The extracellular proteases produced by Vibrio parahaemolyticus

Environmental magnesium ion affects global gene expression, motility, biofilm formation and virulence of Vibrio parahaemolyticus

Vibrio parahaemolyticus is widely distributed in marine ecosystems. Magnesium ion (Mg2+) is the second most abundant metal cation in seawater, and plays important roles in the growth and gene expression of V. parahaemolyticus, but lacks the detailed mechanisms. In this study, the RNA sequencing data demonstrated that a total of 1494 genes was significantly regulated by Mg2+. The majority of the genes associated with lateral flagella, exopolysaccharide, type III secretion system 2, type VI secretion system (T6SS) 1, T6SS2, and thermostable direct hemolysin were downregulated. A total of 18 genes that may be involved in c-di-GMP metabolism and more than 80 genes encoding putative regulators were also significantly and differentially expressed in response to Mg2+, indicating that the adaptation process to Mg2+ stress may be strictly regulated by complex regulatory networks. In addition, Mg2+ promoted the proliferative speed, swimming motility and cell adhesion of V. parahaemolyticus, but inhibited the swarming motility, biofilm formation, and c-di-GMP production. However, Mg2+ had no effect on the production of capsular polysaccharide and cytoxicity against HeLa cells. Therefore, Mg2+ had a comprehensive impact on the physiology and gene expression of V. parahaemolyticus.

Unveiling the positive impact of biofloc culture on Vibrio parahaemolyticus infection of Pacific white shrimp by reducing quorum sensing and virulence gene expression and enhancing immunity

This study aimed to evaluate and unveil the positive impact of biofloc culture on Vibrio parahaemolyticus infection of Pacific white shrimp by reducing quorum sensing (QS) and virulence gene expression and enhancing shrimp's immunity. The shrimp with an average body weight of 0.50 ± 0.09 g were reared in containers with a volume of 2.5 L, 21 units, and a density of 20 shrimp L-1. The shrimp were cultured for 5 days, with each treatment including biofloc system maintenance with a C/N ratio of 10 and a control treatment without biofloc, followed by a challenge test through immersion using V. parahaemolyticus at densities of 103, 105, and 107 CFU mL-1 initially. The results of the in vitro experiment showed that biofloc suspension can inhibit and disperse biofilm formation, as well as reduce the exo-enzyme activity (amylase, protease, and chitinase) of V. parahaemolyticus. Furthermore, the biofloc treatment significantly reduced the expression of the QS regulatory gene OpaR, the PirB toxin gene, and the virulence factor genes T6SS1 and T6SS2 in both in vitro and in vivo. The biofloc system also increased the expression of shrimp immunity-related genes (LGBP, proPO, SP, and PE) and the survival rate of white shrimp challenged with V. parahaemolyticus.

The effect of environmental calcium on gene expression, biofilm formation and virulence of Vibrio parahaemolyticus

Calcium (Ca2+) can regulate the swarming motility and virulence of Vibrio parahaemolyticus BB22. However, the effects of Ca2+ on the physiology of V. parahaemolyticus RIMD2210633, whose genomic composition is quite different with that of BB22, have not been investigated. In this study, the results of phenotypic assays showed that the biofilm formation, c-di-GMP production, swimming motility, zebrafish survival rate, cytoxicity against HeLa cells, and adherence activity to HeLa cells of V. parahaemolyticus RIMD2210633 were significantly enhanced by Ca2+. However, Ca2+ had no effect on the growth, swarming motility, capsular polysaccharide (CPS) phase variation and hemolytic activity. The RNA sequencing (RNA-seq) assay disclosed 459 significantly differentially expressed genes (DEGs) in response to Ca2+, including biofilm formation-associated genes and those encode virulence factors and putative regulators. DEGs involved in polar flagellum and T3SS1 were upregulated, whereas majority of those involved in regulatory functions and c-di-GMP metabolism were downregulated. The work helps us understand how Ca2+ affects the behavior and gene expression of V. parahaemolyticus RIMD2210633.

