Pathogenicity of Vibrio alginolyticus for cultured gilt-head sea bream (Sparus aurata L.)

Comparative phenotype and transcriptome analysis revealed the role of ferric uptake regulator (Fur) in the virulence of Vibrio harveyi isolated from diseased American eel (Anguilla rostrata)

Vibrio harveyi is commonly found in salt and brackish water and is recognized as a serious bacterial pathogen in aquaculture worldwide. In this study, we cloned the ferric uptake regulator (fur) gene from V. harveyi wild-type strain HA_1, which was isolated from diseased American eels (Anguilla rostrata) and has a length of 450 bp, encoding 149 amino acids. Then, a mutant strain, HA_1-Δfur, was constructed through homologous recombination of a suicide plasmid (pCVD442). The HA_1-Δfur mutant exhibited weaker biofilm formation and swarming motility, and 18-fold decrease (5.5%) in virulence to the American eels; compared to the wild-type strain, the mutant strain showed time and diameter differences in growth and haemolysis, respectively. Additionally, the adhesion ability of the mutant strain was significantly decreased. Moreover, there were 15 different biochemical indicators observed between the two strains. Transcriptome analysis revealed that 875 genes were differentially expressed in the Δfur mutant, with 385 up-regulated and 490 down-regulated DEGs. GO and KEGG enrichment analysis revealed that, compared to the wild-type strain, the type II and type VI secretion systems (T2SS and T6SS), amino acid synthesis and transport and energy metabolism pathways were significantly down-regulated, but the ABC transporters and biosynthesis of siderophore group non-ribosomal peptides pathways were up-regulated in the Δfur strain. The qRT-PCR results further confirmed that DEGs responsible for amino acid transport and energy metabolism were positively regulated, but DEGs involved in iron acquisition were negatively regulated in the Δfur strain. These findings suggest that the virulence of the Δfur strain was significantly decreased, which is closely related to phenotype changing and gene transcript regulation.

Bacteriophages to control Vibrio alginolyticus in live feeds prior to their administration in larviculture

AIMS

This study aimed to evaluate the efficiency of two phages [VB_VaC_TDDLMA (phage TDD) and VB_VaC_SRILMA (phage SRI)] alone and in a cocktail to control Vibrio alginolyticus in brine shrimp before their administration in larviculture.

METHODS AND RESULTS

Phages were isolated from seawater samples and characterized by host spectrum, growth parameters, adsorption rate, genomic analysis, and inactivation efficiency. Both phages belong to the Caudoviricetes class and lack known virulence or antibiotic-resistance genes. They exhibit specificity, infecting only their host, V. alginolyticus CECT 521. Preliminary experiments in a culture medium showed that phage TDD (reduction of 5.8 log CFU ml-1 after 10 h) outperformed phage SRI (reduction of 4.6 log CFU ml-1 after 6 h) and the cocktail TDD/SRI (reduction of 5.2 log CFU ml-1 after 8 h). In artificial marine water experiments with Artemia franciscana, both single phage suspensions and the phage cocktail, effectively inactivated V. alginolyticus in culture water (reduction of 4.3, 2.1, and 1.9 log CFU ml-1 for phages TDD, SRI, and the phage cocktail, respectively, after 12 h) and in A. franciscana (reduction of 51.6%, 87.3%, and 85.3% for phages TDD, SRI, and the phage cocktail, respectively, after 24 h). The two phages and the phage cocktail did not affect A. franciscana natural microbiota or other Vibrio species in the brine shrimp.

CONCLUSIONS

The results suggest that phages can safely and effectively control V. alginolyticus in A. franciscana prior to its administration in larviculture.

Identification and florfenicol-treatment of pseudomonas putida infection in gilthead seabream (Sparus aurata) fed on tilapia-trash-feed

The present study aimed to determine the major cause of the high mortality affecting farmed gilthead seabream (Sparus aurata) and controlling this disease condition. Fifteen diseased S. aurata were sampled from a private fish farm located at Eldeba Triangle, Damietta, fish showed external skin hemorrhages, and ulceration. Bacterial isolates retrieved from the diseased fish were identified biochemically as Pseudomonas putida and then confirmed by phylogenetic analysis of the 16 S rRNA gene sequence. P. putida was also isolated from three batches of tilapia-trash feed given to S. aurata. Biofilm and hemolytic assay indicated that all P. putida isolates produced biofilm, but 61.11% can haemolyse red blood cells. Based on the antibiotic susceptibility test results, P. putida was sensitive to florfenicol with minimum inhibitory concentrations ranging between 0.25 and 1.0 µg mL- 1, but all isolates were resistant to ampicillin and sulfamethoxazole-trimethoprim. Pathogenicity test revealed that P. putida isolate (recovered from the tilapia-trash feed) was virulent for S. aurata with LD50 equal to 4.67 × 107 colony forming unit (CFU) fish- 1. After intraperitoneal (IP) challenge, fish treated with 10 mg kg- 1 of florfenicol showed 16.7% mortality, while no mortality was recorded for the fish group that received 20 mg kg- 1. The non-treated fish group showed 46.7% mortality after bacterial challenge. HPLC analysis of serum florfenicol levels reached 1.07 and 2.52 µg mL- 1 at the 5th -day post-drug administration in the fish groups received 10 and 20 mg kg- 1, respectively. In conclusion, P. putida was responsible for the high mortality affecting cultured S. aurata, in-feed administration of florfenicol (20 mg kg- 1) effectively protected the challenged fish.

Oxidative stress, gene expression and histopathology of cultured gilthead sea bream (Sparus aurata) naturally co-infected with Ergasilus sieboldi and Vibrio alginolyticus

BACKGROUND

Parasitic and bacterial co-infections have been associated with increasing fish mortalities and severe economic losses in aquaculture through the past three decades. The aim of this study was to evaluate the oxidative stress, histopathology, and immune gene expression profile of gilthead sea bream (Sparus aurata) co-infected with Ergasilus sieboldi and Vibrio alginolyticus.

RESULTS

Vibrio alginolyticus and Ergasilus sieboldi were identified using 16 S rRNA and 28 S rRNA sequencing, respectively. The collagenase virulence gene was found in all Vibrio alginolyticus isolates, and the multiple antimicrobial resistance index ranged from 0.286 to 0.857. Oxidant-antioxidant parameters in the gills, skin, and muscles of naturally infected fish revealed increased lipid peroxidation levels and a decrease in catalase and glutathione antioxidant activities. Moreover, naturally co-infected gilthead sea bream exhibited substantial up-regulation of il-1β, tnf-α, and cyp1a1. Ergasilus sieboldi encircled gill lamellae with its second antennae, exhibited severe gill architectural deformation with extensive eosinophilic granular cell infiltration. Vibrio alginolyticus infection caused skin and muscle necrosis in gilthead sea bream.

CONCLUSION

This study described some details about the gill, skin and muscle tissue defense mechanisms of gilthead sea bream against Ergasilus sieboldi and Vibrio alginolyticus co-infections. The prevalence of co-infections was 100%, and no resistant fish were detected. These co-infections imbalance the health status of the fish by hampering the oxidant-antioxidant mechanisms and proinflammatory/inflammatory immune genes to a more detrimental side. Our results suggest that simultaneous screening for bacterial and parasitic pathogens should be considered.

