Lethal attribute of serine protease secreted by Vibrio alginolyticus strains in kuruma prawn Penaeus japonicus

Dietary supplementation of Polygonum chinense improves the immunity of Asian seabass, Lates calcarifer (Bloch, 1790) against Vibrio harveyi infection

Aquaculture plays a significant role in the overall fish production in Malaysia, contributing a substantial quantity of food-fish amounting to roughly 573,683 tonnes with an estimated economic value of US$860 million in 2022. However, diseases have become a significant limitation for aquaculture production. Therefore, herbal immunostimulant has been considered a natural and practical approach of preventing disease infection in fish. The ability of Polygonum chinense extract (PCE) on haemato-biochemistry parameters, immunomodulatory properties, and disease resistance of Lates calcarifer (Asian seabass) under Vibrio harveyi challenge was evaluated in this study, with a focus on dose-response associations and variability over various exposure durations (0-, 7- and 14-day post-infection). A total of 480 Asian seabass (9.5 ± 0.2 g) were distributed in 12 aquaria and fed four diets supplemented with 0 (control), 2, 5 and 10 g/kg diet for 60 days before being challenged with V. harveyi. Dietary PCE significantly improved (P < 0.05) survival, with the dose of 10 g/kg showing the highest survival rate (90 %) when compared to the control (60 %). Additionally, hematological (red and white blood cell counts, hemoglobulin, packed cell volume, and mean corpuscular volume) and immunological (activities of lysozyme, phagocytic activity and respiratory burst, and serum total immunoglobulin) properties were significantly increased (P < 0.05) in comparison to the control group. In contrast, serum aspartate aminotransferase and alanine aminotransferase levels, as well as glucose level were significantly reduced (P < 0.05) in PCE-fed fish compared to the control group. Conclusively, the current study discovered that supplementing fish feed with P. chinense extract improves fish haemato-biochemical profile, immunocompetence and disease resistance to V. harveyi infection.

Simple, rapid and on spot dye-based sensor for the detection of Vibrio load in shrimp culture farms

For the detection of Vibrio bacteria, a kit involving two-step method was developed. In the in first step, a specific media was added in the water sample which selectively promote the growth of vibrios and inhibit the growth of other bacteria. The second step involved addition of dye-based sensor (already developed in our previous work) in the sample which detect the active Vibrio and changed the colour of the sample to red/pink. The vibrio detection kit was optimised on five different species of Vibrio (V. cholerae, V. parahaemolyticus, V. campbellii, V. harveyi and V. proteolyticus) and two negative control bacteria (Escherichia coli and Bacillus subtilis). The kit was further evaluated on aquaculture pond water and probiotics used in aquaculture farms. It successfully estimated Vibrio concentration of all the five strains in aquaculture ponds. The negative control bacteria and probiotics were not sensed by the kit. Hence, the kit developed here is perfect for the detection of Vibrio, especially in aquaculture farms.

Antigenicity of hypothetical protein HP33 of Vibrio harveyi Y6 causing scale drop and muscle necrosis disease in Asian sea bass

A unique strain of Vibrio harveyi is the causative agent of scale drop and muscle necrosis disease (SDMND) in Asian sea bass (Lates calcarifer). This study investigated the protein profiles of SDMND-causing Vibrio harveyi isolates compared to the reference V. harveyi ATCC 14126 strain. A distinct protein band of 33 kDa, namely HP33, found from only V. harveyi SDMND was subjected to analysis by LC-MS/MS and the identified peptide sequences matched to an unknown hypothetical protein. Detection of HP33 coding sequence was investigated at both genomic and transcriptional levels and the results consistently supported the protein analysis. Recombinant HP33 protein was then produced using Escherichia coli system. The rHP33 protein did not cause mortality or visible clinical signs to Asian sea bass. However, the rHP33 protein was able to stimulate antibody response in Asian sea bass as evidenced by Western blotting and agglutination tests. Here, we proposed that rHP33 might be a good protein target for development of subunit vaccine and/or immunostimulant to protect Asian sea bass from SDMND.

