LuxO controls extracellular protease, haemolytic activities and siderophore production in fish pathogen Vibrio alginolyticus

Vibrio harveyi uses both type III secretion system and quorum sensing for the colonization of the European abalone

The vibriosis of the European abalone, Haliotis tuberculata, is characterized by the rapidity of the infection by the pathogen Vibrio harveyi ORM4, leading to death of animals only after two days. The lethality of the pathogen is linked to the production of the type III secretion system (T3SS) and to genes regulated by quorum sensing (QS). The aim of this study was to investigate the colonization of the European abalone by both virulent and avirulent V. harveyi strains, as well as the involvement of T3SS and QS during infection. Our results emphasize the importance of gills for the bacterial establishment as the bacterial concentration of the avirulent V. harveyi strain significantly decreased from 189.3 ± 98.6 CFU/mg to 0.8 ± 0.5 CFU/mg between 24 and 48 h post-infection (hpi). In opposition, the pathogen V. harveyi ORM4 was able to maintain itself on the gills, with a concentration of 461.9 ± CFU/mg at 48 hpi, which was allowed by the production of T3SS and a functional QS. Following the infection cycle of V. harveyi ORM4 inside H. tuberculata, we also demonstrated that QS is essential for the ability of V. harveyi ORM4 to colonize the abalone hemolymph and to maintain in it. In response to the presence of V. harveyi, an increase in reactive oxygen species production was recorded, while the phagocytosis activity remained unchanged. We also highlighted the involvement of both QS and T3SS to escape the immune system activity, and that an overproduction of T3SS induced hemocyte mortality. This study provides the evidence that both T3SS and QS are essential for the establishment of V. harveyi ORM4 inside the European abalone.

Quorum sensing positively regulates CPS-dependent Autographiviridae phage infection in Vibrio alginolyticus

Quorum sensing (QS) orchestrates many bacterial behaviors, including virulence and biofilm formation, across bacterial populations. Nevertheless, the underlying mechanism by which QS regulates capsular polysaccharide (CPS)-dependent phage-bacterium interactions remains unclear. In this study, we report that QS upregulates the expression of CPS-dependent phage receptors, thus increasing phage adsorption and infection rates in Vibrio alginolyticus. We found that QS upregulated the expression of the ugd gene, leading to increased synthesis of Autographiviridae phage receptor CPS synthesis in V. alginolyticus. The signal molecule autoinducer-2 released by Vibrio from different sources can potentially enhance CPS-dependent phage infections. Therefore, our data suggest that inhibiting QS may reduce, rather than improve, the therapeutic efficacy of CPS-specific phages.

IMPORTANCE

Phage resistance is a direct threat to phage therapy, and understanding phage-host interactions, especially how bacteria block phage infection, is essential for developing successful phage therapy. In the present study, we demonstrate for the first time that Vibrio alginolyticus uses quorum sensing (QS) to promote capsular polysaccharide (CPS)-specific phage infection by upregulating ugd expression, which is necessary for the synthesis of Autographiviridae phage receptor CPS. Although increased CPS-specific phage susceptibility is a novel trade-off mediated by QS, it results in the upregulation of virulence factors, promoting biofilm development and enhanced capsular polysaccharide production in V. alginolyticus. This suggests that inhibiting QS may improve the effectiveness of antibiotic treatment, but it may also reduce the efficacy of phage therapy.

BolA-like protein (IbaG) promotes biofilm formation and pathogenicity of Vibrio parahaemolyticus

Vibrio parahaemolyticus is a gram-negative halophilic bacterium widespread in temperate and tropical coastal waters; it is considered to be the most frequent cause of Vibrio-associated gastroenteritis in many countries. BolA-like proteins, which reportedly affect various growth and metabolic processes including flagellar synthesis in bacteria, are widely conserved from prokaryotes to eukaryotes. However, the effects exerted by BolA-like proteins on V. parahaemolyticus remain unclear, and thus require further investigation. In this study, our purpose was to investigate the role played by BolA-like protein (IbaG) in the pathogenicity of V. parahaemolyticus. We used homologous recombination to obtain the deletion strain ΔibaG and investigated the biological role of BolA family protein IbaG in V. parahaemolyticus. Our results showed that IbaG is a bacterial transcription factor that negatively modulates swimming capacity. Furthermore, overexpressing IbaG enhanced the capabilities of V. parahaemolyticus for swarming and biofilm formation. In addition, inactivation of ibaG in V. parahaemolyticus SH112 impaired its capacity for colonizing the heart, liver, spleen, and kidneys, and reduced visceral tissue damage, thereby leading to diminished virulence, compared with the wild-type strain. Finally, RNA-sequencing revealed 53 upregulated and 71 downregulated genes in the deletion strain ΔibaG. KEGG enrichment analysis showed that the two-component system, quorum sensing, bacterial secretion system, and numerous amino acid metabolism pathways had been altered due to the inactivation of ibaG. The results of this study indicated that IbaG exerts a considerable effect on gene regulation, motility, biofilm formation, and pathogenicity of V. parahaemolyticus. To the best of our knowledge, this is the first systematic study on the role played by IbaG in V. parahaemolyticus infections. Thus, our findings may lead to a better understanding of the metabolic processes involved in bacterial infections and provide a basis for the prevention and control of such infections.

Exploring the Potential Molecular Mechanisms of Interactions between a Probiotic Consortium and Its Coral Host

Beneficial microorganisms for corals (BMCs) have been demonstrated to be effective probiotics to alleviate bleaching and mitigate coral mortality in vivo. The selection of putative BMCs is traditionally performed manually, using an array of biochemical and molecular tests for putative BMC traits. We present a comprehensive genetic survey of BMC traits using a genome-based framework for the identification of alternative mechanisms that can be used for future in silico selection of BMC strains. We identify exclusive BMC traits associated with specific strains and propose new BMC mechanisms, such as the synthesis of glycine betaine and ectoines. Our roadmap facilitates the selection of BMC strains while increasing the array of genetic targets that can be included in the selection of putative BMC strains to be tested as coral probiotics. IMPORTANCE Probiotics are currently the main hope as a potential medicine for corals, organisms that are considered the marine "canaries of the coal mine" and that are threatened with extinction. Our experiments have proved the concept that probiotics mitigate coral bleaching and can also prevent coral mortality. Here, we present a comprehensive genetic survey of probiotic traits using a genome-based framework. The main outcomes are a roadmap that facilitates the selection of coral probiotic strains while increasing the array of mechanisms that can be included in the selection of coral probiotics.