Exploring bacterial community composition and immune gene expression of European eel larvae (Anguilla anguilla) in relation to first-feeding diets

European eel (Anguilla anguilla) is a commercially important species for fisheries and aquaculture in Europe and the attempt to close the lifecycle in captivity is still at pioneering stage. The first feeding stage of this species is characterized by a critical period between 20 to 24 days post hatch (dph), which is associated with mortalities, indicating the point of no return. We hypothesized that this critical period might also be associated with larvae-bacterial interactions and the larval immune status. To test this, bacterial community composition and expression of immune and stress-related genes of hatchery-produced larvae were explored from the end of endogenous feeding (9 dph) until 28 dph, in response to three experimental first-feeding diets (Diet 1, Diet 2 and Diet 3). Changes in the water bacterial community composition were also followed. Results revealed that the larval stress/repair mechanism was activated during this critical period, marked by an upregulated expression of the hsp90 gene, independent of the diet fed. At the same time, a shift towards a potentially detrimental larval bacterial community was observed in all dietary groups. Here, a significant reduction in evenness of the larval bacterial community was observed, and several amplicon sequence variants belonging to potentially harmful bacterial genera were more abundant. This indicates that detrimental larvae-bacteria interactions were likely involved in the mortality observed. Beyond the critical period, the highest survival was registered for larvae fed Diet 3. Interestingly, genes encoding for pathogen recognition receptor TLR18 and complement component C1QC were upregulated in this group, potentially indicating a higher immunocompetency that facilitated a more successful handling of the harmful bacteria that dominated the bacterial community of larvae on 22 dph, ultimately leading to better survival, compared to the other two groups.

Purification and Characterization of the Lecithin-Dependent Thermolabile Hemolysin Vhe1 from the Vibrio sp. Strain MA3 Associated with Mass Mortality of Pearl Oyster (Pinctada fucata)

In a previous study, we isolated a Vibrio sp. strain MA3 and its virulence factor, a hemolysin encoded by vhe1. This strain is associated with mass mortalities of the pearl oyster Pinctada fucata. In the present study, the vhe1 gene from strain MA3 was cloned and its encoded product was purified and characterized. Our results show that the vhe1 gene encodes a protein of 417 amino acids with an estimated molecular mass of 47.2 kDa and a pI of 5.14. The deduced protein, Vhe1, was found to contain the conserved amino acid sequence (GDSL motif) of the hydrolase/esterase superfamily and five conserved blocks characteristic of SGNH hydrolases. A BLAST homology search indicated that Vhe1 belongs the lecithin-dependent hemolysin/thermolabile hemolysin (LDH/TLH) family. In activity analyses, the optimal temperature for both the hemolytic and phospholipase activities of Vhe1 was 50 °C. Vhe1 hemolytic activity and phospholipase activity were highest at pH 8.5 and pH 8.0, respectively. However, both enzymatic activities sharply decreased at high temperature (> 50 °C) and pH < 7.0. Compared with previously reported hemolysins, Vhe1 appeared to be more thermal- and pH-labile. Both its hemolytic activity and phospholipase activity were significantly inhibited by CuCl2, CdCl2, ZnCl2, and NiCl2, and slightly inhibited by MnCl2 and CoCl2. Vhe1 showed higher phospholipase activity toward medium-chain fatty acids (C8-C12) than toward shorter- and longer-chain fatty acids. These results accumulate knowledge about the LDH/TLH of V. alginolyticus, which detailed characterization has not been reported, and contribute to solving of the mass mortality of pearl oyster.

Coral Disease and Ingestion: Investigating the Role of Heterotrophy in the Transmission of Pathogenic Vibrio spp. using a Sea Anemone (Exaiptasia pallida) Model System

Understanding disease transmission in corals can be complicated given the intricacy of the holobiont and difficulties associated with ex situ coral cultivation. As a result, most of the established transmission pathways for coral disease are associated with perturbance (i.e., damage) rather than evasion of immune defenses. Here, we investigate ingestion as a potential pathway for the transmission of coral pathogens that evades the mucus membrane. Using sea anemones (Exaiptasia pallida) and brine shrimp (Artemia sp.) to model coral feeding, we tracked the acquisition of the putative pathogens, Vibrio alginolyticus, V. harveyi, and V. mediterranei using GFP-tagged strains. Vibrio sp. were provided to anemones using 3 experimental exposures (i) direct water exposure alone, (ii) water exposure in the presence of a food source (non-spiked Artemia), and (iii) through a "spiked" food source (Vibrio-colonized Artemia) created by exposing Artemia cultures to GFP-Vibrio via the ambient water overnight. Following a 3 h feeding/exposure duration, the level of acquired GFP-Vibrio was quantified from anemone tissue homogenate. Ingestion of spiked Artemia resulted in a significantly greater burden of GFP-Vibrio equating to an 830-fold, 3,108-fold, and 435-fold increase in CFU mL^-1 when compared to water exposed trials and a 207-fold, 62-fold, and 27-fold increase in CFU mL^-1 compared to water exposed with food trials for V. alginolyticus, V. harveyi, and V. mediterranei, respectively. These data suggest that ingestion can facilitate delivery of an elevated dose of pathogenic bacteria in cnidarians and may describe an important portal of entry for pathogens in the absence of perturbing conditions. IMPORTANCE The front line of pathogen defense in corals is the mucus membrane. This membrane coats the surface body wall creating a semi-impermeable layer that inhibits pathogen entry from the ambient water both physically and biologically through mutualistic antagonism from resident mucus microbes. To date, much of the coral disease transmission research has been focused on mechanisms associated with perturbance of this membrane such as direct contact, vector lesions (predation/biting), and waterborne exposure through preexisting lesions. The present research describes a potential transmission pathway that evades the defenses provided by this membrane allowing unencumbered entry of bacteria as in association with food. This pathway may explain an important portal of entry for emergence of idiopathic infections in otherwise healthy corals and can be used to improve management practices for coral conservation.

Functional characterization of Labeo rohita muscle cell line for in vitro research

BACKGROUND

Labeo rohita represents the most dominant fish species in Indian aquaculture and the fish cell lines have been used as an excellent in vitro platform for performing various biological research.

METHODS AND RESULTS

The LRM cell culture developed from the muscle tissue of L. rohita was used to study the in vitro applications. The developed muscle cells were maintained in a Leibovitz's-15 (L-15) supplemented with 10% FBS (Fetal Bovine Serum) and 10 ng/ml bFGF at 28 ^oC temperature. The LRM cells showed fibroblastic-like morphology and was authenticated by sequencing mitochondrial gene 16S rRNA. The expression of myogenic regulatory factors (MRFs) was studied in different stages of LRM cells; however, the expression patterns varied at different passages. The MEF2A, Mrf-4, and Myogenin expressions were higher in passage 25, while the expression of MyoD was maximum in passage 15, and the expression of Myf-5 was highest in passage 1. The transfection efficiency of LRM cells revealed 14 % of the GFP expression with a pmaxGFP vector DNA. The LRM cells were susceptible to the extracellular products prepared from Aeromonas hydrophilla and Edwardsiella tarda. The acute cytotoxicity of six heavy metals (Hg, Cd, Zn, Cu, Pb, Ni) was assessed in LRM cells by a dose-dependent manner in comparison to IC₅₀ values obtained from MTT and NR assays. A revival rate of 70-75% was achieved when the LRM cells were cryopreserved at - 196 °C using liquid nitrogen.

CONCLUSION

The developed muscle cells serve as an functional in vitro tool for toxicological and biotechnological studies.