QsvR and OpaR coordinately regulate the transcription of cpsS and cpsR in Vibrio parahaemolyticus

Vibrio parahaemolyticus, the leading cause of seafood-associated gastroenteritis, has a strong capacity to form biofilms on surfaces, which is strictly regulated by the CpsS-CpsR-CpsQ regulatory cascade. OpaR, a master regulator of quorum sensing, is a global regulator that controls multiple cellular pathways including biofilm formation and virulence. QsvR is an AraC-type regulator that works coordinately with OpaR to control biofilm formation and virulence gene expression of V. parahaemolyticus. QsvR and OpaR activate cpsQ transcription. OpaR also activates cpsR transcription, but lacks the detailed regulatory mechanisms. Furthermore, it is still unknown whether QsvR regulates cpsR transcription, as well as whether QsvR and OpaR regulate cpsS transcription. In this study, the results of quantitative real-time PCR and LacZ fusion assays demonstrated that deletion of qsvR and/or opaR significantly decreased the expression levels of cpsS and cpsR compared to the wild-type strain. However, the results of two-plasmid lacZ reporter and electrophoretic mobility-shift assays showed that both QsvR and OpaR were unable to bind the regulatory DNA regions of cpsS and cpsR. Therefore, transcription of cpsS and cpsR was coordinately and indirectly activated by QsvR and OpaR. This work enriched our knowledge on the regulatory network of biofilm formation in V. parahaemolyticus.

Genomic and Proteomic Analyses of Extracellular Products Reveal Major Virulence Factors Likely Accounting for Differences in Pathogenicity to Bivalves between Vibrio mediterranei Strains

Vibrio mediterranei, a bacterial pathogen of bivalves, has exhibited strain-dependent virulence. The mechanisms behind the variations in bivalve pathogenicity between V. mediterranei strains have remained unclear. However, a preliminary analysis of the extracellular product (ECP) proteomes has revealed differences in protein compositions between low- and high-virulence strains; in addition to 1265 shared proteins, 127 proteins have been identified to be specific to one low-virulence strain and 95 proteins to be specific to two high-virulence strains. We further studied the ECP proteins of the three V. mediterranei strains from functional perspectives using integrated genomics and proteomics approaches. The results showed that lipid metabolism, transporter activity and membrane transporter pathways were more enriched in the ECPs of the two high-virulence strains than in those of the low-virulence strain. Additionally, 73 of the 95 high-virulence strain-specific proteins were found to have coding genes in the genome but were not expressed in the low-virulence strain. Moreover, comparisons with known virulence factors in the Virulence Factor Database (VFDB) and the Pathogen-Host Interactions Database (PHI-base) allowed us to predict more than 10 virulence factors in the categories of antimicrobial activity/competitive advantage, the effector delivery system and immune modulation, and the high-virulence strain-specific ECP proteins consisted of a greater percentage of known virulence factors than the low-virulence strain. Particularly, two virulence factors, MtrC and KatG, were identified in the ECPs of the two high-virulence strains but not in those of the low-virulence strain. Most coding genes of the ECP proteins including known virulence factors were identified on chromosome 1 of V. mediterranei. Our findings indicate that variations in virulence factor composition in the bacterial ECPs may partially account for the differences in the bivalve pathogenicity between V. mediterranei strains.

Bovine Lactoferrin and Hen Ovotransferrin Affect Virulence Factors of Acute Hepatopancreatic Necrosis Disease (AHPND)-Inducing Vibrio parahaemolyticus Strains

Acute Hepatopancreatic Necrosis Disease (AHPND), a highly destructive shrimp disease, has inflicted severe setbacks on the shrimp farming industry worldwide. As the use of antibiotics is discouraged due to emerging antibiotic-resistant bacteria and the pollution of ecosystems, there is a pressing demand for novel, sustainable alternatives. Hence, the influence of bovine lactoferrin (bLF) and hen ovotransferrin (OT), two natural antimicrobial proteins, on the growth of three AHPND-causing Vibrio parahaemolyticus (Vp) strains (M0904, TW01 and PV1) was examined. Additionally, we explored their potential to affect selected Vp virulence factors such as biofilm formation, swimming and swarming, cell surface hydrophobicity, and activity of released lipases and caseinases. Lag phases of all bacterial growth curves were significantly prolonged in the presence of bLF or OT (1, 5 and 10 mg/mL), and bLF (5 and 10 mg/mL) completely inhibited growth of all strains. In addition, bLF or OT significantly reduced biofilm formation (all tested bLF and OT concentrations for Vp M0904 and Vp PV1), bacterial swimming motility (0.5 mg/mL bLF and OT for Vp M0904 and Vp TW01; 1 mg/mL bLF and OT for all strains), cell surface hydrophobicity (for all strains, all bLF and OT concentrations tested except for 0.125 mg/mL OT for Vp PV1) and lipase activity (1 mg/mL bLF and OT for all strains and 0.5 mg/mL bLF and OT for Vp PV1). These promising in vitro results suggest that bLF and/or OT might be used as novel agents for combating AHPND and warrant further research to elucidate the underlying mechanisms of action to fully unlock their potential for AHPND disease management.