Transcriptomic analysis of Manila clam Ruditapes philippinarum under lipopolysaccharide challenge provides molecular insights into immune response

The Manila clam, Ruditapes philippinarum, is an economically important shellfish in marine aquaculture. A better understanding of the immune system in R. philippinarum will provide the basis for the development of strategies to mitigate the impact of infectious diseases affecting this species but can also be of relevance for other bivalves of commercial interest. In this study, the transcriptional response of the Manila clam under lipopolysaccharide (LPS) challenge was characterized using RNA sequencing. The transcriptomes of LPS challenged group of clams (LH1, LH2 and LH3), and the PBS control group (CH1, CH2 and CH3), were sequenced with the Illumina HiSeq platform. Compared with the unigene expression profile of the control group, 223 unigenes were up-regulated and 389 unigenes were down-regulated in the LPS challenged group. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that signal transduction, defense response, and immune-related pathways such as Chemokine signaling pathway, Complement and coagulation cascades, NOD-like receptor signaling pathway, and Inflammatory mediator regulation of TRP channels in sensory system were the most highly enriched pathways among the genes that were differentially expressed under LPS challenge. This study present understanding of the molecular basis underpinning response to LPS challenge and provides useful information for future work on the molecular mechanism of pathogen resistance and immunity in Manila clam.

Mechanism of the Potential Therapeutic Candidate Bacillus subtilis BSXE-1601 Against Shrimp Pathogenic Vibrios and Multifunctional Metabolites Biosynthetic Capability of the Strain as Predicted by Genome Analysis

The global shrimp industry has suffered bacterial diseases caused mainly by Vibrio species. The typical vibriosis, acute hepatopancreatic necrosis disease (AHPND), has resulted in mass mortality and devastating economic losses. Thus, therapeutic strategies are highly needed to decrease the risk of vibriosis outbreaks. Herein, we initially identified that the growth of the causative agent of AHPND, Vibrio parahaemolyticus (VP AHPND ) and other vibrios in Pacific white shrimp (Litopenaeus vannamei) was inhibited by a Bacillus subtilis strain BSXE-1601. The natural products amicoumacins A, B, and C were purified from the cell-free supernatant from the strain BSXE-1601, but only amicoumacin A was demonstrated to be responsible for this anti-Vibrio activity. Our discovery provided the first evidence that amicoumacin A was highly active against shrimp pathogens, including the representative strain VP AHPND . Furthermore, we elucidated the amicoumacin A biosynthetic gene cluster by whole genome sequencing of the B. subtilis strain BSXE-1601. In addition to amicoumacin A, the strain BSXE-1601 genome harbored other genes encoding bacillibactin, fengycin, surfactin, bacilysin, and subtilosin A, all of which have previously reported antagonistic activities against pathogenic strains. The whole-genome analysis provided unequivocal evidence in support of the huge potential of the strain BSXE-1601 to produce diverse biologically antagonistic natural products, which may facilitate further studies on the effective therapeutics for detrimental diseases in shrimp.

Spotlight on a novel bactericidal mechanism and a novel SXT/R391-like integrative and conjugative element, carrying multiple antibiotic resistance genes, in Pseudoalteromonas flavipulchra strain CDM8

Many Pseudoalteromonas strains can produce bioactive compounds with antimicrobial activities. This study focused on a probiotic candidate P.flavipulchra CDM8 to reveal its novel antibacterial mechanism and risks for antibiotic resistance dissemination. Strain CDM8 could form floating biofilm, displayed strikingly broad antibacterial activities against multiple Vibrio and Bacillus species, and decreased the competitor's concentration in their co-cultures in the microtiter plate tests. It could also form vesicle/pilus-like structures on the outer surface, which were indicated to participate in the bactericidal activity and represent a novel antibacterial mechanism of CDM8, according to the scanning electron microscopic observation. However, CDM8 displayed multi-antibiotic resistance, conferred by the multidrug resistance regions in hotspot 4 and variable region III of a novel SXT/R391-like integrative and conjugative element (ICEPflCDM8). Summing up, our results provided a better understanding of the bactericidal mechanism of P. flavipulchra and highlighted the role of SXT/R391-like ICEs in conferring multidrug resistance phenotype of probiotic P. flavipulchra candidates.

Strain variability and pathogenic potential of tdhcarrying Vibrio parahaemolyticus isolated from diseased Penaeus monodon

A disease outbreak in 42-d-old black tiger shrimp Penaeus monodon juveniles from a commercial aquaculture farm in Kerala, India, was investigated. The cause of the disease outbreak was confirmed as Vibrio parahaemolyticus by biochemical tests, PCR targeting the toxR gene and pathogenicity testing of the isolates. All of the isolates tested negative by PCR specific for V. parahaemolyticus associated with acute hepatopancreatic necrosis disease (AHPND), implicating vibriosis unrelated to AHPND as the cause of mortality. Among the 19 isolates obtained, 2 possessed the tdh gene (coding for thermo-stable hemolysin), whereas none of the isolates possessed trh. The LD50 value of 8 isolates of V. parahaemolyticus from diseased and apparently healthy shrimp ranged from 2.7 × 104 to 4.9 × 105 CFU ml-1 by immersion challenge of P. monodon postlarvae. BOX-PCR and dendrogram analysis of the bacterial isolates revealed that the isolates from moribund and apparently healthy shrimp formed separate clusters, indicating that these isolates originate from separate clones. The isolates from moribund shrimp including tdh-positive V. parahaemolyticus clustered together. The present study represents the first report of tdh-positive V. parahaemolyticus causing disease in a shrimp farm.