Isolation and characterization of a novel phage vB_ValP_VA-RY-3 infecting Vibrio alginolyticus

Vibrio alginolyticus is a common foodborne pathogen existing both in contaminated seafood and the environment and can cause serious mortality in aquaculture facilities. Bacteriophages can be used as an alternative bio-control agent to eliminate and reduce pathogens. In this study, a novel lytic phage, designated vB_ValP_VA-RY-3 (referred to as S1R3Y), was isolated from sewage collected in Dalian, China. The linear double-stranded DNA genome of phage S1R3Y is 40.271 kb, which has a mol% G + C content of 43.98, containing 51 ORFs with a T7-like genomic organization. It shared the closest relationship with phage vB_CsaP_Ss1, but the homology coverage is just 6%. S1R3Y lacks tRNA and no known virulence or lysogenic genes were found. S1R3Y had a burst size of 147 PFU/cell and is stable under different temperatures (4-56 °C) and pH (5.0-7.0). A comparison of its genomic features and phylogenetic analysis revealed that phage S1R3Y is a novel member of the order Caudovirales, family Podoviridae. Our results suggest that phage S1R3Y may represent a potential therapeutic agent against Vibrio alginolyticus.

Vibrio pathogenicity island and phage CTX genes in Vibrio alginolyticus isolated from different aquatic environments

In the present study, we investigated the presence of four Vibrio cholerae virulence genes (ctxA, VPI, Zot and ace) in 36 Vibrio alginolyticus isolates obtained from different seawater, sediments and aquatic organisms. We tested the virulence of 13 V. alginolyticus strains against juveniles of Sparus aurata and this virulence was correlated with the presence of V. cholerae virulence genes. A positive amplification for the virulence pathogenicity island was produced by five V. alginolyticus strains and four for cholerae toxin. Some of the V. alginolyticus strains are pathogenic to aquatic animals and might have derived their virulence genes from V. cholerae. V. alginolyticus strains can be considered as a possible reservoir of V. cholerae virulence genes.

Effects of a commercial feed additive (Sanacore® GM ) on immune-antioxidant profile and resistance of gilthead seabream (Sparus aurata) against Vibrio alginolyticus infection

The effects of a functional additive (Sanacore® GM; SAN ) on immune and antioxidant indices, and the resistance of gilthead seabream (Sparus aurata) against Vibrio alginolyticus infection. For this, four diets containing 0% (the control), 0.1%, 0.2%, and 0.4% SAN were offered to triplicated groups of fish (20 - 23 g) for ten weeks. Subsequently, fish were injected intraperitoneally with V. alginolyticus and monitored for further ten days. Feeding the fish on SAN-supplemented diets showed positive effects on leukocyte counts and its differential percentages. Serum lysozyme activity and total immunoglobulin values, as well as phagocytic activity and indices, were linearly and quadratically higher in SAN-fed fish; especially at the 0.4% SAN diet. Similarly, linear and quadratic increases in catalase, superoxide dismutase, and total antioxidant capacity were observed in SAN-fed fish, particularly at the 0.4% SAN diet. Conversely, serum malondialdehyde values decreased in SAN-fed fish compared with the control group, which showed its highest value. The highest expression of the IL-1β gene coupled with the lowest TNF-α and HSP70 genes expressions was found in the fish fed with the 0.4% SAN. On the other hand, fish fed on the control diet showed the lowest IL-1β gene coupled with the highest TNF-α and HSP70 genes expressions. After bacterial infection, most of the control fish died with a relative percent of survival of 5.0%; meanwhile feeding gilthead seabream on SAN-enriched diets significantly enhanced their protection against V. alginolyticus infection. Fish fed on the 0.4% SAN diet showed 100% survival. The SAN administration to gilthead seabream especially at the 0.4% level led to significant promotions in antioxidative and immune responses and augment the fish resistance to V. alginolyticus infection.

Vibrio Phage Artemius, a Novel Phage Infecting Vibrio alginolyticus

Vibrio alginolyticus is an important pathogen of marine animals and has been the target of phage therapy applications in marine aquaculture for many years. Here, we report the isolation and partial characterization of a novel species of the Siphoviridae family, the Vibrio phage Artemius. The novel phage was species-specific and could only infect strains of V. alginolyticus. It could efficiently reduce the growth of the host bacterium at various multiplicities of infection as assessed by an in vitro lysis assay. It had a genome length of 43,349 base pairs. The complete genome has double-stranded DNA with a G + C content of 43.61%. In total, 57 ORFs were identified, of which 19 were assigned a predicted function. A genomic analysis indicated that Vibrio phage Artemius is lytic and does not harbor genes encoding toxins and antibiotic resistance determinants.

Vibriosis Outbreaks in Aquaculture: Addressing Environmental and Public Health Concerns and Preventive Therapies Using Gilthead Seabream Farming as a Model System

Bacterial and viral diseases in aquaculture result in severe production and economic losses. Among pathogenic bacteria, species belonging to the Vibrio genus are one of the most common and widespread disease-causing agents. Vibrio infections play a leading role in constraining the sustainable growth of the aquaculture sector worldwide and, consequently, are the target of manifold disease prevention strategies. During the early, larval stages of development, Vibrio species are a common cause of high mortality rates in reared fish and shellfish, circumstances under which the host organisms might be highly susceptible to disease preventive or treatment strategies such as vaccines and antibiotics use, respectively. Regardless of host developmental stage, Vibrio infections may occur suddenly and can lead to the loss of the entire population reared in a given aquaculture system. Furthermore, the frequency of Vibrio-associated diseases in humans is increasing globally and has been linked to anthropic activities, in particular human-driven climate change and intensive livestock production. In this context, here we cover the current knowledge of Vibrio infections in fish aquaculture, with a focus on the model species gilthead seabream (Sparus aurata), a highly valuable reared fish in the Mediterranean climatic zone. Molecular methods currently used for fast detection and identification of Vibrio pathogens and their antibiotic resistance profiles are addressed. Targeted therapeutic approaches are critically examined. They include vaccination, phage therapy and probiotics supplementation, which bear promise in supressing vibriosis in land-based fish rearing and in mitigating possible threats to human health and the environment. This literature review suggests that antibiotic resistance is increasing among Vibrio species, with the use of probiotics constituting a promising, sustainable approach to prevent Vibrio infections in aquaculture.

Mass kills in hatchery-reared European seabass (Dicentrarchus labrax) triggered by concomitant infections of Amyloodinium ocellatum and Vibrio alginolyticus

Amyloodiniosis and vibriosis are serious diseases in European seabass (Dicentrarchus labrax) hatcheries with noticeable high mortality. This study was conducted on tank-cultured D. labrax frys at a private marine hatchery near Mariout Lake (Alexandria, Egypt). Frys showed a high mortality rate (70%), lethargy, darkening, asphyxia, ascites, and velvety skin appearance. Both infectious agents were presumptively identified in all investigated frys. The identities of the two recovered agents were confirmed by molecular assay and phylogenetic analysis. On the tissue level, histopathological examination of skin, splenic, and renal tissue indicated severe alterations due to the direct impacts of both infections. On the cellular level, scanning electron micrographs showed both protozoal and bacterial pathogens on/in gill epithelial cells in solitary and colonial forms. Vibrio alginolyticus showed variable results for tested antibiotics, with a higher sensitivity to florfenicol. A successful control strategy was strictly adopted to overcome infections and stop mortalities. Copper sulphate and hydrogen peroxide were efficiently applied to tank water to overcome A. ocellatum infections. Further, florfenicol was effectively used to overcome systemic V. alginolyticus infections. The efficacy of treatments was confirmed by the absence of infectious agents in randomly collected fish samples. To the best of the authors’ knowledge, this study is one of the earliest Egyptian studies that dealt with the dilemma of mass kills associated with external parasitic/systemic bacterial infections among hatchery-reared European seabass.