The phase variation between wrinkly and smooth colony phenotype affects the virulence of Vibrio parahaemolyticus

Vibrio parahaemolyticus, the causative agent of seafood-associated gastroenteritis, undergoes wrinkly and smooth colony switching on the plate. The wrinkly spreader grew faster, had stronger motility and biofilm capacity when compared with the smooth one. However, whether the two phenotypes differ in their virulence still needs to be further investigated. In this study, the data showed that the smooth spreader had stronger virulence phenotypes, including the cytotoxicity against HeLa cells, antibacterial activity against E. coli, adhesive capacity toward HeLa cells, and lethality in zebrafish, relative to the wrinkly one. However, the colony morphology variation had no influence on the haemolytic activity. The mRNA levels of major virulence genes including T3SS1, T6SS1, and T6SS2 were significantly enhanced in the smooth colonies relative to those in the wrinkly colonies. Taken together, the presented work highlighted the different virulence profiles of the wrinkly and smooth colony phenotypes.

The Pleiotropic Phenotypes Caused by an hfq Null Mutation in Vibrio harveyi

Hfq is a global regulator and can be involved in multiple cellular processes by assisting small regulatory RNAs (sRNAs) to target mRNAs. To gain insight into the virulence regulation of Hfq in Vibrio harveyi, the hfq null mutant, ∆hfq, was constructed in V. harveyi strain 345. Compared with the wild-type strain, the mortality of pearl gentian sharply declined from 80% to 0% in ∆hfq when infected with a dose that was 7.5-fold the median lethal dose (LD50). Additionally, ∆hfq led to impairments of bacterial growth, motility, and biofilm formation and resistance to reactive oxygen species, chloramphenicol, and florfenicol. A transcriptome analysis indicated that the expression of 16.39% genes on V. harveyi 345 were significantly changed after the deletion of hfq. Without Hfq, the virulence-related pathways, including flagellar assembly and bacterial chemotaxis, were repressed. Moreover, eleven sRNAs, including sRNA0405, sRNA0078, sRNA0419, sRNA0145, and sRNA0097, which, respectively, are involved in chloramphenicol/florfenicol resistance, outer membrane protein synthesis, electron transport, amino acid metabolism, and biofilm formation, were significantly down-regulated. In general, Hfq contributes to the virulence of V. harveyi 345 probably via positively regulating bacterial motility and biofilm formation. It is involved in flagellar assembly and bacterial chemotaxis by binding sRNAs and regulating the target mRNAs.

Transcriptomic Profiles of Vibrio parahaemolyticus During Biofilm Formation

Vibrio parahaemolyticus, the leading cause of bacterial seafood-associated gastroenteritis, can form biofilms. In this work, the gene expression profiles of V. parahaemolyticus during biofilm formation were investigated by transcriptome sequencing. A total of 183, 503, and 729 genes were significantly differentially expressed in the bacterial cells at 12, 24 and 48 h, respectively, compared with that at 6 h. Of these, 92 genes were consistently activated or repressed from 6 to 48 h. The genes involved in polar flagellum, chemotaxis, mannose-sensitive haemagglutinin type IV pili, capsular polysaccharide, type III secretion system 1 (T3SS1), T3SS2, thermostable direct hemolysin (TDH), type VI secretion system 1 (T6SS1) and T6SS2 were downregulated, whereas those involved in V. parahaemolyticus pathogenicity island (Vp-PAI) (except for T3SS2 and TDH) and membrane fusion proteins were upregulated. Three extracellular protease genes (vppC, prtA and VPA1071) and a dozen of outer membrane protein encoding genes were also significantly differentially expressed during biofilm formation. In addition, five putative c-di-GMP metabolism-associated genes were significantly differentially expressed, which may account for the drop in c-di-GMP levels after the beginning of biofilm formation. Moreover, many putative regulatory genes were significantly differentially expressed, and more than 1000 putative small non-coding RNAs were detected, suggesting that biofilm formation was tightly regulated by complex regulatory networks. The data provided a global view of gene expression profiles during biofilm formation, showing that the significantly differentially expressed genes were involved in multiple cellular pathways, including virulence, biofilm formation, metabolism, and regulation.