Potential dsRNAs can be delivered to aquatic for defense pathogens

The use of antibiotics to facilitate resistance to pathogens in aquatic animals is a traditional method of pathogen control that is harmful to the environment and human health. RNAi is an emerging technology in which homologous small RNA molecules target specific genes for degradation, and it has already shown success in laboratory experiments. However, further research is needed before it can be applied in aquafarms. Many laboratories inject the dsRNA into aquatic animals for RNAi, which is obviously impractical and very time consuming in aquafarms. Therefore, to enable the use of RNAi on a large scale, the methods used to prepare dsRNA need to be continuously in order to be fast and efficient. At the same time, it is necessary to consider the issue of biological safety. This review summarizes the key harmful genes associated with aquatic pathogens (viruses, bacteria, and parasites) and provides potential targets for the preparation of dsRNA; it also lists some current examples where RNAi technology is used to control aquatic species, as well as how to deliver dsRNA to the target hydrobiont.

Molecular Mechanism of Iron Transport Systems in Vibrio

The ability to acquire iron from the environment is often an important virulence factor for pathogenic bacteria and Vibrios are no exception to this. Vibrios are reported mainly from marine habitats and most of the species are pathogenic. Among those, the pathogenic vibrios eg. V cholerae, V. parahaemolyticus, V. vulnificus causes foodborne illnesses. Vibrios are capable of producing all different classes of siderophores like hydroxamate (aerobactin), catecholate (vibriobactin, fluvibactin), carboxylate (vibrioferrin), and amphiphilic (amphibactin). Every different species of vibrios are capable of utilizing some endogenous or xenosiderophores. Being Gram-negative bacteria, Vibrios import iron siderophore via TonB dependent transport system and unlike other Gamma proteobacteria these usually possess two or even three partially redundant TonB systems for iron siderophore transport. Other than selected few iron siderophores, most pathogenic Vibrios are known to be able to utilize heme as the sole iron source, while some species are capable of importing free iron from the environment. As per the present knowledge, the spectrum of iron compound transport and utilization in Vibrios is better understood than the siderophore biosynthetic capability of individual species.

Regulatory small RNA, Qrr2 is expressed independently of sigma factor-54 and can function as the sole Qrr sRNA to control quorum sensing in Vibrio parahaemolyticus

Bacterial cells alter gene expression in response to changes in population density in a process called quorum sensing (QS). In Vibrio harveyi, LuxO, a low cell density activator of sigma factor-54 (RpoN), is required for transcription of five non-coding regulatory sRNAs, Qrr1-Qrr5, which each repress translation of the master QS regulator LuxR. Vibrio parahaemolyticus, the leading cause of bacterial seafood-borne gastroenteritis, also contains five Qrr sRNAs that control OpaR (the LuxR homolog), controlling capsule polysaccharide (CPS), motility, and metabolism. We show that in a ΔluxO deletion mutant, opaR was de-repressed and CPS and biofilm were produced. However, in a ΔrpoN mutant, opaR was repressed, no CPS was produced, and less biofilm production was observed compared to wild type. To determine why opaR was repressed, expression analysis in ΔluxO showed all five qrr genes were repressed, while in ΔrpoN the qrr2 gene was significantly de-repressed. Reporter assays and mutant analysis showed Qrr2 sRNA can act alone to control OpaR. Bioinformatics analysis identified a sigma-70 (RpoD) -35 -10 promoter overlapping the canonical sigma-54 (RpoN) -24 -12 promoter in the qrr2 regulatory region. The qrr2 sigma-70 promoter element was also present in additional Vibrio species indicating it is widespread. Mutagenesis of the sigma-70 -10 promoter site in the ΔrpoN mutant background, resulted in repression of qrr2. Analysis of qrr quadruple deletion mutants, in which only a single qrr gene is present, showed that only Qrr2 sRNA can act independently to regulate opaR. Mutant and expression data also demonstrated that RpoN and the global regulator, Fis, act additively to repress qrr2. Our data has uncovered a new mechanism of qrr expression and shows that Qrr2 sRNA is sufficient for OpaR regulation. Importance The quorum sensing non-coding sRNAs are present in all Vibrio species but vary in number and regulatory roles among species. In the Harveyi clade, all species contain five qrr genes, and in V. harveyi these are transcribed by sigma-54 and are additive in function. In the Cholerae clade, four qrr genes are present, and in V. cholerae the qrr genes are redundant in function. In V. parahaemolyticus, qrr2 is controlled by two overlapping promoters. In an rpoN mutant, qrr2 is transcribed from a sigma-70 promoter that is present in all V. parahaemolyticus strains and in other species of the Harveyi clade suggesting a conserved mechanism of regulation. Qrr2 sRNA can function as the sole Qrr sRNA to control OpaR.

Binding site profiles and N-terminal minor groove interactions of the master quorum-sensing regulator LuxR enable flexible control of gene activation and repression

LuxR is a TetR family master quorum sensing (QS) regulator activating or repressing expression of hundreds of genes that control collective behaviors in Vibrios with underlying mechanism unknown. To illuminate how this regulator controls expression of various target genes, we applied ChIP-seq and DNase I-seq technologies. Vibrio alginolyticus LuxR controls expression of ∼280 genes that contain either symmetric palindrome (repDNA) or asymmetric (actDNA) binding motifs with different binding profiles. The median number of LuxR binding sites for activated genes are nearly double for that of repressed genes. Crystal structures of LuxR in complex with the respective repDNA and actDNA motifs revealed a new mode of LuxR DNA binding that involves contacts of its N-terminal extension to the minor groove. The N-terminal contacts mediated by Arginine-9 and Arginine-11 differ when LuxR binds to repDNA vs actDNA, leading to higher binding affinity at repressed targets. Moreover, modification of LuxR binding sites, binding profiles, and N-terminal extension have important consequences on QS-regulated phenotypes. These results facilitate fundamental understanding of the high flexibility of mechanisms of LuxR control of gene activation and repression in Vibrio QS, which may facilitate to design QS inhibiting chemicals that interfere with LuxR regulation to effectively control pathogens.

Roles of rpoN in biofilm formation of Vibrio alginolyticus HN08155 at different cell densities

RpoN (δ54) as a global regulator controls crucialvirulence-associated phenotype, which can regulate flagellum and exopolysaccharides (EPS) during pathogenic biofilm formation. However, the knowledge of the roles of rpoN in biofilm formation of V. alginolyticus is limited, especially at different cell densities. Herein, deletion mutant strain ΔrpoN, complementary strain ΔrpoN-C and negative control strain ΔrpoN-Z were constructed to investigate the effects of rpoN on biofilm formation of V. alginolyticus HN08155 based on flagellum and EPS at different cell density conditions. The results showed that all of strains can form biofilm, and biofilms of strains with rpoN were formed at low cell density (LCD) and detached at high cell density (HCD), while those of ΔrpoN and ΔrpoN-Z were absent at LCD and accumulated excessively with a spotty pellicle at HCD without detaching. The EPS contents of strains with rpoN was greater than that of ΔrpoN and ΔrpoN-Z at LCD, while the opposite trends were observed at HCD. The expression levels of rpoN were quantified, which were consistent with the trend of biofilm formation. It's worth noting that absence of rpoN resulted in the failure of biofilm detachment, lacking of flagellum and decreasing motility, indicating that rpoN was not necessary for biofilm formation, but it was essential for biofilm detachment.