Intestinal morphology and microflora to Vibrio alginolyticus in pacific white shrimp (Litopenaeus vannamei)

In recent years, the shrimp farming industry encountered significant economic losses induced by Vibrio alginolyticus. In this study, the influence of Vibrio alginolyticus on intestinal histomorphology and microbiome composition in Litopenaeus vannamei were studied. The results showed that the intestinal mucosal epithelial cells of Vibrio group (VA group) injected only with Vibrio alginolyticus showed large area exfoliation at 12 h, and the tissue morphology of intestine recovered at 48 h. Compared with the control group (CK group), the abundance of Proteobacteria was significantly higher (P < 0.05), while the abundance of Actinobacteria was significantly lower after infection with Vibrio alginolyticus. The abundance of Shewanella in intestinal microbiome of Litopenaeus vannamei was significantly higher at 12 h (P < 0.05), but the abundance of Candidatus_Bacilloplasma was significantly lower at 48 h after infection (P < 0.05). In VA group, the diversity of intestinal microbiome was significantly lower at 12 h, which could be caused by the proliferation of Candidatus_Bacilloplasma and Shewanella. All above findings suggested that the stability of the dynamic balance of microbiome in the intestine helped Litopenaeus vannamei to resist pathogen colonization.

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.

Histopathological and immunological changes in green mussel, Perna viridis, challenged with Vibrio alginolyticus

Understanding of pathogenicity and immunity is crucial in producing disease-resistant cultured mollusk varieties. This study aimed to isolate pathogenic Vibrio alginolyticus from naturally infected Perna viridis, and to determine histopathological and immunological changes after challenge test with the same bacteria. Biochemical tests and 16S rDNA identified the pathogen as V. alginolyticus (99%). Antibiotic susceptibility test showed ampicillin resistance of the pathogen. Pathogenicity assay was conducted by immersing P. viridis in 1.5 × 10⁶ CFU mL^-1V. alginolyticus for 60 min and observed for 5 days. Clinical signs, histopathological and immunological alterations were observed and monitored. Infected groups showed 60% mortality and decreased immunity factors, including total hemocyte count and lysozymes activity. Histopathological examination revealed pathological lesions in the hepatopancreas at 24 h post-challenge and hemocyte proliferation as part of a severe inflammatory reaction. Karyomegaly in the hepatopancreas tissue, concomitant with necrosis demolition of tubules cells, was also observed. V. alginolyticus was determined to be pathogenic to P. viridis, causing mortality as a result of multiple organ lesions and dysfunction in digestive gland and immune organs. This study demonstrated the role of histopathological and immunological parameters as potential biomarkers in assessing vibriosis caused by Vibrio species in green mussel, P. viridis.

Stress adaptation and virulence in Vibrio alginolyticus is mediated by two (p)ppGpp synthetase genes, relA and spoT

Vibrio alginolyticus belongs to gram-negative opportunistic pathogen realm infecting humans and aquatic animals causing severe economic losses. The (p)ppGpp-mediated stringent response is corroborated to stress adaptation and virulence of pathogenic mechanisms. Limited reports are documented for the intricate assessment of (p)ppGpp synthetase genes in combating various stress adaptation and elucidation of virulence in V. alginolyticus remains unraveled. The present assessment comprises of generation of deletion mutants in the (p)ppGpp-deficient strains, ΔrelA (relA gene single mutant) and ΔrelAΔspoT (relA and spoT genes double mutant), and the complemented strains, ΔrelA+ and ΔrelAΔspoT+, were constructed to investigate the pivotal roles of (p)ppGpp synthetase genes in V. alginolyticus, respectively. Amino acid sequence alignment analysis initially revealed that RelA and SpoT possess relatively conserved domains and synthetase activity. Hydrolase activity was emancipated by SpoT alone showing variant mode of action. Compared with the wild type and complemented strains, the relA-deficient strain was more sensitive to amino acid starvation and mupirocin. Interestingly, the deletion of spoT resulted in a significant growth deficiency supplemented with bile salts, 3 % ethanol and heat shock. Rapid growth was observed in the stationary phase upon exposure to cold stress and lower doses of ethanol. Subsequently, disruption of (p)ppGpp synthetase genes caused the decline in swimming motility, enhanced biofilm formation, cell aggregation of V. alginolyticus, and reduced mortality of Litopenaeus vannamei. The expression levels of some virulence-associated genes were quantified affirming consistency established by pleiotropic phenotypes. The results are evident for putative roles of (p)ppGpp synthetase genes attributing essential roles for environmental adaption and virulence regulation in V. alginolyticus.

Mediterranean Aquaculture in a Changing Climate: Temperature Effects on Pathogens and Diseases of Three Farmed Fish Species

Climate change is expected to have a drastic effect on aquaculture worldwide. As we move forward with the agenda to increase and diversify aquaculture production, rising temperatures will have a progressively relevant impact on fish farming, linked to a multitude of issues associated with fish welfare. Temperature affects the physiology of both fish and pathogens, and has the potential to lead to significant increases in disease outbreaks within aquaculture systems, resulting in severe financial impacts. Significant shifts in future temperature regimes are projected for the Mediterranean Sea. We therefore aim to review and discuss the existing knowledge relating to disease outbreaks in the context of climate change in Mediterranean finfish aquaculture. The objective is to describe the effects of temperature on the physiology of both fish and pathogens, and moreover to list and discuss the principal diseases of the three main fish species farmed in the Mediterranean, namely gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax), and meagre (Argyrosomus regius). We will attempt to link the pathology of each disease to a specific temperature range, while discussing potential future disease threats associated with the available climate change trends for the Mediterranean Sea.

Construction and Characterization of an Aeromonas hydrophila Multi-Gene Deletion Strain and Evaluation of Its Potential as a Live-Attenuated Vaccine in Grass Carp

Aeromonas hydrophila is an important pathogen that causes motile Aeromonas septicemia (MAS) in the aquaculture industry. Aerolysin, hemolysin, serine protease and enterotoxins are considered to be the major virulence factors of A. hydrophila. In this study, we constructed a five-gene (aerA, hly, ahp, alt and ast) deletion mutant strain (named Aeromonas hydrophila five-gene deletion strain, AHFGDS) to observe the biological characteristics and detect its potential as a live-attenuated vaccine candidate. AHFGDS displayed highly attenuated and showed increased susceptibility to fish blood and skin mucus killing, while the wild-type strain ZYAH72 was highly virulent. In zebrafish (Danio rerio), AHFGDS showed a 240-fold higher 50% lethal dose (LD50) than that of the wild-type strain. Immunization with AHFGDS by intracelomic injection or immersion routes both provided grass carp (Ctenopharyngodon idella) significant protection against the challenge of the strain ZYAH72 or J-1 and protected the fish organs from serious injury. Further agglutinating antibody titer test supported that AHFGDS could elicit a host-adaptive immune response. These results suggested the potential of AHFGDS to serve as a live-attenuated vaccine to control A. hydrophila infection in aquaculture.