L-arabinose affects the growth, biofilm formation, motility, c-di-GMP metabolism, and global gene expression of Vibrio parahaemolyticus

The L-arabinose inducible pBAD vectors are commonly used to turn on and off the expression of specific genes in bacteria. The utilization of certain carbohydrates can influence bacterial growth, virulence factor production, and biofilm formation. Vibrio parahaemolyticus, the causative agent of seafood-associated gastroenteritis, can grow in media with L-arabinose as the sole carbon source. However, the effects of L-arabinose on V. parahaemolyticus physiology have not been investigated. In this study, we show that the growth rate, biofilm formation capacity, capsular polysaccharide production, motility, and c-di-GMP production of V. parahaemolyticus are negatively affected by L-arabinose. RNA-seq data revealed significant changes in the expression levels of 752 genes, accounting for approximately 15.6% of V. parahaemolyticus genes in the presence of L-arabinose. The affected genes included those associated with L-arabinose utilization, major virulence genes, known key biofilm-related genes, and numerous regulatory genes. In the majority of type III secretion system, two genes were upregulated in the presence of L-arabinose, whereas in those of type VI secretion system, two genes were downregulated. Ten putative c-di-GMP metabolism-associated genes were also significantly differentially expressed, which may account for the reduced c-di-GMP levels in the presence of L-arabinose. Most importantly, almost 40 putative regulators were significantly differentially expressed due to the induction by L-arabinose, indicating that the utilization of L-arabinose is strictly regulated by regulatory networks in V. parahaemolyticus. The findings increase the understanding of how L-arabinose affects the physiology of V. parahaemolyticus. Researchers should use caution when considering the use of L-arabinose inducible pBAD vectors in V. parahaemolyticus. IMPORTANCE The data in this study show that L-arabinose negatively affects the growth rate, biofilm formation, capsular polysaccharide production, motility, and c-di-GMP production of V. parahaemolyticus. The data also clarify the gene expression profiles of the bacterium in the presence of L-arabinose. Significantly differentially expressed genes in response to L-arabinose were involved in multiple cellular pathways, including L-arabinose utilization, virulence factor production, biofilm formation, motility, adaptation, and regulation. The collective findings indicate the significant impact of L-arabinose on the physiology of V. parahaemolyticus. There may be similar effects on other species of bacteria. Necessary controls should be established when pBAD vectors must be used for ectopic gene expression.

Effect of sublethal dose of chloramphenicol on biofilm formation and virulence in Vibrio parahaemolyticus

Vibrio parahaemolyticus isolates are generally very sensitive to chloramphenicol. However, it is usually necessary to transfer a plasmid carrying a chloramphenicol resistance gene into V. parahaemolyticus to investigate the function of a specific gene, and the effects of chloramphenicol on bacterial physiology have not been investigated. In this work, the effects of sublethal dose of chloramphenicol on V. parahaemolyticus were investigated by combined utilization of various phenotypic assays and RNA sequencing (RNA-seq). The results showed that the growth rate, biofilm formation capcity, c-di-GMP synthesis, motility, cytoxicity and adherence activity of V. parahaemolyticus were remarkably downregulated by the sublethal dose of chloramphenicol. The RNA-seq data revealed that the expression levels of 650 genes were significantly differentially expressed in the response to chloramphenicol stress, including antibiotic resistance genes, major virulence genes, biofilm-associated genes and putative regulatory genes. Majority of genes involved in the synthesis of polar flagellum, exopolysaccharide (EPS), mannose-sensitive haemagglutinin type IV pilus (MSHA), type III secretion systems (T3SS1 and T3SS2) and type VI secretion system 2 (T6SS2) were downregulated by the sublethal dose of chloramphenicol. Five putative c-di-GMP metabolism genes were significantly differentially expressed, which may be the reason for the decrease in intracellular c-di-GMP levels in the response of chloramphenicol stress. In addition, 23 genes encoding putative regulators were also significantly differentially expressed, suggesting that these regulators may be involved in the resistance of V. parahaemolyticus to chloramphenicol stress. This work helps us to understand how chloramphenicol effect on the physiology of V. parahaemolyticus.

