phospholipase that possesses a conserved lipase motif, Gly-X-Ser-X-Gly (X is for any amino acid). Tle1^AH is secreted by T6SS, and this secretion requires its interaction with an associated VgrG (valine-glycine repeat protein G). In conclusion, we identified a T6SS effector-immunity pair and verified its function, which lays the foundation for future research on the role of T6SS in the pathogenic mechanism of A. hydrophila.

Obesity Affects the Microbiota-Gut-Brain Axis and the Regulation Thereof by Endocannabinoids and Related Mediators

The hypothalamus regulates energy homeostasis by integrating environmental and internal signals to produce behavioral responses to start or stop eating. Many satiation signals are mediated by microbiota-derived metabolites coming from the gastrointestinal tract and acting also in the brain through a complex bidirectional communication system, the microbiota–gut–brain axis. In recent years, the intestinal microbiota has emerged as a critical regulator of hypothalamic appetite-related neuronal networks. Obesogenic high-fat diets (HFDs) enhance endocannabinoid levels, both in the brain and peripheral tissues. HFDs change the gut microbiota composition by altering the Firmicutes:Bacteroidetes ratio and causing endotoxemia mainly by rising the levels of lipopolysaccharide (LPS), the most potent immunogenic component of Gram-negative bacteria. Endotoxemia induces the collapse of the gut and brain barriers, interleukin 1β (IL1β)- and tumor necrosis factor α (TNFα)-mediated neuroinflammatory responses and gliosis, which alter the appetite-regulatory circuits of the brain mediobasal hypothalamic area delimited by the median eminence. This review summarizes the emerging state-of-the-art evidence on the function of the “expanded endocannabinoid (eCB) system” or endocannabinoidome at the crossroads between intestinal microbiota, gut-brain communication and host metabolism; and highlights the critical role of this intersection in the onset of obesity.

Diverse effects of nitric oxide reductase NorV on Aeromonas hydrophila virulence-associated traits under aerobic and anaerobic conditions

NorV has been known to be an anaerobic nitric oxide reductase associated with nitric oxide (NO) detoxification. Recently, we showed that the norV gene of Aeromonas hydrophila was highly upregulated after co-culturing with Tetrahymena thermophila. Here, we demonstrated that the transcription and expression levels of norV were upregulated in a dose-dependent manner after exposure to NO under aerobic and anaerobic conditions. To investigate the roles of norV in resisting predatory protists and virulence of A. hydrophila, we constructed the norV gene-deletion mutant (ΔnorV). Compared to the wild type, the ΔnorV mutant showed no significant difference in growth at various NO concentrations under aerobic conditions but significantly stronger NO-mediated growth inhibition under anaerobic conditions. The deletion of norV exhibited markedly decreased cytotoxicity, hemolytic and protease activities under aerobic and anaerobic conditions. Also, the hemolysin co-regulated protein (Hcp) in the ΔnorV mutant showed increased secretion under aerobic conditions but decreased secretion under anaerobic conditions as compared to the wild-type. Moreover, the inactivation of norV led to reduced resistance to predation by T. thermophila, decreased survival within macrophages and highly attenuated virulence in zebrafish. Our data indicate a diverse role for norV in the expression of A. hydrophila virulence-associated traits that is not completely dependent on its function as a nitric oxide reductase. This study provides insights into an unexplored area of NorV, which will contribute to our understanding of bacterial pathogenesis and the development of new control strategies for A. hydrophila infection.

Roles of three TonB systems in the iron utilization and virulence of the Aeromonas hydrophila Chinese epidemic strain NJ-35