Functional characterization of T3SS C-ring component VscQ and evaluation of its mutant as a live attenuated vaccine in zebrafish (Danio rerio) model

Vibrio alginolyticus, a Gram-negative bacterium, has been recognized as an opportunistic pathogen in marine animals as well as humans. Type III secretion system (T3SS) is critical for pathogen virulence and disease development. However, no more information is known about the C-ring component VscQ and its physiological role. In this study, gene vscQ was cloned from V. alginolyticus wild-type strain HY9901 and the mutant strain HY9901ΔvscQ was constructed by the in-frame deletion method. The HY9901ΔvscQ mutant showed an attenuated swarming phenotype and a closely 4.6-fold decrease in the virulence to Danio rerio. However, the HY9901ΔvscQ mutant showed no difference in growth, biofilm formation and ECPase activity. HY9901ΔvscQ reduces the release of LDH, NO and caspase-3 activity of infected FHM cell, which are involved in fish cell apoptosis. Deletion of gene vscQ downregulates the expression level of T3SS-related genes including vscL, vopB, hop, vscO, vscK, vopD, vcrV and vopS and flagellum-related genes (flaA and fliG). And Danio rerio vaccinated via i.m injection with HY9901ΔvscQ induced a relative percent survival (RPS) value of 71% after challenging with the wild-type HY9901. Real-time PCR assays showed that vaccination with HY9901ΔvscQ enhanced the expression of immune-related genes, including TNF-α, TLR5, IL-6R, IgM and c/ebpβ in liver and spleen after vaccination, indicating that it is able to induce humoral and cell-mediated immune response in zebrafish. These results demonstrate that the HY9901ΔvscQ mutant could be used as an effective live vaccine to combat V. alginolyticus infection.

tepR encoding a bacterial enhancer-binding protein orchestrates the virulence and interspecies competition of Burkholderia glumae through qsmR and a type VI secretion system

The pathogenesis of the rice pathogenic bacterium Burkholderia glumae is under the tight regulation of the tofI/tofR quorum-sensing (QS) system. tepR, encoding a group I bacterial enhancer-binding protein, negatively regulates the production of toxoflavin, the phytotoxin acting as a major virulence factor in B. glumae. In this study, through a transcriptomic analysis, we identified the genes that were modulated by tepR and/or the tofI/tofR QS system. More than half of the differentially expressed genes, including the genes for the biosynthesis and transport of toxoflavin, were significantly more highly expressed in the ΔtepR mutant but less expressed in the ΔtofI-tofR (tofI/tofR QS-defective) mutant. In consonance with the transcriptome data, other virulence-related functions of B. glumae, extracellular protease activity and flagellum-dependent motility, were also negatively regulated by tepR, and this negative regulatory function of tepR was dependent on the IclR-type transcriptional regulator gene qsmR. Likewise, the ΔtepR mutant exhibited a higher level of heat tolerance in congruence with the higher transcription levels of heat shock protein genes in the mutant. Interestingly, tepR also exhibited its positive regulatory function on a previously uncharacterized type VI secretion system (denoted as BgT6SS-1). The survival of the both ΔtepR and ΔtssD (BgT6SS-1-defective) mutants was significantly compromised compared to the wild-type parent strain 336gr-1 in the presence of the natural rice-inhabiting bacterium, Pantoea sp. RSPAM1. Taken together, this study revealed pivotal regulatory roles of tepR in orchestrating multiple biological functions of B. glumae, including pathogenesis, heat tolerance, and bacterial interspecies competition.

Characterization of a cell density-dependent sRNA, Qrr, and its roles in the regulation of the quorum sensing and metabolism in Vibrio alginolyticus

Vibrio alginolyticus is an important fish pathogen causing pandemic diseases in marine animals. Small noncoding RNAs (sRNAs) are important posttranscriptional modulators of gene expression and involved in the pathogenesis of bacterial pathogens. Thus far, no cell density-dependent sRNA has been reported in V. alginolyticus. In this study, a cell density-dependent sRNA, Qrr, predicted based on the previous RNA-Seq analysis of V. alginolyticus cultured at low cell density (LCD) and high cell density (HCD), was characterized. The Qrr mutant showed significantly impaired growth and decreased swimming and swarming ability, and increased biofilm formation, extracellular polysaccharide content, serine protease production, and LD₅₀ values during zebrafish infection in contrast to the wild-type strain. Qrr modulates the master regulators LuxR and AphA in quorum sensing (QS) pathways possibly at the posttranscriptional level by base pairing with the 5'-untranslated regions (5'-UTRs). Meanwhile, both LuxR and AphA could directly bind to the promoter of qrr to activate or repress its transcription, respectively. Moreover, our unbiased metabolic approaches revealed that Qrr modulates a large quantity of metabolic and lipidomic pathways, including amino acids, purine and pyrimidine derivatives, tricarboxylic acid cycle (TCA cycle) intermediates, and lipids. Collectively, this work contributes to a systematic understanding of regulatory roles of the cell density-dependent sRNA, Qrr, in V. alginolyticus.

Alternative Sigma Factor RpoX Is a Part of the RpoE Regulon and Plays Distinct Roles in Stress Responses, Motility, Biofilm Formation, and Hemolytic Activities in the Marine Pathogen Vibrio alginolyticus

Vibrio alginolyticus is one of the most abundant microorganisms in marine environments and is also an opportunistic pathogen mediating high-mortality vibriosis in marine animals. Alternative sigma factors play essential roles in bacterial pathogens in the adaptation to environmental changes during infection and the adaptation to various niches, but little is known about them for V. alginolyticus Our previous investigation indicated that the transcript level of the gene rpoX significantly decreased in an RpoE mutant. Here, we found that rpoX was highly expressed in response to high temperature and low osmotic stress and was under the direct control of the alternative sigma factor RpoE and its own product RpoX. Moreover, transcriptome sequencing (RNA-seq) results showed that RpoE and RpoX had different regulons, although they coregulated 105 genes at high temperature (42°C), including genes associated with biofilm formation, motility, virulence, regulatory factors, and the stress response. RNA-seq and chromatin immunoprecipitation sequencing (ChIP-seq) analyses as well as electrophoretic mobility shift assays (EMSAs) revealed the distinct binding motifs of RpoE and RpoX proteins. Furthermore, quantitative real-time reverse transcription-PCR (qRT-PCR) analysis also confirmed that RpoX can upregulate genes associated with flagella, biofilm formation, and hemolytic activities at higher temperatures. rpoX abrogation does not appear to attenuate virulence toward model fish at normal temperature. Collectively, data from this study demonstrated the regulatory cascades of RpoE and an alternative sigma factor, RpoX, in response to heat and osmotic stresses and their distinct and overlapping roles in pathogenesis and stress responses in the marine bacterium V. alginolyticus IMPORTANCE The alternative sigma factor RpoE is essential for the virulence of Vibrio alginolyticus toward marine fish, coral, and other animals in response to sea surface temperature increases. In this study, we characterized another alternative sigma factor, RpoX, which is induced at high temperatures and under low-osmotic-stress conditions. The expression of rpoX is under the tight control of RpoE and RpoX. Although RpoE and RpoX coregulate 105 genes, they are programming different regulatory functions in stress responses and virulence in V. alginolyticus These findings illuminated the RpoE-RpoX-centered regulatory cascades and their distinct and overlapping regulatory roles in V. alginolyticus, which facilitates unraveling of the mechanisms by which the bacterium causes diseases in various sea animals in response to temperature fluctuations as well as the development of appropriate strategies to tackle infections by this bacterium.