OTUs and ASVs Produce Comparable Taxonomic and Diversity from Shrimp Microbiota 16S Profiles Using Tailored Abundance Filters

The interplay between shrimp immune system, its environment, and microbiota contributes to the organism’s homeostasis and optimal production. The metagenomic composition is typically studied using 16S rDNA profiling by clustering amplicon sequences into operational taxonomic units (OTUs) and, more recently, amplicon sequence variants (ASVs). Establish the compatibility of the taxonomy, α, and β diversity described by both methods is necessary to compare past and future shrimp microbiota studies. Here, we used identical sequences to survey the V3 16S hypervariable-region using 97% and 99% OTUs and ASVs to assess the hepatopancreas and intestine microbiota of L. vannamei from two ponds under standardized rearing conditions. We found that applying filters to retain clusters >0.1% of the total abundance per sample enabled a consistent taxonomy comparison while preserving >94% of the total reads. The three sets turned comparable at the family level, whereas the 97% identity OTU set produced divergent genus and species profiles. Interestingly, the detection of organ and pond variations was robust to the clustering method’s choice, producing comparable α and β-diversity profiles. For comparisons on shrimp microbiota between past and future studies, we strongly recommend that ASVs be compared at the family level to 97% identity OTUs or use 99% identity OTUs, both using tailored frequency filters.

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.

The Role of Feed in Aquatic Laboratory Animal Nutrition and the Potential Impact on Animal Models and Study Reproducibility

Feed plays a central role in the physiological development of terrestrial and aquatic animals. Historically, the feeding practice of aquatic research species derived from aquaculture, farmed, or ornamental trades. These diets are highly variable, with limited quality control, and have been typically selected to provide the fastest growth or highest fecundity. These variations of quality and composition of diets may affect animal/colony health and can introduce confounding experimental variables into animal-based studies that impact research reproducibility.

Intestinal Explant Cultures from Gilthead Seabream (Sparus aurata, L.) Allowed the Determination of Mucosal Sensitivity to Bacterial Pathogens and the Impact of a Plant Protein Diet

The interaction between diet and intestinal health has been widely discussed, although in vivo approaches have reported limitations. The intestine explant culture system developed provides an advantage since it reduces the number of experimental fish and increases the time of incubation compared to similar methods, becoming a valuable tool in the study of the interactions between pathogenic bacteria, rearing conditions, or dietary components and fish gut immune response. The objective of this study was to determine the influence of the total substitution of fish meal by plants on the immune intestinal status of seabream using an ex vivo bacterial challenge. For this aim, two growth stages of fish were assayed (12 g): phase I (90 days), up to 68 g, and phase II (305 days), up to 250 g. Additionally, in phase II, the effects of long term and short term exposure (15 days) to a plant protein (PP) diet were determined. PP diet altered the mucosal immune homeostasis, the younger fish being more sensitive, and the intestine from fish fed short-term plant diets showed a higher immune response than with long-term feeding. Vibrio alginolyticus (V. alginolyticus) triggered the highest immune and inflammatory response, while COX-2 expression was significantly induced by Photobacterium damselae subsp. Piscicida (P. damselae subsp. Piscicida), showing a positive high correlation between the pro-inflammatory genes encoding interleukin 1β (IL1-β), interleukin 6 (IL-6) and cyclooxygenase 2(COX-2).

Identification of Vibrio alginolyticus virulent strain-specific DNA regions by suppression subtractive hybridization and PCR

AIMS

Vibrio alginolyticus was frequently isolated from diseased farmed fish in the coaster waters of Hainan Island over the past two decades. In this study, we attempted to identify candidates of virulent strain-specific DNA regions for this pathogen.

METHODS AND RESULTS

Suppression subtractive hybridization (SSH) and PCR were successively performed between the typical virulent strain and avirulent strain of V. alginolyticus, in which they shared 99·54% homology of 16S rDNAs. Out of 2873 subtracted clones, nine clones were finally indicated to harbour virulent strain-specific DNA fragments. The receivable functions of the major fragments in the nine clones were believed to encode methyl-accepting chemotaxis protein (n = 1), type VI secretion system-associated FHA domain protein TagH (n = 1), diguanylate cyclase (n = 1), AraC family transcriptional regulator (n = 1), ABC-type uncharacterized transport system permease component (n = 1) and hypothetical proteins (n = 4). Two hypothetical proteins contain several disordered regions.

CONCLUSIONS

Some specific DNA regions existed in the virulent strain of V. alginolyticus, and the SSH assay could be a highly sensitive method for identifying virulent regions in pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

This report is the first to describe the identification of virulent strain-specific DNA regions in the V. alginolyticus genome, which is helpful in developing virulent strain-specific rapid detection methods and is a pivotal precondition for clarifying the molecular virulence mechanism of V. alginolyticus.

Metabolic modulation of redox state confounds fish survival against Vibrio alginolyticus infection

Vibrio alginolyticus threatens both humans and marine animals, but hosts respond to V. alginolyticus infection is not fully understood. Here, functional metabolomics was adopted to investigate the metabolic differences between the dying and surviving zebrafish upon V. alginolyticus infection. Tryptophan was identified as the most crucial metabolite, whose abundance was decreased in the dying group but increased in the survival group as compared to control group without infection. Concurrently, the dying zebrafish displayed excessive immune response and produced higher level of reactive oxygen species (ROS). Interestingly, exogenous tryptophan reverted dying rate through metabolome re‐programming, thereby enhancing the survival from V. alginolyticus infection. It is preceded by the following mechanism: tryptophan fluxed into the glycolysis and tricarboxylic acid cycle (TCA cycle), promoted adenosine triphosphate (ATP) production and further increased the generation of NADPH. Meanwhile, tryptophan decreased NADPH oxidation. These together ameliorate ROS, key molecules in excessive immune response. This is further supported by the event that the inhibition of pyruvate metabolism and TCA cycle by inhibitors decreased D. reiro survival. Thus, our data indicate that tryptophan is a key metabolite for the host to fight against V. alginolyticus infection, representing an alternative strategy to treat bacterial infection in an antibiotic‐independent way.

Construction and evaluation of Vibrio alginolyticus ΔclpP mutant, as a safe live attenuated vibriosis vaccine

Vibrio alginolyticus is a common and serious pathogen threatening the progress of coastal aquaculture. ClpP protease has been proved to be closely associated with biofilm formation, stress tolerance, autolysis and virulence in several pathogens. Hence, targeting ClpP may be a potentially viable, attractive option for the preparation of vaccine in preventing vibriosis. In this study, an in-frame deleted mutant strain (ΔclpP) was constructed by allelic exchange mutagenesis to investigate physiological role of clpP in pathogenicity of V. alginolyticus and evaluate its potential as a live attenuated vaccine. The results exhibited that ΔclpP showed no differences in external morphology, growth, swarming motility and ECPase activity. However, ΔclpP represented an increment in biofilm formation, and a decrement in adherence to CIK cells. In addition, virulence of ΔclpP was examined in pearl gentian grouper and was found to be seriously attenuated. ΔclpP induced high antibody titers and provided a valid protection with a relative percent survival value of 83.8% without histopathologic abnormality. Our results indicated ΔclpP showed a great potential to be a live attenuated vaccine.