Evaluating the efficacy of endolysins and membrane permeabilizers against Vibrio parahaemolyticus in marine conditions

Endolysins have garnered significant attention as a potential alternative to antibiotics in aquaculture, mainly for combating Vibrio spp., Gram-negative pathogens responsible for infectious outbreaks. However, endolysin effectiveness against Gram-negative bacteria is limited due to the outer membrane's poor permeability. The combat against marine pathogens poses an additional challenge of finding endolysins that retain their activity in high ionic strength conditions. Thus, this study aimed to demonstrate that certain endolysins retain muralytic activity in seawater and also evaluated outer membrane permeabilizers as endolysin adjuvants. The effectiveness of KZ144 and LysPA26 endolysins, along with EDTA and oregano essential oil, was evaluated against Vibrio parahaemolyticus ATCC-17802 in natural seawater. Results revealed the muralytic activity of both endolysins in seawater. However, the endolysins appeared to counteract the permeabilizers' effect during the initial bactericidal assays. Further investigations revealed that the observed effect was not antagonistic. After the permeabilizer action, V. parahaemolyticus likely used endolysins as a growth substrate. Endolysins may not play an indifferent role if they fail to exert a bactericidal effect. Instead, they can serve as a substrate for fast-growing bacteria, such as V. parahaemolyticus, increasing bacterial density. It should be considered a potential drawback of endolysins' proteinaceous nature as bactericidal agents.

Antibacterial and Antibiofilm Efficacy and Mechanism of Ginger (Zingiber officinale) Essential Oil against Shewanella putrefaciens

Ginger (Zingiber officinale) has unique medicinal value and can be used to treat colds and cold-related diseases. The chemical composition and antibacterial activity of ginger essential oil (GEO) against Shewanella putrefaciens were determined in the present study. Zingiberene, α-curcumene, and zingerone were the main active compounds of GEO. GEO displayed significant antibacterial activity against S. putrefaciens, with a minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 2.0 and 4.0 μL/mL, respectively. Changes in intracellular ATP content, nucleic acid and protein structure, exopolysaccharides (EPS) content, and extracellular protease production indicated that GEO disrupted the membrane integrity of S. putrescens. At the same time, changes in biofilm metabolic activity content and the growth curve of biofilm showed that GEO could destroy the biofilm. Both scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) observations confirmed that GEO destroyed the cell membrane and lead to the leakage of the constituents. The above results indicate that GEO entered the cells via contact with bacterial membranes, and then inhibited the growth of S. putrefaciens and its biofilms by increasing membrane permeability and inhibiting various virulence factors such as EPS. The findings showed that GEO could destroy the structure of cell membrane and biofilm of tested S. putrefaciens, indicating its potential as a natural food preservative.

Transcriptomic Analysis of Vibrio parahaemolyticus Underlying the Wrinkly and Smooth Phenotypes

Vibrio parahaemolyticus, a causative agent of seafood-associated gastroenteritis, undergoes opaque-translucent (OP-TR) colony switching associated with capsular polysaccharide (CPS) production. Here, we showed that V. parahaemolyticus was also able to naturally and reversibly switch between wrinkly and smooth phenotypes. More than 1,000 genes were significantly differentially expressed during colony morphology switching, including the major virulence gene loci and key biofilm-related genes. The genes responsible for type III secretion system 1 (T3SS1), type VI secretion systems (T6SS1 and T6SS2), and flagellar synthesis were downregulated in the wrinkly spreader phenotype, whereas genes located on the pathogenicity island Vp-PAI and those responsible for chitin-regulated pili (ChiRP) and Syp exopolysaccharide synthesis were upregulated. In addition, we showed that the wrinkly spreader grew faster, had greater motility and biofilm capacities, and produced more c-di-GMP than the smooth type. A dozen genes potentially associated with c-di-GMP metabolism were shown to be significantly differentially expressed, which may account for the differences in c-di-GMP levels between the two phenotypes. Most importantly, dozens of putative regulators were significantly differentially expressed, and hundreds of noncoding RNAs were detected during colony morphology switching, indicating that phenotype switching is strictly regulated by a complex molecular regulatory network in V. parahaemolyticus. Taken together, the presented work highlighted the gene expression profiles related to wrinkly-smooth switching, showing that the significantly differentially expressed genes were involved in various biological behaviors, including virulence factor production, biofilm formation, metabolism, adaptation, and colonization. IMPORTANCE We showed that Vibrio parahaemolyticus was able to naturally and reversibly switch between wrinkly and smooth phenotypes and disclosed the gene expression profiles related to wrinkly-smooth switching, showing that the significantly differentially expressed genes between the two colony morphology phenotypes were involved in various biological behaviors, including virulence factor production, biofilm formation, metabolism, adaptation, and colonization.