VqsA controls exotoxin production by directly binding to the promoter of asp in the pathogen Vibrio alginolyticus

Quorum sensing (QS) system is an important bacterial cell-to-cell signaling system controlling expression of various genes in response to cell densities. In vibrios, LuxR/AphA are two established master QS regulators (MQSRs), and VqsA is recently identified to be the third putative MQSR. As a novel LysR-type regulator, the regulon and the underlying regulation mechanisms of VqsA remains to be elucidated. Here our investigation indicated that the yields of alkaline serine protease (Asp), the exotoxin in Vibrio alginolyticus was dependent on both LuxR and VqsA in growth phase dependent manner. Various in vivo and in vitro analyses including electrophoretic mobility shift assays (EMSA) along with DNase I footprinting investigations demonstrated that VqsA positively controls asp expression through directly binding to the partially palindromic 29 bp binding motif in the promoter region of asp. Moreover, RNA-seq analysis validated the regulatory roles of VqsA in various processes in the organism. Collectively, our data showed that VqsA positively regulates the expression of exotoxin and other virulence-associated genes and is essential for the QS regulation in V. alginolyticus.

Mechanisms underlying the virulence regulation of new Vibrio alginolyticus ncRNA Vvrr1 with a comparative proteomic analysis

The incidence of Vibrio alginolyticus infections has increased in recent years due to the influence of climate change and rising sea temperature. Vibrio virulence regulatory RNA 1 (Vvrr1) is a newly found noncoding RNA (ncRNA) predicted to be closely related to the adhesion ability of V. alginolyticus based on the previous RNA-seq. In this study, the target genes of Vvrr1 were fully screened and verified by constructing Vvrr1-overexpressing strains and using the proteome sequencing technology. Pyruvate kinase I (pykF) gene was predicted to be a chief target gene of Vvrr1 involved in virulence regulation. The adhesion ability, biofilm formation and virulence were significantly reduced in the Vvrr1-overexpressing and the pykF-silenced strain compared with the wild strains. Similar to the overexpression of Vvrr1, the silencing of pykF also reduced the expression level of virulence genes, such as ndk, eno, sdhB, glpF, and cysH. Meanwhile, by constructing the "pykF-GFP" fusion expression plasmid and using the GFP reporter gene analysis in Escherichia coli, the fluorescence intensity of the strain containing Vvrr1 whole ncRNA sequence vector was found to be significantly weakened. These indicated that Vvrr1 participated in the virulence regulation mechanism of V. alginolyticus by interacting with the virulence gene pykF.

VqsA, a Novel LysR-Type Transcriptional Regulator, Coordinates Quorum Sensing (QS) and Is Controlled by QS To Regulate Virulence in the Pathogen Vibrio alginolyticus

The quorum sensing (QS) system controls bacterial group behaviors in response to cell density. In vibrios, LuxR and AphA are two master QS regulators (MQSRs) controlling gene expression in response to high or low cell density. Other regulators involved in the regulation of these two MQSRs and QS pathways remain to be determined. Here, we performed bacterial one-hybrid (B1H)-assay-based screens of transcriptional factors (TFs) to identify TFs that can directly regulate the expression of luxR and aphA from a library of 285 TFs encoded by the fish pathogen Vibrio alginolyticus A total of 7 TFs were identified to bind to the promoters of both luxR and aphA Among these TFs, the novel LysR-type transcriptional regulator (LTTR) VqsA could activate LuxR and repress AphA transcription. Meanwhile, LuxR and AphA exerted feedback inhibition and activation of vqsA expression, respectively, indicating that VqsA coordinates QS and is also regulated by QS. In addition, VqsA inhibited its own expression by directly binding to its own promoter region. The VqsA-binding sites in the promoter regions of luxR and aphA as well as the binding sites of LuxR, AphA, and VqsA in the vqsA gene were uncovered by electrophoretic mobility shift assays (EMSAs) and DNase I footprinting analysis. Finally, VqsA was verified to play essential roles in QS-regulated phenotypes, i.e., type VI secretion system 2 (T6SS2)-dependent interbacterial competition, biofilm formation, exotoxin production, and in vivo virulence of V. alginolyticus Collectively, our data showed that VqsA is an important QS regulator in V. alginolyticusIMPORTANCE Investigation of the mechanism of regulation of quorum sensing (QS) systems will facilitate an understanding of bacterial pathogenesis and the identification of effective QS interference (QSI) targets. Here, we systematically screened transcriptional factors (TFs) that modulate the expression of the master QS regulators (MQSRs) LuxR and AphA, and a novel LysR-type transcriptional regulator, VqsA, was identified. Our data illuminated the mechanisms mediating the interaction among LuxR, AphA, and VqsA as well as the effects of these regulators on the expression and output of QS. The impaired expression of virulence genes as a result of vqsA disruption demonstrated that VqsA is an important player in QS regulation and pathogenesis and may be the third MQSR involved in sensing environmental signals by vibrios to coordinate QS responses. This study will facilitate the development of strategies to interfere with QS and effectively control this pathogen that plagues the aquaculture industry.

Copper nanoparticles toxicity: Laboratory strains verses environmental bacterial isolates

Nanoparticles have emerged as significant environmental contaminants and their impact has been studied using laboratory strains of bacteria. This study focuses on investigating the response of environmental isolate and laboratory strains of E. coli to 50 and 100 nm size of copper nanoparticles (CuNPs). The laboratory cultures included pathogenic and non-pathogenic strains. The environmental isolate and the non-pathogenic E. coli strain showed different inactivation patterns. After 2 h exposure to 50 nm CuNPs, the environmental isolate and the lab strain of E. coli lost 7.22 and 6.47 log; whereas the reduction of 6.16 and 6.68 log resulted after exposure to 100 nm CuNPs, respectively. The pathogenic E. coli O157:H7 exposed to 50 and 100 nm CuNPs for 2 h resulted in 5.24 and 6.54 log reduction, respectively. Although the environmental isolate and the laboratory strains of E. coli showed similar inactivation trends; they exhibited different toxicity elicitation mechanisms after exposure to the CuNPs. The pathogenic and non-pathogenic strains elicited significantly different levels of glutathione reductase (GR) activities, an enzyme critical for protection against radicals. Similarly, the environmental isolate and the lab strains of E. coli exhibited opposite trend in GR activities. These results clearly indicate divergence in the toxicity elicitation in the environmental isolate versus the laboratory strains from exposure to CuNPs, which highlights the need for an in-depth investigation of the impact of NPs on the biological processes and long-term effect of high load of NPs on the stability of aquatic and terrestrial ecologies.