Saline Environments as a Source of Potential Quorum Sensing Disruptors to Control Bacterial Infections: A Review

Saline environments, such as marine and hypersaline habitats, are widely distributed around the world. They include sea waters, saline lakes, solar salterns, or hypersaline soils. The bacteria that live in these habitats produce and develop unique bioactive molecules and physiological pathways to cope with the stress conditions generated by these environments. They have been described to produce compounds with properties that differ from those found in non-saline habitats. In the last decades, the ability to disrupt quorum-sensing (QS) intercellular communication systems has been identified in many marine organisms, including bacteria. The two main mechanisms of QS interference, i.e., quorum sensing inhibition (QSI) and quorum quenching (QQ), appear to be a more frequent phenomenon in marine aquatic environments than in soils. However, data concerning bacteria from hypersaline habitats is scarce. Salt-tolerant QSI compounds and QQ enzymes may be of interest to interfere with QS-regulated bacterial functions, including virulence, in sectors such as aquaculture or agriculture where salinity is a serious environmental issue. This review provides a global overview of the main works related to QS interruption in saline environments as well as the derived biotechnological applications.

In vitro antagonistic activity and the protective effect of probiotic Bacillus licheniformis Dahb1 in zebrafish challenged with GFP tagged Vibrio parahaemolyticus Dahv2

In vitro antagonistic activity and the protective effect of probiotic Bacillus licheniformis Dahb1 in zebrafish (Danio rerio) challenged with GFP tagged Vibrio parahaemolyticus Dahv2 was studied. The cell free extract of probiotic B. licheniformis Dahb1 at 100 μg mL^-1 showed growth inhibition of V. parahaemolyticus Dahv2 in vitro. B. licheniformis Dahb1 also inhibited the biofilm growth of GFP tagged V. parahaemolyticus Dahv2 at 100 μg mL^-1 in vitro. The growth and survival of zebrafish was tested using probiotic B. licheniformis Dahb1. Weight (1.28 g) of zebrafish that received the cell free extract was much higher than in control (1.04 g). The mortality of zebrafish infected with GFP tagged V. parahaemolyticus Dahv2 at 10⁷ Cfu mL^-1 (Group IV) was 100%, whereas a complete survival of zebrafish that received the cell free extract of B. licheniformis Dahb1 at 10⁷ Cfu mL^-1 (Group VII) was observed after 30 days. The number of GFP tagged V. parahaemolyticus Dahv2 colonies in the intestine and gills significantly reduced after treatment with the cell free extract of B. licheniformis Dahb1. Furthermore, a significant decrease in the fluorescent colonies of GFP tagged V. parahaemolyticus Dahv2 was observed after treatment with the cell free extract of B. licheniformis Dahb1 under confocal laser scanning microscopy (CLSM). In conclusion, the cell free extract of B. licheniformis Dahb1 could prevent Vibrio infection by enhancing the growth and survival of zebrafish.

Isolation, identification and phenotypic and molecular characterization of pathogenic Vibrio vulnificus isolated from Litopenaeus vannamei

The morphology and the drug sensitivity of the strain GYX2014-1 isolated from the hepatic pancreatic tissue of moribund Litopenaeus vannamei were evaluated by conventional culture characteristics, physical and chemical characteristics, and molecular biology methods. Detection of extracellulase and hemolysin activity shows that the isolated GYX2014-1 has protease, lipase, gelatinase activity, but none of amylase, or lecithinase activity. The 16S rRNA gene (GenBank accession number: KT781675) was analyzed, and a phylogenetic tree analysis showed that the isolated pathogen was most closely related to V. vulnificus (GenBank accession number: NR 118570)—a match of more than 99%. The phenotypic traits and molecular biology of isolated bacteria, determined their identity as Vibrio vulnificus (V. vulnificus). In addition, artificially infected L. vannamei with Vibrio vulnificus appeared with the same disease symptoms as those of naturally infected shrimp. Drug sensitivity tests showed that V. vulnificus is highly sensitive to fosfomycin, cefradine and sinomin, and was resistant to penicillin, amikacin and kanamycin. This experiment is the first to separate V. vulnificus from L. vannamei, and the findings of this study can be used as a reference for disease control and health management.