Characterization of Vibrio parahaemolyticus isolated from stool specimens of diarrhea patients in Nantong, Jiangsu, China during 2018–2020

Vibrio parahaemolyticus is the leading cause of acute seafood-associated gastroenteritis worldwide. The aim of this study was to investigate the presence of virulence genes, biofilm formation, motor capacities and antimicrobial resistance profile of V. parahaemolyticus isolates isolated from clinical samples in Nantong during 2018–2020. Sixty-six V. parahaemolyticus strains isolated from stool specimens of diarrheal patients were examined. The PCR results showed that there were two tdh+trh+ isolates, four tdh-trh- isolates and sixty tdh+trh- isolates, accounting for 3.0%, 6.1% and 90.9%, respectively. All the tdh carrying isolates manifested the positive reactions for the Kanagawa phenomenon (KP) test. Most of the isolates harbored at least one of the specific DNA markers of ‘pandemic group’ strains, suggesting that the dominant isolates of V. parahaemolyticus in Nantong might belong to the new O3: K6 or its serovariants. All tdh+ isolates possessed the Vp-PAI genes, but no tdh-trh- isolates carried the T3SS2 genes. All isolates were biofilm producers and had relatively strong motor capacities. In addition, the V. parahaemolyticus isolates were resistant to ampicillin (98.5%), cefuroxime (75.6%), cefepime (66.7%), piperacillin (59.1%) and ampicillin/sulbactam (50.0%), but sensitive to ciprofloxacin (100.0%), levofloxacin (100.0%), trimethoprim-sulfamethoxazole (98.5%), gentamicin (98.5%), amikacin (97%), meropenem (71.2%), and ceftazidime (56.1%). Multidrug-resistant isolates in clinical might be related to the inappropriate use of antimicrobials in aquaculture.

Antagonistic activity and mode of action of trypacidin from marine-derived Aspergillus fumigatus against Vibrio parahaemolyticus

This study aimed to investigate the antagonistic activity and mode of action of trypacidin from marine-derived Aspergillus fumigatus against Vibrio parahaemolyticus. Results indicated that the minimal inhibitory concentration and minimal bactericidal concentration of trypacidin against V. parahaemolyticus were 31.25 and 62.5 μg/mL, respectively, which was better than that of streptomycin sulfate. Trypacidin remarkably inhibited the growth of V. parahaemolyticus and had a strong destructive effect on cell wall permeability and integrity, cell membrane permeability, and morphological alterations. Its potential as an antibacterial agent for aquatic products must be further explored.

The role of dctP gene in regulating colonization, adhesion and pathogenicity of Vibrio alginolyticus strain HY9901

Vibriosis caused by Vibrio alginolyticus has severely affected the development of mariculture industry in recent decades. DctP, a tripartite ATP-independent periplasmic transporter solute-binding subunit, is thought to be one of the virulence factors in Vibrio. In this study, the results displayed no difference in morphological characteristics and growth between ΔdctP (dctP mutant strain) and WT (wild-type strain). Nevertheless, the ability of swarming motility, biofilm formation, ECPase formation, cell adhesion and colonized ability of ΔdctP significantly decreased compared to those of WT. The LD₅₀ of ΔdctP significantly increased by 40-fold compared to that of WT. The transcriptome analysis demonstrated the deletion mutation of dctP could regulate the expression levels of 22 genes related to colonization, adhesion and pathogenicity in V. alginolyticus. The analysis of qRT-PCR showed the transcriptome data were reliable. These results reveal the effect of attenuated function of DctP on colonization, adherence and pathogenicity by controlling the expression of related gene.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

The Effect of Salinity on Biofilm Formation and c-di-GMP Production in Vibrio parahaemolyticus

Vibrio parahaemolyticus is a moderately halophilic, salt-requiring organism that exhibits optimal growth at approximately 3% salt. Thus, salinity stress is one of the most important stimuli during its lifecycle. The bacterium possesses a strong ability to form biofilms on surfaces, which are thought to be involved in protecting it from adverse environmental conditions. In the present study, salinity-dependent biofilm formation by V. parahaemolyticus was investigated by combining crystal violet staining, colony morphology, intracellular c-di-GMP quantification and quantitative PCR. The results showed that biofilm formation by V. parahaemolyticus was significantly enhanced in low salinity growth conditions and was affected by incubation time. In addition, low salinity reduced intracellular c-di-GMP degradation in V. parahaemolyticus. Transcription of genes encoding ScrABC and ScrG proteins, which are involved in intracellular c-di-GMP metabolism, was inhibited by low salinity growth conditions. Thus, reduced intracellular c-di-GMP degradation in V. parahaemolyticus in low salinity growth conditions may be mediated by repression of scrG and scrABC transcription. Taken together, these results demonstrated for the first time that salinity regulates biofilm formation and c-di-GMP production in V. parahaemolyticus.