Serine/threonine kinase PpkA coordinates the interplay between T6SS2 activation and quorum sensing in the marine pathogen Vibrio alginolyticus

Type VI secretion systems (T6SS) are multiprotein secretion machines that can mediate killing of bacterial cells and thereby modify the composition of bacterial communities. The mechanisms that control the production of and secretion of these killing machines are incompletely understood, although quorum sensing (QS) and the PpkA kinase modulate T6SS activity in some organisms. Here we investigated control the T6S in the marine organism Vibrio alginolyticus EPGS, which encodes two T6SS systems (T6SS1 and T6SS2). We found that the organism principally relies on T6SS2 for interbacterial competition. We further carried out a phosphoproteomic screen to identify substrates of the T6SS2-linked PpkA2 kinase. Substrates of PpkA2 encoded within the T6SS2 cluster as well proteins that are apparently not linked to T6SS-related processes were identified. Similar to other organisms, PpkA2 autophosphorylation was critical for T6SS2 function. Notably, phosphorylation of a polypeptide encoded outside of the T6SS2 cluster, VtsR, was critical for T6SS2 expression and function because it augments the expression of luxR, a key regulator of QS that also promotes T6SS2 gene expression. Thus, PpkA2 controls a phosphorylation cascade that mediates a positive regulatory loop entwining T6SS and QS, thereby coordinating these pathways to enhance the competitive fitness of V. alginolyticus.

Identification of the Regulon of AphB and Its Essential Roles in LuxR and Exotoxin Asp Expression in the Pathogen Vibrio alginolyticus

In Vibrio species, AphB is essential to activate virulence cascades by sensing low-pH and anaerobiosis signals; however, its regulon remains largely unknown. Here, AphB is found to be a key virulence regulator in Vibrio alginolyticus, a pathogen for marine animals and humans. Chromatin immunoprecipitation followed by high-throughput DNA sequencing (ChIP-seq) enabled the detection of 20 loci in the V. alginolyticus genome that contained AphB-binding peaks. An AphB-specific binding consensus was confirmed by electrophoretic mobility shift assays (EMSAs), and the regulation of genes flanking such binding sites was demonstrated using quantitative real-time PCR analysis. AphB binds directly to its own promoter and positively controls its own expression in later growth stages. AphB also activates the expression of the exotoxin Asp by binding directly to the promoter regions of asp and the master quorum-sensing (QS) regulator luxR DNase I footprinting analysis uncovered distinct AphB-binding sites (BBS) in these promoters. Furthermore, a BBS in the luxR promoter region overlaps that of LuxR-binding site I, which mediates the positive control of luxR promoter activity by AphB. This study provides new insights into the AphB regulon and reveals the mechanisms underlying AphB regulation of physiological adaptation and QS-controlled virulence in V. alginolyticusIMPORTANCE In this work, AphB is determined to play essential roles in the expression of genes associated with QS, physiology, and virulence in V. alginolyticus, a pathogen for marine animals and humans. AphB was found to bind directly to 20 genes and control their expression by a 17-bp consensus binding sequence. Among the 20 genes, the aphB gene itself was identified to be positively autoregulated, and AphB also positively controlled asp and luxR expression. Taken together, these findings improve our understanding of the roles of AphB in controlling physiological adaptation and QS-controlled virulence gene expression.

Chromatin Immunoprecipitation Sequencing Technology Reveals Global Regulatory Roles of Low-Cell-Density Quorum-Sensing Regulator AphA in the Pathogen Vibrio alginolyticus

Quorum sensing (QS) is an important regulatory system in virulence expression and environmental adaptation in bacteria. The master QS regulators (MQSR) LuxR and AphA reciprocally control QS gene expression in vibrios. However, the molecular basis for the regulatory functions of AphA remains undefined. In this study, we characterized its regulatory roles in Vibrio alginolyticus, an important zoonotic pathogen causing diseases in marine animals as well as in humans. AphA is involved in the motility ability, biofilm formation, and in vivo survival of V. alginolyticus Specifically, AphA is expressed at low-cell-density growth phases. In addition, AphA negatively regulates the expression of the main virulence factor, alkaline serine protease (Asp), through LuxR. Chromatin immunoprecipitation (ChIP) followed by high-throughput DNA sequencing (ChIP-seq) detected 49 enriched loci harboring AphA-binding peaks across the V. alginolyticus genome. An AphA-specific binding motif was identified and further confirmed by electrophoretic mobility shift assay (EMSA) and mutagenesis analysis. A quantitative real-time PCR (qRT-PCR) assay further validated the regulation of AphA on these genes. AphA binds directly to the aphA promoter and negatively regulates its own expression. Moreover, AphA directly regulates genes encoding adenylate cyclase, anti-σ^D, FabR, and the small RNA CsrB, revealing versatile regulatory roles of AphA in its physiology and virulence. Furthermore, our data indicated that AphA modulates motility through the coordinated function of LuxR and CsrB. Collectively, the findings of this work contribute to better understanding of the regulatory roles of AphA in QS and non-QS genes.

IMPORTANCE

In this work, we determined that AphA, the master regulator of QS at low cell density, plays essential roles in expression of genes associated with physiology and virulence in V. alginolyticus, a Gram-negative pathogen for humans and marine animals. We further uncovered that 49 genes could be directly regulated by AphA and a 19-bp consensus binding sequence was identified. Among the 49 genes, the QS and other non-QS-associated genes were identified to be regulated by AphA. Besides, the small RNA CsrB was negatively regulated by AphA, and AphA regulate motility abilities through both CsrB and LuxR. Taken together, the findings of this study improve our understanding of the complex regulation network of AphA and QS.

A σE-Mediated Temperature Gauge Controls a Switch from LuxR-Mediated Virulence Gene Expression to Thermal Stress Adaptation in Vibrio alginolyticus

In vibrios, the expression of virulence factors is often controlled by LuxR, the master quorum-sensing regulator. Here, we investigate the interplay between LuxR and σ^E, an alternative sigma factor, during the control of virulence-related gene expression and adaptations to temperature elevations in the zoonotic pathogen Vibrio alginolyticus. An rpoE null V. alginolyticus mutant was unable to adapt to various stresses and was survival-deficient in fish. In wild type V. alginolyticus, the expression of LuxR-regulated virulence factors increased as the temperature was increased from 22°C to 37°C, but mutants lacking σ^E did not respond to temperature, indicating that σ^E is critical for the temperature-dependent upregulation of virulence genes. Further analyses revealed that σ^E binds directly to -10 and -35 elements in the luxR promoter that drive its transcription. ChIP assays showed that σ^E binds to the promoter regions of luxR, rpoH and rpoE at high temperatures (e.g., 30°C and 37°C). However, at higher temperatures (42°C) that induce thermal stress, σ^E binding to the luxR promoter decreased, while its binding to the rpoH and rpoE promoters was unchanged. Thus, the temperature-dependent binding of σ^E to distinct promoters appears to underlie a σ^E-controlled switch between the expression of virulence genes and adaptation to thermal stress. This study illustrates how a conserved temperature response mechanism integrates into quorum-sensing circuits to regulate both virulence and stress adaptation.