In vitro and in vivo exploration of palmitic acid from Synechococcus elongatus as an antibiofilm agent on the survival of Artemia franciscana against virulent vibrios

Biofilm formation of Vibrio spp. has been demonstrated as a potentially important mechanism contributing antibiotic treatment failure in aquaculture. In the present study, the effect of palmitic acid (PA) identified from Synechococcus elongatus was assessed for the inhibition of quorum sensing (QS) regulated biofilm formation in aquatic bacterial pathogens. The biofilm inhibitory concentration (BIC) of PA against Vibrio spp. was found to be 100µgml^-1. In this concentration, PA exhibited a significant inhibition in biofilm biomass of Vibrio harveyi MTCC 3438, V. parahaemolyticus ATCC 17802, V. vulnificus MTCC 1145 and V. alginolyticus ATCC 17749 without hindering their planktonic growth. Also, PA displayed gradual decrease in bioluminescence production of V. harveyi. The results of extracellular polymeric substances quantification, microbial adhesion to hydrocarbons and Fourier transform infrared spectroscopic (FT-IR) analyses suggested that PA positively interferes with the initial adhesion stages of biofilm formation. In addition, confocal and scanning electron microscopic analysis substantiates the antibiofilm efficacy of the PA. The transcriptomic analysis revealed the down-regulation of QS mediated response regulator genes expression in V. harveyi. Concomitantly, PA reduced the intestinal colonization of vibrios in brine shrimp larvae and thereby attenuates the biofilm assemblage and its associated virulence. In vivo studies using brine shrimp larvae manifested the reduction in adherence and virulence, which prompts further investigation about the potential of PA for the treatment of vibriosis.

Culture conditions improvement of Crassostrea gigas using a potential probiotic Bacillus sp strain

It is well demonstrated that some probiotics improve rearing water quality and thereby have beneficial effects on reared organisms. We conducted this study to determine the effect of Bacillus consortium on Crassostrea gigas reared in contemned seawater with indigo dye priory treated with Bacillus or no treated. This effect was studied by assessing hemocytes death using flow cytometry analysis. We found that the percentage of decolorization of indigo dye in polluted seawater in presence of C. gigas increased from 41% to 90% when using Bacillus consortium. In these conditions, the hemocytes mortality of reared C. gigas decreased from 87% to 56%. We have demonstrated also that seawater contemned with priory treated indigo with Bacillus consortium is less toxic than seawater contemned with the no treated indigo. The percentage of hemocytes death is 81% for the contemned seawater with indigo and 56% for no contemned seawater. This consortium shows a protector effect of C. gigas against Vibrio harveyi contemning reared seawater.

The GRIM-19 plays a vital role in shrimps' responses to Vibrio alginolyticus

GRIM-19 (gene associated with retinoid-interferon-induced mortality 19), a novel cell death regulatory gene, plays important roles in cell apoptosis, mitochondrial respiratory chain and immune response. It has been reported to interact physically with STAT3 and inhibit STAT3-dependent signal transduction. In this study, a new GRIM-19 gene, which is a 789-bp gene encoding a 149 amino acids protein, is identified and characterized from Litopenaeus vannamei. The tissue distribution patterns showed that LvGRIM-19 was widely expressed in all examined tissues, with the highest expression in muscle. Quantitative real-time PCR revealed that LvGRIM-19 was down-regulated in hepatopancreas after infection with the Vibrio alginolyticus. Knockdown of LvGRIM-19 by RNA interference resulted in a lower mortality of L. vannamei under V. alginolyticus infection, as well as an enhancement in the protein expression of STAT gene and JAK gene. V. alginolyticus infection caused an increase apoptotic cell ratio and ROS production of L. vannamei, while LvGRIM-19 silenced shrimps showed significantly lower than GFP group. Our results suggest that the GRIM-19 plays a vital role in shrimps' responses to V. alginolyticus. Interferenced LvGRIM-19 treatment during V. alginolyticus infection could increase 12 h survival rate, which might indicated that LvGRIM-19 is closely related to death of shrimps.

A prophenoloxidase from Artemia sinica: cDNA cloning, expression and activity analysis during early development

To understand the defense mechanisms of Crustacean animals, brine shrimp Artemia sinica prophenoloxidase (AsproPO) cDNA was cloned and its expression at early developmental stages was examined by reverse-transcription PCR (RT-PCR) and semi-quantitative RT-PCR, and activity of phenoloxidase (PO) at different developmental stages was further detected by using l-3,4-dihydroxyphenylalanine (l-DOPA) as a specific substrate in this study. It was found that the full-length of AsproPO cDNA is 2125 bp and it contains an open reading frame of 2100 bp encoding a protein of 699 amino acids. The deduced amino acid sequence of AsproPO has two putative copper binding sites highly conserved in Arthropods. Semi-quantitative RT-PCR analyses showed that the gene of AsproPO expressed at Emergence, Instar I and Instar II stages but did not at 0 h and 6 h stages. Activity measurement showed that PO activity could only be detected at Instar II stage but the other measured stages. All these implied that Artemia proPO immune system was complexly modulated during early development.