LuxS modulates motility and secretion of extracellular protease in fish pathogen Vibrio harveyi

In this study, an in-frame deletion of the luxS gene was constructed to reveal the role of LuxS in the physiology and virulence of V. harveyi. The statistical analysis showed no significant differences in the growth ability, biofilm formation, antibiotic susceptibility, virulence by intraperitoneal injection, and the ability of V. harveyi to colonize the spleen and liver of the pearl gentian grouper between the wild-type (WT) and the luxS mutant. However, the deletion of luxS decreased the secretion of extracellular protease, while increased the ability of swimming and swarming. Simultaneously, a luxS-deleted mutant showed overproduction of lateral flagella, and an intact luxS complemented the defect. Since motility is flagella dependent, 16 of V. harveyi flagella biogenesis related genes were selected for further analysis. Based on quantitative real-time reverse transcription-PCR (qRT-PCR), the expression levels of these genes, including the polar flagella genes flaB, flhA, flhF, flhB, flhF, fliS, and flrA and the lateral flagella genes flgA, flgB, fliE, fliF, lafA, lafK, and motY, were significantly up-regulated in the ΔluxS: pMMB207 (ΔluxS+) strain as compared with the V. harveyi 345: pMMB207 (WT+) and C-ΔluxS strains during the early, mid-exponential, and stationary growth phase.

ToxR modulates biofilm formation in fish pathogen Vibrio harveyi

Vibrio harveyi is a common aquaculture pathogen causing diseases in a variety of aquatic animals. toxR, a conserved virulence-associated gene in vibrios, is identified in V. harveyi 345, a pathogenic strain isolated from diseased fish. In this study, to gain insight into function of ToxR in V. harveyi, an in-frame deletion of the toxR gene was constructed to reveal the role of ToxR in the physiology and virulence of V. harveyi. The statistical analysis showed no significant differences in the growth ability, motility, extracellular protease secretion, antibiotic susceptibility, virulence by intraperitoneal injection and the ability of V. harveyi to colonize the spleen and liver tissues of the pearl gentian grouper between the wild-type (WT) and the toxR mutant. However, the deletion of toxR increased the biofilm formation. The structure of the V. harveyi biofilm was further analysed by using scanning electron microscopy (SEM) and confocal laser scanning microscopy, and the results showed that deletion of toxR increased the number and density of V. harveyi biofilm. Since biofilm production is flagella, exopolysaccharide (EPS) and lipopolysaccharide dependent, 16 of V. harveyi biofilm-related genes were selected for further analysis. Based on quantitative real-time reverse transcription-PCR, the expression levels of these genes, including genes flrB, motY and mshA, flaE, flrA and gmhD, were significantly up-regulated in the ΔtoxR⁺ strain as compared with the WT⁺ and C-ΔtoxR strains during the early and mid-exponential, while epsG, flaA, flaE, flgD, flgE, flrB, flrC, lpxB, motY, mshA and scrG genes were inhibited because of deletion of the toxR gene in the stationary growth phase. Our results indicate that ToxR plays an important role in controlling the biofilm in V. harveyi.

Acute Hepatopancreatic Necrosis Disease (AHPND): Virulence, Pathogenesis and Mitigation Strategies in Shrimp Aquaculture

Shrimp, as a high-protein animal food commodity, are one of the fastest growing food producing sectors in the world. It has emerged as a highly traded seafood product, currently exceeding 8 MT of high value. However, disease outbreaks, which are considered as the primary cause of production loss in shrimp farming, have moved to the forefront in recent years and brought socio-economic and environmental unsustainability to the shrimp aquaculture industry. Acute hepatopancreatic necrosis disease (AHPND), caused by Vibrio spp., is a relatively new farmed penaeid shrimp bacterial disease. The shrimp production in AHPND affected regions has dropped to ~60%, and the disease has caused a global loss of USD 43 billion to the shrimp farming industry. The conventional approaches, such as antibiotics and disinfectants, often applied for the mitigation or cure of AHPND, have had limited success. Additionally, their usage has been associated with alteration of host gut microbiota and immunity and development of antibiotic resistance in bacterial pathogens. For example, the Mexico AHPND-causing V. parahaemolyticus strain (13-306D/4 and 13-511/A1) were reported to carry tetB gene coding for tetracycline resistance gene, and V. campbellii from China was found to carry multiple antibiotic resistance genes. As a consequence, there is an urgent need to thoroughly understand the virulence mechanism of AHPND-causing Vibrio spp. and develop novel management strategies to control AHPND in shrimp aquaculture, that will be crucially important to ensure food security in the future and offer economic stability to farmers. In this review, the most important findings of AHPND are highlighted, discussed and put in perspective, and some directions for future research are presented.