Zoonotic Vibrio outbreaks are believed to be closely associated with increases in environmental temperature. The mechanisms underlying this phenomenon have not been defined. Here, we show that the expression of the V. alginolyticus exotoxin Asp and other quorum-sensing (QS)-regulated virulence factors are induced by increasing temperatures, with the maximum expression observed at approximately 37°C. σ^E plays an essential role in regulating the QS master regulator LuxR in response to temperature shifts by binding directly to the -10 and -35 regions of the luxR promoter to drive its transcription. However, at higher thermal stress temperatures, σ^E binding to the luxR promoter decreased, resulting in a reduction in luxR transcription. This change underlies a binomial switch mechanism that regulates σ^E-controlled virulence gene expression patterns. Furthermore, we found that anti-σ^E signaling was involved in this stress and virulence reciprocal switch. This study suggests that a common temperature response mechanism is integrated into QS circuits to regulate both virulence and adaptation in related Vibrio taxa.

Microbial Surface Colonization and Biofilm Development in Marine Environments

Biotic and abiotic surfaces in marine waters are rapidly colonized by microorganisms. Surface colonization and subsequent biofilm formation and development provide numerous advantages to these organisms and support critical ecological and biogeochemical functions in the changing marine environment. Microbial surface association also contributes to deleterious effects such as biofouling, biocorrosion, and the persistence and transmission of harmful or pathogenic microorganisms and their genetic determinants. The processes and mechanisms of colonization as well as key players among the surface-associated microbiota have been studied for several decades. Accumulating evidence indicates that specific cell-surface, cell-cell, and interpopulation interactions shape the composition, structure, spatiotemporal dynamics, and functions of surface-associated microbial communities. Several key microbial processes and mechanisms, including (i) surface, population, and community sensing and signaling, (ii) intraspecies and interspecies communication and interaction, and (iii) the regulatory balance between cooperation and competition, have been identified as critical for the microbial surface association lifestyle. In this review, recent progress in the study of marine microbial surface colonization and biofilm development is synthesized and discussed. Major gaps in our knowledge remain. We pose questions for targeted investigation of surface-specific community-level microbial features, answers to which would advance our understanding of surface-associated microbial community ecology and the biogeochemical functions of these communities at levels from molecular mechanistic details through systems biological integration.

Mechanisms of quorum sensing and strategies for quorum sensing disruption in aquaculture pathogens

In many countries, infectious diseases are a considerable threat to aquaculture. The pathogenicity of micro-organisms that infect aquaculture systems is closely related to the release of virulence factors and the formation of biofilms, both of which are regulated by quorum sensing (QS). Thus, QS disruption is a potential strategy for preventing disease in aquaculture systems. QS inhibitors (QSIs) not only inhibit the expression of virulence-associated genes but also attenuate the virulence of aquaculture pathogens. In this review, we discuss QS systems in important aquaculture pathogens and focus on the relationship between QS mechanisms and bacterial virulence in aquaculture. We further elucidate QS disruption strategies for targeting aquaculture pathogens. Four main types of QSIs that target aquaculture pathogens are discussed based on their mechanisms of action.

Growth inhibition of bacterial fish pathogens and quorum-sensing blocking by bacteria recovered from chilean salmonid farms

The main goal of this study was to find bacterial isolates with the ability to inhibit the growth of the fish pathogens Aeromonas hydrophila, Vibrio anguillarum, and Flavobacterium psychrophilum and to inhibit the blockage of the quorum-sensing (QS) system. A total of 80 gram-negative strains isolated from various freshwater Chilean salmonid farms were studied. We determined that 10 strains belonging to the genus Pseudomonas inhibited at least one of the assayed fish pathogens. Of these, nine strains were able to produce siderophores and two strains were able to inhibit the growth of all assayed pathogenic species. When the 80 strains were examined for QS-blocking activity, only the strains Pseudomonas sp. FF16 and Raoultella planticola R5B1 were identified as QS blockers. When the QS-blocker strains were analyzed for their ability to produce homoserine lactone (HSL) molecules, thin-layer chromatography analysis showed that both strains were able to produce C6-HSL- and C8-HSL-type molecules. Strain R5B1 did not show growth inhibition properties, but strain FF16 also led to inhibition of growth in A. hydrophila and F. psychrophilum as well as to siderophore production. Pseudomonas sp. FF16 exhibited potentially useful antagonistic properties and could be a probiotic candidate for the salmon farming industry.

Highly Potent, Chemically Stable Quorum Sensing Agonists for Vibrio Cholerae

In the Vibrio cholerae pathogen, initiation of bacterial quorum sensing pathways serves to suppress virulence. We describe herein a potent and chemically stable small molecule agonist of V. cholerae quorum sensing, which was identified through rational drug design based on the native quorum sensing signal. This novel agonist may serve as a useful lead compound for the control of virulence in V. cholerae.

Diversity of Vibrio spp. isolated at ambient environmental temperature in the Eastern English Channel as determined by pyrH sequencing

AIMS

To describe the diversity of the culturable mesophilic and potentially pathogenic vibrios isolated at 22 and 37°C on TCBS medium, in September 2009 from seawater and surface sediments.

METHODS AND RESULTS

q-PCR assays previously selected for the identification of bacterial strains isolated at 37°C were used in combination with the partial sequencing of two housekeeping genes, pyrH and toxR, to identify 315 strains isolated at 22°C. The great majority of the 37°C strains was identified by q-PCR assays, (five of the six species) with the predominance of Vibrio alginolyticus (85·9%) and V. harveyi (10·7%). The human pathogens V. parahaemolyticus and V. cholerae were rarely detected (two strains each). The 22°C strains were successfully identified by the phylogeny analysis of pyrH and toxR genes, revealing 20 Vibrio species, with the predominance of the clam pathogen V. celticus (36·8%). The Splendidus and the Harveyi groups represented the main Vibrio group at 22°C (80%) and 37°C (99·5%), respectively.

CONCLUSIONS

The combination of q-PCR assays and the sequencing of pyrH and toxR genes highlighted two different Vibrio communities at 22 and 37°C both dominated by pathogenic species for marine organisms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The sequencing of the pyrH gene revealed to be a valuable tool to identify environmental Vibrio spp. strains isolated at 22°C, as 92·3% of them were identified in this study.