Passive immune-protection of small abalone against Vibrio alginolyticus infection by anti-Vibrio IgY-encapsulated feed

Small abalone (Haliotis diversicolor supertexta) is a high value-added shellfish. It however has been suffering Vibrio alginolyticus infections, which cause mass death of small abalone and thus great economic losses, particularly in artificial aquaculture. In this study, we attempted to treat small abalone with anti-Vibrio IgY to elicit a passive immunity directly against V. alginolyticus infections. Anti-Vibrio IgY was alginate encapsulated in egg powders as feed, which may avoid antibody inactivation in the gastrointestinal tract of small abalone. The feed was tested for the stability of anti-Vibrio IgY in a gastrointestinal mimic environment. The result showed anti-Vibrio IgY retained activity as high as 90% after 4 h exposure to pancreatic enzymes. Addition of 0, 5 or 10% anti-Vibrio IgY-encapsulated egg powders into a basal diet to form abalone diet formulae. Small abalones fed with the anti-Vibrio IgY formulae showed a relatively high respiratory burst activity than those without anti-Vibrio IgY treatments. The survival rates of small abalones fed with 5 or 10% anti-Vibrio IgY egg powders were in the range of 65-70% 14 days post-V. alginolyticus challenge (1 x 10⁶ c.f.u.), which was significantly higher than 0% of those fed without anti-Vibrio IgY. The anti-Vibrio IgY-encapsulated formulae were thus concluded to be an effective means to prevent small abalone from V. alginolyticus infection, and may be practical in use in abalone aquaculture.

The extracellular metalloprotease of Vibrio tubiashii is a major virulence factor for pacific oyster (Crassostrea gigas) larvae

Vibrio tubiashii is a recently reemerging pathogen of larval bivalve mollusks, causing both toxigenic and invasive disease. Marine Vibrio spp. produce an array of extracellular products as potential pathogenicity factors. Culture supernatants of V. tubiashii have been shown to be toxic to oyster larvae and were reported to contain a metalloprotease and a cytolysin/hemolysin. However, the structural genes responsible for these proteins have yet to be identified, and it is uncertain which extracellular products play a role in pathogenicity. We investigated the effects of the metalloprotease and hemolysin secreted by V. tubiashii on its ability to kill Pacific oyster (Crassostrea gigas) larvae. While V. tubiashii supernatants treated with metalloprotease inhibitors severely reduced the toxicity to oyster larvae, inhibition of the hemolytic activity did not affect larval toxicity. We identified structural genes of V. tubiashii encoding a metalloprotease (vtpA) and a hemolysin (vthA). Sequence analyses revealed that VtpA shared high homology with metalloproteases from a variety of Vibrio species, while VthA showed high homology only to the cytolysin/hemolysin of Vibrio vulnificus. Compared to the wild-type strain, a VtpA mutant of V. tubiashii not only produced reduced amounts of protease but also showed decreased toxicity to C. gigas larvae. Vibrio cholerae strains carrying the vtpA or vthA gene successfully secreted the heterologous protein. Culture supernatants of V. cholerae carrying vtpA but not vthA were highly toxic to Pacific oyster larvae. Together, these results suggest that the V. tubiashii extracellular metalloprotease is important in its pathogenicity to C. gigas larvae.

Two novel species of tetrodotoxin-producing bacteria isolated from toxic marine puffer fishes

Out of eight dominant discrete bacterial colonies isolated and purified from the toxic marine puffer fishes collected in Hong Kong waters, two novel species of non-sporing, non-acid-fast and chemoorganotrophic bacteria capable of producing tetrodotoxin (TTX, a potent non-protein neurotoxin), as well as one previously reported and confirmed TTX-producing bacterium. They were identified as Microbacterium arabinogalactanolyticum, Serratia marcescens and Vibrio alginolyticus, respectively, all of which are widely distributed in soils, sewage or marine environments. Each bacterial isolate (500 ml broth medium cultured in darkness without aeration for 10 days at 25 degrees C) could produce an amount of toxicity, after extraction and purification, ranging from 78.3 to 105.3 mouse units (MU) in 500 ml of broth medium by mouse bioassay. The principal toxic component in the bacterial cultures was determined to be TTX by thin layer chromatography and mass spectrometry.