Quorum-Sensing Regulator OpaR Directly Represses Seven Protease Genes in Vibrio parahaemolyticus

Proteases play a key role in numerous bacterial physiological events. Microbial proteases are used in the pharmaceutical industry and in biomedical applications. The genus Vibrio comprises protease-producing bacteria. Proteases transform polypeptides into shorter chains for easier utilization. They also function as a virulence factor in pathogens. The mechanism by which protease genes are regulated in Vibrio parahaemolyticus, an emerging world-wide human pathogen, however, still remains unclear. Quorum sensing is the communication system of bacteria. OpaR is the master quorum-sensing regulator in V. parahaemolyticus. In the present study, quantitative reverse transcriptase-polymerase chain reaction and protease gene promoter-fusion reporter assays revealed that OpaR represses seven protease genes—three metalloprotease genes and four serine protease genes—which are involved in environmental survival and bacterial virulence. Furthermore, the electrophoresis mobility shift assay demonstrated that OpaR is bound directly to the promoter region of each of the seven protease genes. DNase I footprinting identified the sequence of these OpaR-binding sites. ChIP-seq analyses revealed 435 and 835 OpaR-binding sites in the late-log and stationary phases, respectively. These OpaR-binding sequences indicated a conserved OpaR-binding motif: TATTGATAAAATTATCAATA. These results advance our understanding of the protease regulation system in V. parahaemolyticus. This study is the first to reveal the OpaR motif within V. parahaemolyticus in vivo, using ChIP-seq, and to provide a database for OpaR direct regulon.

Role of LuxR-type regulators in fish pathogenic Aeromonas hydrophila

LuxR-type transcriptional factors are essential in many bacterial physiological processes. However, there have been no reports on their roles in Aeromonas hydrophila. In this study, six stable silent strains were constructed using shRNA. Significant decreases in the expression levels of luxR₀₅ , luxR₀₈ , luxR₁₉ , luxR₁₁ , luxR₁₆₄ and luxR₁₆₅ were shown in their respective strains by qRT-PCR. The luxR₀₅ -RNAi and luxR₁₆₄ -RNAi exhibit the most significant changes in sensitivity to kanamycin and gentamicin. The luxR₀₅ -RNAi showed minimum biofilm formation and the least motility, while luxR₁₆₄ -RNAi showed minimum biofilm formation, adhesion, growth and extracellular protease activity compared to the wild-type strain. In summary, the results of this paper suggest that all six luxR genes are involved in multiple physiological processes in A. hydrophila and that the roles of luxR₀₅ and luxR₁₆₄ are highly significant. The sensitivity of luxR₀₅ -RNAi and luxR₁₆₄ -RNAi to drugs may be closely related to biofilm formation. The luxR₀₅ may play an important role in the pathogenicity of A. hydrophila by regulating the movement, adhesion and biofilm formation of bacteria, whereas luxR₁₆₄ may be involved in similar functions by regulating bacterial adhesion, extracellular enzyme activity and growth. These results help further our understanding of the drug resistance and pathogenesis of A. hydrophila.

Molecular mechanisms of Vibrio parahaemolyticus pathogenesis

Vibrio parahaemolyticus is a Gram-negative halophilic bacterium that is mainly distributed in the seafood such as fish, shrimps and shellfish throughout the world. V. parahaemolyticus can cause diseases in marine aquaculture, leading to huge economic losses to the aquaculture industry. More importantly, it is also the leading cause of seafood-borne diarrheal disease in humans worldwide. With the development of animal model, next-generation sequencing as well as biochemical and cell biological technologies, deeper understanding of the virulence factors and pathogenic mechanisms of V. parahaemolyticus has been gained. As a globally transmitted pathogen, the pathogenicity of V. parahaemolyticus is closely related to a variety of virulence factors. This article comprehensively reviewed the molecular mechanisms of eight types of virulence factors: hemolysin, type III secretion system, type VI secretion system, adhesion factor, iron uptake system, lipopolysaccharide, protease and outer membrane proteins. This review comprehensively summarized our current understanding of the virulence factors in V. parahaemolyticus, which are potentially new targets for the development of therapeutic and preventive strategies.