Connecting type VI secretion, quorum sensing, and c-di-GMP production in fish pathogen Vibrio alginolyticus through phosphatase PppA

Vibrio alginolyticus, a Gram-negative marine bacterium, has brought about severe economic damage to the mariculture industry by causing vibriosis in various fish species. We are intrigued in the regulation of the pathogenesis in this bacterium. Here, we reported a complex regulatory connection among the newly defined type VI secretion system (T6SS), quorum sensing (QS), and 3',5'-cyclic diguanylic acid (c-di-GMP) signal through the phosphatase PppA encoded in the T6SS gene cluster of V. alginolyticus. Whole-genome transcriptome analysis revealed various regulatory targets of PppA including the T6SS substrate hemolysin coregulated protein (Hcp), quorum sensing regulator LuxR, exotoxin alkaline serine protease (Asp), flagellar proteins, as well as proteins involved in polysaccharide biosynthesis and transport. Western blot analysis showed PppA served as a negative regulator of the expression and secretion of Hcp1. Mutation of pppA resulted in an increased level of the intracellular second messenger c-di-GMP and a decreased expression of the QS regulator LuxR as well as exotoxin Asp. Complementation of intact pppA gene in ΔpppA mutant restored the production of c-di-GMP, LuxR, and Asp to the wild-type level. Phenotypic studies suggested that PppA takes part in the modulation of biofilm formation, motility, and cell aggregation. These results demonstrated new roles of PppA in controlling virulence factors and pleiotropic phenotypes and contributed to our understanding of the regulation of pathogenesis in V. alginolyticus.

Quorum-sensing regulates biofilm formation in Vibrio scophthalmi

BACKGROUND

In a previous study, we demonstrated that Vibrio scophthalmi, the most abundant Vibrio species among the marine aerobic or facultatively anaerobic bacteria inhabiting the intestinal tract of healthy cultured turbot (Scophthalmus maximus), contains at least two quorum-sensing circuits involving two types of signal molecules (a 3-hydroxy-dodecanoyl-homoserine lactone and the universal autoinducer 2 encoded by luxS). The purpose of this study was to investigate the functions regulated by these quorum sensing circuits in this vibrio by constructing mutants for the genes involved in these circuits.

RESULTS

The presence of a homologue to the Vibrio harveyi luxR gene encoding a main transcriptional regulator, whose expression is modulated by quorum-sensing signal molecules in other vibrios, was detected and sequenced. The V. scophthalmi LuxR protein displayed a maximum amino acid identity of 82% with SmcR, the LuxR homologue found in Vibrio vulnificus. luxR and luxS null mutants were constructed and their phenotype analysed. Both mutants displayed reduced biofilm formation in vitro as well as differences in membrane protein expression by mass-spectrometry analysis. Additionally, a recombinant strain of V. scophthalmi carrying the lactonase AiiA from Bacillus cereus, which causes hydrolysis of acyl homoserine lactones, was included in the study.

CONCLUSIONS

V. scophthalmi shares two quorum sensing circuits, including the main transcriptional regulator luxR, with some pathogenic vibrios such as V. harveyi and V. anguillarum. However, contrary to these pathogenic vibrios no virulence factors (such as protease production) were found to be quorum sensing regulated in this bacterium. Noteworthy, biofilm formation was altered in luxS and luxR mutants. In these mutants a different expression profile of membrane proteins were observed with respect to the wild type strain suggesting that quorum sensing could play a role in the regulation of the adhesion mechanisms of this bacterium.

Single cell analysis of Vibrio harveyi uncovers functional heterogeneity in response to quorum sensing signals

Background

Vibrio harveyi and closely related species are important pathogens in aquaculture. A complex quorum sensing cascade involving three autoinducers controls bioluminescence and several genes encoding virulence factors. Single cell analysis of a V. harveyi population has already indicated intercellular heterogeneity in the production of bioluminescence. This study was undertaken to analyze the expression of various autoinducer-dependent genes in individual cells.

Results

Here we used reporter strains bearing promoter::gfp fusions to monitor the induction/repression of three autoinducer-regulated genes in wild type conjugates at the single cell level. Two genes involved in pathogenesis - vhp and vscP, which code for an exoprotease and a component of the type III secretion system, respectively, and luxC (the first gene in the lux operon) were chosen for analysis. The lux operon and the exoprotease gene are induced, while vscP is repressed at high cell density. As controls luxS and recA, whose expression is not dependent on autoinducers, were examined. The responses of the promoter::gfp fusions in individual cells from the same culture ranged from no to high induction. Importantly, simultaneous analysis of two autoinducer induced phenotypes, bioluminescence (light detection) and exoproteolytic activity (fluorescence of a promoter::gfp fusion), in single cells provided evidence for functional heterogeneity within a V. harveyi population.

Conclusions

Autoinducers are not only an indicator for cell density, but play a pivotal role in the coordination of physiological activities within the population.

Interactions between diatoms and bacteria

Diatoms and bacteria have cooccurred in common habitats for hundreds of millions of years, thus fostering specific associations and interactions with global biogeochemical consequences. Diatoms are responsible for one-fifth of the photosynthesis on Earth, while bacteria remineralize a large portion of this fixed carbon in the oceans. Through their coexistence, diatoms and bacteria cycle nutrients between oxidized and reduced states, impacting bioavailability and ultimately feeding higher trophic levels. Here we present an overview of how diatoms and bacteria interact and the implications of these interactions. We emphasize that heterotrophic bacteria in the oceans that are consistently associated with diatoms are confined to two phyla. These consistent bacterial associations result from encounter mechanisms that occur within a microscale environment surrounding a diatom cell. We review signaling mechanisms that occur in this microenvironment to pave the way for specific interactions. Finally, we discuss known interactions between diatoms and bacteria and exciting new directions and research opportunities in this field. Throughout the review, we emphasize new technological advances that will help in the discovery of new interactions. Deciphering the languages of diatoms and bacteria and how they interact will inform our understanding of the role these organisms have in shaping the ocean and how these interactions may change in future oceans.

Broad spectrum pro-quorum-sensing molecules as inhibitors of virulence in vibrios

Quorum sensing (QS) is a bacterial cell-cell communication process that relies on the production and detection of extracellular signal molecules called autoinducers. QS allows bacteria to perform collective activities. Vibrio cholerae, a pathogen that causes an acute disease, uses QS to repress virulence factor production and biofilm formation. Thus, molecules that activate QS in V. cholerae have the potential to control pathogenicity in this globally important bacterium. Using a whole-cell high-throughput screen, we identified eleven molecules that activate V. cholerae QS: eight molecules are receptor agonists and three molecules are antagonists of LuxO, the central NtrC-type response regulator that controls the global V. cholerae QS cascade. The LuxO inhibitors act by an uncompetitive mechanism by binding to the pre-formed LuxO-ATP complex to inhibit ATP hydrolysis. Genetic analyses suggest that the inhibitors bind in close proximity to the Walker B motif. The inhibitors display broad-spectrum capability in activation of QS in Vibrio species that employ LuxO. To the best of our knowledge, these are the first molecules identified that inhibit the ATPase activity of a NtrC-type response regulator. Our discovery supports the idea that exploiting pro-QS molecules is a promising strategy for the development of novel anti-infectives.