Outer membrane protein A (OmpA) is a potential virulence factor of Vibrio alginolyticus strains isolated from diseased fish

Vibrio alginolyticus is one of the most serious causative agents of diseases in cultured marine fish and shellfish. However, the characteristics of virulence factors in pathogenic V. alginolyticus are poorly known. To gain insight into fish diseases caused by V. alginolyticus, we carried out two-dimensional gel electrophoresis (2-DE) combined with MALDI-TOF mass spectrometry to identify uniquely expressed proteins in the disease-causing V. alginolyticus. V. alginolyticus strains were isolated from marine environments and diseased fish obtained from southern Thailand. We identified seven unique proteins in the disease-causing V. alginolyticus strain. Among those, the outer membrane protein A (OmpA) had the strongest expression. Therefore, the function of this protein was further analysed. To investigate the role of OmpA protein, an in-frame deletion mutant of ompA was constructed using the homologous recombination method. Although the ompA mutant V. alginolyticus strain (ΔompA) grew normally, the mutant exhibited a significant defect in the swarming ability and the biofilm formation. Furthermore, Galleria mellonella larvae injected with the mutant bacteria had a significantly greater survival percentage than those injected with the wild-type strain, demonstrating that OmpA protein is required for the pathogenicity of V. alginolyticus. Together, this study suggests a potential target for vaccine development against pathogenic V. alginolyticus strain.

Virulence properties of three new Photobacterium species affecting cultured fish

AIMS

Several virulence factors of three new Photobacterium species: Photobacterium toruni, Photobacterium malacitanum and Photobacterium andalusiense associated with diseases of cultured redbanded seabream (Pagrus auriga) were studied. The exoenzymatic activities, adherence and cytotoxic capabilities, and iron-uptake mechanisms were determined both in bacterial extracellular products (ECP) and whole bacterial cells. The histopathology damages provoked on redbanded seabream by the ECP was also studied.

METHODS AND RESULTS

The highest exoenzymatic activities of the ECP were alkaline- and acid-phosphatase, phosphohydrolase and lipase. The ECP were strongly lethal for fish at 4-96 h post-inoculation (p.i). Histological changes were evident at 96 hpi of ECP, affecting head kidney, splenic parenchyma and heart. Cytotoxicity assays, on three fish lines and one human cell line, were conducted using whole bacterial cells and their ECP. The new species tested were cytotoxic only for fish cell lines using whole bacterial cells. Bacterial adherence showed an adherence index moderate on CHSE-214 cell line. All strains showed variable haemolytic activity, and were able to grow under iron-limiting conditions, although the CAS reactivitiy was very low. However, all strains produced high amounts of extracelullar citrate that could be used as iron carrier, and use haem as iron source, except the P. toruni strains because a deletion in the genomic region encoding this ability in all Vibrionaceae members.

CONCLUSIONS

The toxic activity of the bacterial ECPs was thermolabile, and not associated with their thermoresistant lipopolysaccharide content. The virulence of the strains tested could not be related to the haemolytic activity. Iron uptake could be based on the use of endogenous citrate as iron carrier and P. toruni lacks the ability to use haem as iron source.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study analyses for the first time the virulence properties of three new species of Photobacterium pathogenic for fish.

Vibrio Proteases for Biomedical Applications: Modulating the Proteolytic Secretome of V. alginolyticus and V. parahaemolyticus for Improved Enzymes Production

Proteolytic enzymes are of great interest for biotechnological purposes, and their large-scale production, as well as the discovery of strains producing new molecules, is a relevant issue. Collagenases are employed for biomedical and pharmaceutical purposes. The high specificity of collagenase-based preparations toward the substrate strongly relies on the enzyme purity. However, the overall activity may depend on the cooperation with other proteases, the presence of which may be essential for the overall enzymatic activity, but potentially harmful for cells and tissues. Vibrios produce some of the most promising bacterial proteases (including collagenases), and their exo-proteome includes several enzymes with different substrate specificities, the production and relative abundances of which strongly depend on growth conditions. We evaluated the effects of different media compositions on the proteolytic exo-proteome of Vibrio alginolyticus and its closely relative Vibrio parahaemolyticus, in order to improve the overall proteases production, as well as the yield of the desired enzymes subset. Substantial biological responses were achieved with all media, which allowed defining culture conditions for targeted improvement of selected enzyme classes, besides giving insights in possible regulatory mechanisms. In particular, we focused our efforts on collagenases production, because of the growing biotechnological interest due to their pharmaceutical/biomedical applications.

Selection and characterization of ssDNA aptamers specifically recognizing pathogenic Vibrio alginolyticus

Vibrio alginolyticus (V. alginolyticus) is a major opportunistic pathogen to both marine animals and humans, which has also caused heavy economic losses to mariculture. The aim of this study was to develop highly specific aptamers for V. alginolyticus. Single-stranded DNA (ssDNA) aptamers with high binding affinity to viable V. alginolyticus were generated by Systematic Evolution of Ligands by Exponential Enrichment (SELEX) and identified by flow cytometric analysis in this study. The selected aptamers showed high specificity for V. alginolyticus and low apparent binding for other bacteria. The aptamers formed distinct stem-loop structures, which could form the basis of aptamers' specific binding to the target V. alginolyticus. Aptamer VA2 and VA8 showed particularly high binding affinity constant (Kd) of 14.31 ± 4.26 and 90.00 ± 13.51 nM, respectively. The aptamers produced no cytotoxic effects in vitro and in vivo. ssDNA aptamers were successfully selected against the viable bacteria pathogen V. alginolyticus by SELEX. The aptamers selected in this study could be not only applied as specific chemical molecular probes for studying V. alginolyticus pathogenesis to Trachinotus ovatus, but also developing rapid convenient diagnosis assay for V. alginolyticus infection, even when applied to the complex sample matrix, such as food and environment samples.

Isolation and characterization of a virulence related Vibrio alginolyticus strain Wz11 pathogenic to cuttlefish, Sepia pharaonis

Vibrio alginolyticus is a ubiquitous marine opportunistic pathogen that can infect various hosts in marine environment. In the present study, V. alginolyticus strain Wz11 was isolated from diseased cuttlefish, Sepia pharaonis, with 20% of promoted death and high survival capacity in skin mucus and tissue liquid. Its growth, siderophore production, and expressions of haemolysin and swarming related genes were characterized under iron limited conditions. The minimal inhibitory concentration (MIC) of 2,2'-dipyridyl (DP) to V. alginolyticus strain Wz11 was 640 μM. While growth of V. alginolyticus strain Wz11 was inhibited by DP, production of iron-seizing substances, haemolytic activity and swarming motility were increased. Moreover, expressions of haemolysin related genes tlh, tdh and vah and flagellar related genes flgH, fliC, fliD and fliS were also characterized using real-time reverse transcriptase PCR. Expression of tdh was up-regulated to 7.7-fold, while expressions of tlh and vah were down-regulated to 0.016-fold and 0.03-fold, respectively. The expression of fliC, flgH, fliD and fliS was up-regulated to 4.9-, 3.8-, 8.6- and 4.5-fold, respectively. Concluded from our results suggested that V. alginolyticus strain Wz11 was considered as a potential pathogen of S. pharaonis, and iron level played an important role in the production of iron-seizing substances, and activities of haemolysin and bacterial swarming as well as their related gene expressions.