The TonB system functions in iron transport and has been identified in certain Gram-negative bacteria. Recently, we reported three TonB systems in the Aeromonas hydrophila Chinese epidemic strain NJ-35, but the functions of these systems have not been thoroughly elucidated to date. In this study, we investigated the role of these TonB systems in A. hydrophila iron utilization and virulence. We found that tonB1 and tonB2 were preferentially transcribed in iron-chelated conditions, where gene expression levels were approximately 8- and 68-fold higher compared with iron-rich conditions, respectively; tonB3 was consistently transcribed at a low level under iron-repleted and iron-depleted conditions. Only the TonB2 system was required to utilize iron-binding proteins. The tonB123 mutant showed increased susceptibility to erythromycin and roxithromycin. In addition, all three tonB genes were involved in A. hydrophila virulence in zebrafish, and various phenotypes associated with environmental survival were changed with varying degrees in each tonB mutant. TonB2 plays a relatively major role in adhesion, motility, and biofilm formation, while TonB3 is more involved in the anti-phagocytosis of A. hydrophila. In each observed phenotype, no significant difference was found between the single- and double-deletion mutants, whereas the triple-deletion mutant exhibited the most serious defects, indicating that all three TonB systems of A. hydrophila coordinately complement one another. In conclusion, this study elucidates the importance of TonB in iron acquisition and virulence of A. hydrophila, which lays the foundation for future studies regarding the survival mechanisms of this bacterium in iron-restricted environments.

The stress hormone norepinephrine increases the growth and virulence of Aeromonas hydrophila

Stress is an important contributing factor in the outbreak of infectious fish diseases. To comprehensively understand the impact of catecholamine stress hormone norepinephrine (NE) on the pathogenicity of Aeromonas hydrophila, we assessed variations in bacterial growth, virulence‐related genes expression and virulence factors activity after NE addition in serum‐SAPI medium. Further, we assessed the effects of NE on A. hydrophila virulence in vivo by challenging fish with pathogenic strain AH196 and following with or without NE injection. The NE‐associated stimulation of A. hydrophila strain growth was not linear‐dose‐dependent, and only 100 μM, or higher concentrations, could stimulate growth. Real‐time PCR analyses revealed that NE notably changed 13 out of the 16 virulence‐associated genes (e.g. ompW, ahp, aha, ela, ahyR, ompA, and fur) expression, which were all significantly upregulated in A. hydrophila AH196 (p < 0.01). NE could enhance the protease activity, but not affect the lipase activity, hemolysis, and motility. Further, the mortality of crucian carp challenged with A. hydrophila AH196 was significantly higher in the group treated with NE (p < 0.01). Collectively, our results showed that NE enhanced the growth and virulence of pathogenic bacterium A. hydrophila.

Comparative transcriptomic and proteomic analyses reveal upregulated expression of virulence and iron transport factors of Aeromonas hydrophila under iron limitation

Background

Iron plays important roles in the growth, reproduction and pathogenicity of Aeromonas hydrophila. In this study, we detected and compared the mRNA and protein expression profiles of A. hydrophila under normal and iron restricted medium with 200 μM 2,2-Dipyridyl using RNA Sequencing (RNA-seq) and isobaric tags for relative and absolute quantification (iTRAQ) analyses.

Results

There were 1204 genes (601 up- and 603 down-regulated) and 236 proteins (90 up- and 146 down-regulated) shown to be differentially expressed, and 167 genes and proteins that showed consistent expression. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed that the differentially expressed genes and proteins were mainly involved in iron ion transport, protein activity, energy metabolism and virulence processes. Further validation of the RNA-seq and iTRAQ results by quantitative real-time PCR (qPCR) revealed that 18 of the 20 selected genes were consistently expressed. The iron-ion absorption and concentration of A. hydrophila under iron-limited conditions were enhanced, and most virulence factors (protease activity, hemolytic activity, lipase activity, and swimming ability) were also increased. Artificial A. hydrophila infection caused higher mortality in cyprinid Megalobrama amblycephala under iron-limited conditions.

Conclusion

Understanding the responses of pathogenic Aeromonas hydrophila within the hostile environment of the fish host, devoid of free iron, is important to reveal bacterial infection and pathogenesis. This study further confirmed the previous finding that iron-limitation efficiently enhanced the virulence of A. hydrophila using multi-omics analyses. We identified differentially expressed genes and proteins, related to enterobactin synthesis and virulence establishment, that play important roles in addressing iron scarcity.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1178-8) contains supplementary material, which is available to authorized users.