Identification of qseEGF genetic locus and its roles in controlling hemolytic activity and invasion in fish pathogen Edwardsiella tarda

AIMS

The aims of this study were to reveal the roles of the gene locus qseEGF in the pathogenesis of Edwardsiella tarda.

METHODS AND RESULTS

Genome sequencing of fish pathogen E. tarda EIB202 reveals that the gene locus qseEGF, which encodes a novel two-component system QseEF, were located in E. tarda. The transcription of qseE, qseF and qseG was firstly characterized to be cotranscribed by reverse-transcribed PCR (RT-PCR). The mutant strains ΔqseE, ΔqseF and ΔqseG were constructed with in-frame deletion strategy. Compared with the wild type, all of the mutants showed attenuated virulence and impaired intracellular survival capabilities. Deletion in qseE, qseF and qseG resulted in different effects on hemolysin production in E. tarda. qRT-PCR results indicated that QseEF played a role in regulation of secretion systems, which in turn affected the virulence of E. tarda.

CONCLUSIONS

The results manifested that QseEF system affected the virulence in E. tarda EIB202 by controlling the secretion system and hemolysin production. QseE, QseG and QseF in E. tarda serve for the physiological fitness and pathogenesis related to the bacterial survival in macrophage and in vivo of fish.

SIGNIFICANCE AND IMPACT

The present results suggested that the important role of two-component system QseEF in regulation of E. tarda pathogenesis and its potential for attenuated live vaccine construction.

Characterization of a new quorum sensing regulator luxT and its roles in the extracellular protease production, motility, and virulence in fish pathogen Vibrio alginolyticus

Vibrio alginolyticus, an opportunistic pathogen that causes vibriosis in miscellaneous fish species, has brought about serious economic damage to the mariculture industry in South China. The mechanism of virulence regulation in V. alginolyticus is yet not known except a Vibrio harveyi-like quorum sensing (QS) system that is established to manipulate the expression of diverse genes including those encoding virulence determinants. In this study, a new TetR family QS regulator, luxT, was identified and characterized in V. alginolyticus. The transcription of luxT gene was cell density dependent and was positively regulated by LuxU, an established QS component relaying the signal from three paralleled QS regulatory systems in V. harveyi. In addition, luxT positively regulated both luxO at transcriptional level and luxR at post-transcriptional level, which is thoroughly different from the established QS regulation mode in V. harveyi and Vibrio vulnificus. The mutant of luxT deletion produced markedly decreased total extracellular proteases and reduced motility ability compared to the wild type and the complemented strain luxT (+). The fish infection results indicated that mutation of luxT led to marginal attenuation in the virulence of V. alginolyticus, suggesting that LuxT might play a role in the fine-tuning of the virulence via QS in V. alginolyticus.

Modulation of iron-uptake systems by a mutation of luxS encoding an autoinducer-2 synthase in Vibrio vulnificus

Vibrio vulnificus possesses multiple iron-uptake systems which are mediated by VuuA (vulnibactin receptor), IutA (aerobactin receptor) and HupA (heme receptor). In this study, we determined the effect of a mutation of luxS encoding autoinducer-2 (AI-2) synthase on the expressions of the three receptors. A mutation and an in trans complementation of luxS did not affect the growing ability of V. vulnificus in iron-deficient conditions. Nevertheless, the luxS mutation slightly decreased vuuA expression, but slightly increased iutA and hupA expressions in the transcriptional reporter assay or Western blot analysis. These changes were all recovered by the luxS complementation. These results suggest that AI-2-mediated quorum sensing system may be involved in the fine modulation of V. vulnificus iron-uptake systems, positively affecting vuuA expression but negatively affecting iutA and hupA expressions.

Disruption of bacterial cell-to-cell communication by marine organisms and its relevance to aquaculture

Bacterial disease is one of the most critical problems in commercial aquaculture. Although various methods and treatments have been developed to curb the problem, yet they still have significant drawbacks. A novel and environmental-friendly approach in solving this problem is through the disruption of bacterial communication or quorum sensing (QS). In this communication scheme, bacteria regulate their own gene expression by producing, releasing, and sensing chemical signals from the environment. There seems to be a link between QS and diseases through the regulation of certain phenotypes and the induction of virulence factors responsible for pathogen-host association. Several findings have reported that numerous aquatic organisms such as micro-algae, macro-algae, invertebrates, or even other bacteria have the potential to disrupt QS. The mechanism of action varies from degradation of signals through enzymatic or chemical inactivation to antagonistic as well as agonistic activities. This review focuses on the existing marine organisms that are able to interfere with QS with potential application for aquaculture as bacterial control.

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.

Theoretical and structural analysis of the active site of the transcriptional regulators LasR and TraR, using molecular docking methodology for identifying potential analogues of acyl homoserine lactones (AHLs) with anti-quorum sensing activity

In the present study the homology of transcriptional receptors LuxR type were evaluated using as point of reference the receptors TraR and LasR of the bacterial types Agrobacterium tumefaciens and Pseudomonas aureginosa respectively. A series of alignments were performed in order to demonstrate that the active site of the protein is conserved in wide range of gram negative bacteria. Moreover, some docking calculations were carried out for analogs of the acyl homoserin lactones (AHLs) and regulatory proteins LasR and TraR, to understand the complex microenvironment in which the ligands are exposed. The molecular alignments show clearly that there are preserved motifs in the residues (Y53, Y61, W57, D70, W85 to TraR, Y56, Y64, W60, D73, W88 to LasR) analyzed, which may serve as site-specific targets for the development of potential antagonists. In this study was found that the anti-quorum sensing activity of the AHLs molecular analogs appears to depend on; the structure of the lactone ring and on appropriate combination of absolute and relative stereochemistry of the carbonyl (C=O) and amide (NH(2)) groups of the side chain of these AHLs molecular analogs, in combination with the interactions with the conserved amino acids (D73, W60, Y56, S129 to LasR and D70, W57, Y53 to TraR) of the LuxR type protein family.

Role of RpoS in stress survival, synthesis of extracellular autoinducer 2, and virulence in Vibrio alginolyticus

Vibrio alginolyticus, a marine bacterium, is an opportunistic pathogen capable of causing vibriosis with high mortality to fishes in the South China Sea. Stress resistance is very important for its survival in the natural environment and upon infection of the host. RpoS, an alternative sigma factor, is considered as an important regulator involved in stress response and virulence in many pathogens. In this study, the rpoS gene was cloned and characterized to evaluate the role of RpoS in V. alginolyticus. The predicted protein showed high identity with other reported rpoS gene products. The in-frame deleted mutation of rpoS in V. alginolyticus led to sensitivity of the strain to ethanol, hyperosmolarity, heat, and hydrogen peroxide challenges. Further studies showed that extracellular autoinducer 2 level, four of seven detected protease activities, and cytotoxicity of extracellular products were markedly decreased in the rpoS mutant compared with that in the wild-type strain. The results indicated that the global regulator RpoS was part of the regulatory networks of virulence and LuxS quorum sensing system.