Epigallocatechin-3-gallate protects Kuruma shrimp Marsupeneaus japonicus from white spot syndrome virus and Vibrio alginolyticus

Epigallocatechin-3-gallate (EGCG) is the most abundant catechin in green tea and exhibits potential antibacterial and anticancer activities. In this study, EGCG was used in pathogen-challenge experiments in shrimp to discover its effect on the innate immune system of an invertebrate. Kuruma shrimp Marsupeneaus japonicus was used as an experimental model and challenged with white spot syndrome virus (WSSV) and the Gram-negative bacterium Vibrio alginolyticus. Pathogen-challenge experiments showed that EGCG pretreatment significantly delayed and reduced mortality upon WSSV and V. alginolyticus infection, with VP-28 copies of WSSV also reduced. Quantitative reverse transcription polymerase chain reaction revealed the positive influence of EGCG on several innate immune-related genes, including IMD, proPO, QM, myosin, Rho, Rab7, p53, TNF-alpha, MAPK, and NOS, and we observed positive influences on three immune parameters, including total hemocyte count and phenoloxidase and superoxide dismutase activities, by EGCG treatment. Additionally, results showed that EGCG treatment significantly reduced apoptosis upon V. alginolyticus challenge. These results indicated the positive role of EGCG in the shrimp innate immune system as an enhancer of immune parameters and an inhibitor of apoptosis, thereby delaying and reducing mortality upon pathogen challenge. Our findings provide insight into potential therapeutic or preventive functions associated with EGCG to enhance shrimp immunity and protect shrimp from pathogen infection.

Mucus-Pathogen Interactions in the Gastrointestinal Tract of Farmed Animals

Gastrointestinal infections cause significant challenges and economic losses in animal husbandry. As pathogens becoming resistant to antibiotics are a growing concern worldwide, alternative strategies to treat infections in farmed animals are necessary in order to decrease the risk to human health and increase animal health and productivity. Mucosal surfaces are the most common route used by pathogens to enter the body. The mucosal surface that lines the gastrointestinal tract is covered by a continuously secreted mucus layer that protects the epithelial surface. The mucus layer is the first barrier the pathogen must overcome for successful colonization, and is mainly composed of densely glycosylated proteins called mucins. The vast array of carbohydrate structures present on the mucins provide an important setting for host-pathogen interactions. This review summarizes the current knowledge on gastrointestinal mucins and their role during infections in farmed animals. We examine the interactions between mucins and animal pathogens, with a focus on how pathogenic bacteria can modify the mucin environment in the gut, and how this in turn affects pathogen adhesion and growth. Finally, we discuss analytical challenges and complexities of the mucus-based defense, as well as its potential to control infections in farmed animals.

Multilocus sequence analysis of Vibrionaceae isolated from farmed amberjack and the development of a multiplex PCR assay for the detection of pathogenic species

Since 2011, high mortality rates and symptoms consistent with vibriosis have been observed in farmed amberjack (Seriola dumerili) in Japan. To identify 41 strains isolated from diseased amberjack, a multilocus sequence analysis using nine concatenated genes (ftsZ, gapA, gyrB, mreB, pyrH, recA, rpoA, topA and 16S rRNA) was conducted. Twenty-seven strains were identified as Vibrio harveyi, suggesting an epidemic of V. harveyi infection in amberjack farms. Other strains were identified as Vibrio anguillarum, Vibrio owensii and Photobacterium damselae subsp. damselae. To develop an efficient diagnostic method for vibriosis in amberjack, a multiplex PCR system was developed to identify V. anguillarum, V. harveyi and P. damselae subsp. damselae. The method successfully discriminated between these three bacterial species, with amplification products of 350 bp for V. anguillarum, 545 bp for V. harveyi and 887 bp for P. damselae subsp. damselae and can be used for diagnosis in aquaculture farms.

Construction of a Vibrio alginolyticus hopPmaJ (hop) mutant and evaluation of its potential as a live attenuated vaccine in orange-spotted grouper (Epinephelus coioides)

Vibrio alginolyticus, a bacterial pathogen in fish and humans, expresses a type III secretion system (T3SS) that is critical for pathogen virulence and disease development. However, little is known about the associated effectors (T3SEs) and their physiological role. In this study, the T3SE gene hopPmaJ (hop) was cloned from V. alginolyticus wild-type strain HY9901 and the mutant strain HY9901Δhop was constructed by the in-frame deletion method. The results showed that the deduced amino acid sequence of V. alginolyticus HopPmaJ shared 78-98% homology with other Vibrio spp. In addition, the HY9901Δhop mutant showed an attenuated swarming phenotype and a 2600-fold decrease in the virulence to grouper. However, the HY9901Δhop mutant showed no difference in morphology, growth, biofilm formation and ECPase activity. Finally, grouper vaccinated via intraperitoneal (IP) injection with HY9901Δhop induced a high antibody titer with a relative percent survival (RPS) value of 84% after challenging with the wild-type HY9901. Real-time PCR assays showed that vaccination with HY9901Δhop enhanced the expression of immune-related genes, including MHC-Iα, MHC-IIα, IgM, and IL-1β after vaccination, indicating that it is able to induce humoral and cell-mediated immune response in grouper. These results demonstrate that the HY9901Δhop mutant could be used as an effective live vaccine to combat V. alginolyticus in grouper.

Bacteriophage Interactions with Marine Pathogenic Vibrios: Implications for Phage Therapy

A global distribution in marine, brackish, and freshwater ecosystems, in combination with high abundances and biomass, make vibrios key players in aquatic environments, as well as important pathogens for humans and marine animals. Incidents of Vibrio-associated diseases (vibriosis) in marine aquaculture are being increasingly reported on a global scale, due to the fast growth of the industry over the past few decades years. The administration of antibiotics has been the most commonly applied therapy used to control vibriosis outbreaks, giving rise to concerns about development and spreading of antibiotic-resistant bacteria in the environment. Hence, the idea of using lytic bacteriophages as therapeutic agents against bacterial diseases has been revived during the last years. Bacteriophage therapy constitutes a promising alternative not only for treatment, but also for prevention of vibriosis in aquaculture. However, several scientific and technological challenges still need further investigation before reliable, reproducible treatments with commercial potential are available for the aquaculture industry. The potential and the challenges of phage-based alternatives to antibiotic treatment of vibriosis are addressed in this review.

Molecular and phenotypic characterization of Photobacterium damselae among some marine fishes in Lake Temsah

Photobacterium damselae species are one of the most devastating bacterial pathogens in mariculture worldwide. Some species of Photobacterium are pathogenic for marine animals and human. They are the causative agents of photobacteriosis, formerly known as pasteurellosis. A total of (202) marine fishes of three different species were represented as: seabass (Dicentrarchus labrax), seabream (Sparus aurata) and gray mullet (Mugil capitus) randomly collected from Lake Temsah at Ismailia governorate along the parallel Pelagic road to the lake in the governorate from August 2015 to July 2016. The clinical picture and gross lesions of the diseased fishes were recorded. Isolation and identification of suspected bacteria using traditional and molecular methods. Samples from affected organs were collected for studying the histopathological alterations of these pathogens. Fifty one fishes were found to be infected with Photobacterium damselae subsp. Piscicida. Seabass (Dicentrarchus labrax) was the most infected fish species (23), followed by seabream (Sparus aurata) (18) finally gray mullet (Mugil capitus) was (10). 91fishes were found to be infected with P. damselae subsp. damselae, seabass (Dicentrarchus labrax) was the most infected fish sp. (36), followed by seabream (Sparus aurata) (32), then gray mullet (Mugil capitus) (23). The results indicated that, the total prevalence of P. damselae subsp. piscicida in all examined species (25.24%), the highest seasonal prevalence was recorded in summer season (37.09%) followed by autumn (26%) then spring (20.37%) and winter (11.11%). On the other hand, the total prevalence of P. damselae subsp. damselae in all examined species (45.04%), the highest seasonal prevalence was recorded in summer season (67.74%) followed by autumn (52%) then spring (29.62%) and winter (19.44%). Molecular diagnosis with conventional PCR used to confirm the traditional isolation was applied by using specific primers of two genes (polycapsular saccharide gene and urease C gene). The histopathological studies of naturally infected marine fishes showed severe inflammatory reactions in different organs with accumulation of melanomacrophages and necrosis. The results confirm that P. damselae subspecies damsalea is the most prevalent pathogen between marine fishes, and seabass (Dicentrarchus labrax) was the highly affected marine fishes in this study.

Chronic wounds alter the proteome profile in skin mucus of farmed gilthead seabream

Background

Skin and its mucus are known to be the first barrier of defence against any external stressors. In fish, skin wounds frequently appear as a result of intensive culture and also some diseases have skin ulcers as external clinical signs. However, there is no information about the changes produced by the wounds in the mucosae. In the present paper, we have studied the alterations in the proteome map of skin mucus of gilthead seabream during healing of experimentally produced chronic wounds by 2-DE followed by LC-MS/MS. The corresponding gene expression changes of some identified skin proteins were also investigated through qPCR.

Results

Our study has successfully identified 21 differentially expressed proteins involved in immunity and stress processes as well as other metabolic and structural proteins and revealed, for the first time, that all are downregulated in the skin mucus of wounded seabream specimens. At transcript level, we found that four of nine markers (ighm, gst3, actb and krt1) were downregulated after causing the wounds while the rest of them remained unaltered in the wounded fish. Finally, ELISA analysis revealed that IgM levels were significantly lower in wounded fish compared to the control fish.

Conclusions

Our study revealed a decreased-expression at protein and for some transcripts at mRNA levels in wounded fish, which could affect the functionality of these molecules, and therefore, delay the wound healing process and increase the susceptibility to any infection after wounds in the skin of gilthead seabream.

mcp, aer, cheB, and cheV contribute to the regulation of Vibrio alginolyticus (ND-01) adhesion under gradients of environmental factors

Adhesion is a key virulence factor of pathogens and can be affected by the environment. Our previously research with RNA-seq indicated that mcp, aer, cheB, and cheV might play roles in the regulation of adhesion in Vibrio alginolyticus (ND-01). In order to determine whether and how environmental factors affect adhesion through these genes, gene silencing was performed followed by quantitative real-time PCR (qRT-PCR), RNAi, transmission electron microscopy, and adhesion, capillary, and motility assays to verify how these genes influence adhesion. Silencing these genes led to deficiencies in adhesion, chemotaxis, flagellar assembly, and motility. The expression levels of cheA, cheW, and cheY, which are important genes closely related to the functions of mcp, aer, cheV, and cheB, were significantly downregulated in all of the RNAi groups. The expression of mcp, aer, cheV, and cheB under different gradients of temperature, pH, and salinity and after starvation for various durations was also detected, which showed that these genes were sensitive to certain environmental stresses, particularly pH and starvation. Our results indicated that mcp, aer, cheB, and cheV: (1) are necessary for ND-01 adhesion; (2) play key roles in the bacterial chemotaxis pathway by controlling the expression of downstream genes; (3) might affect adhesion by impacting motility, though motility is not the only route through which adhesion is affected; and (4) contribute to the regulation of ND-01 adhesion in natural environments with different temperatures, pH levels, and salinities as well as after various starvation periods.

In vivo adherence of Flavobacterium psychrophilum to mucosal external surfaces of rainbow trout (Oncorhynchus mykiss) fry

The adherence of Flavobacterium psychrophilum to surfaces of epithelial tissues has been inconclusively suggested as a mechanism, which enables the bacterium to invade the host. Hence, the present study aimed to examine the adherence of the cells of two colony phenotypes, smooth and rough, of F. psychrophilum to mucosal tissues of rainbow trout fry and to test the skin mucus as a nutrient for the growth of F. psychrophilum. Fish were immersed in water containing 10⁶  CFU  mL^-1 F. psychrophilum for each colony phenotype. Mucosal tissue samples from fins, gills, skin and eyes, and swab samples from spleen and kidney were taken and inoculated onto TYES agar plates. Colony phenotypes of F. psychrophilum were identified and number of colonies counted. The results showed that cells of both phenotypes initially (0 h) adhered to all mucosal surfaces, but only the rough cells were still present on tissues 1 h post-immersion. Both phenotypes showed a tissue tropism with the fin tissue being the most adhered. Furthermore, skin mucus promoted the growth of both colony phenotypes. We suggest that the growth of F. psychrophilum cells in skin mucus apparently facilitates the bacterial adherence to mucosal surfaces, and the subsequent invasion into the host.

Molecular characterization and phylogenetic analysis of highly pathogenic Vibrio alginolyticus strains isolated during mortality outbreaks in cultured Ruditapes decussatus juvenile

In the summer of 2008 and 2009, a series of mortalities in growing out seeds of R. decussatus juveniles were occurred in the eastern Tunisian littoral. Nine predominant bacterial strains were isolated from dead and moribund juveniles and characterized as Vibrio alginolyticus. These isolates were subjected to biochemical and molecular characterization. All the Vibrio strains were tested for their susceptibility against the most widely used antibiotic in aquaculture as well as, the assessment of the presence of erythromycin (emrB) and tetracycline (tetS) resistance genes among the tested bacteria. The degree of genetic relatedness between V. alginolyticus strains was evaluated on the basis of the Entero-Bacterial Repetitive Intergenic Consensus (ERIC) and the Random Amplification of Polymorphic DNA-PCR (RAPD-PCR) approaches. We also looked for siderophore activity and the ability to grow under iron limitation. Furthermore, the pathogenic potential of the tested isolates was evaluated using R. decussatus larva and juveniles as infection models. On antimicrobial susceptibility test, Vibrio strains exhibited total resistance to at least four antibiotics. The MICs data revealed that flumequine and oxolinic acid were the most effective antibiotics to control the studied bacteria. Results also showed that studied antibiotics resistance genes were widely disseminated in the genome of V. alginolyticus strains. Both ERIC and RAPD-PCR fingerprinting showed the presence of genetic variation among Vibrio isolates. However, RAPD typing exhibited a higher discriminative potential than ERIC-PCR. Besides, we reported here for the first time the co-production of catechol and hydroxamte by V. alginolyticus species. The challenge experiment showed that most of Vibrio isolates caused high mortality rates for both larva and juveniles at 48-h post-exposure to a bacterial concentration of 10⁶ CFU/ml.

Establishment and characterization of a cell line from the head kidney of golden pompano Trachinotus ovatus and its application in toxicology and virus susceptibility

A cell line derived from the head kidney of golden pompano Trachinotus ovatus (TOHK) was established and characterized in this study. The TOHK cells grew most rapidly at 28° C and the optimum foetal bovine serum concentration in L-15 medium was 10%. The TOHK cells have a diploid chromosome number of 2N = 54. The transfection efficiency of TOHK cells was 7·5% at the 15th passage and 72% at the 40th passage. The transfection efficiency in TOHK cells was high, so these cells are suitable for foreign gene expression. The cytotoxic effects of heavy metals and extracellular products from Vibrio anguillarum and Vibrio alginolyticus were demonstrated in TOHK cells, so this TOHK cell line could also be applied in environmental monitoring of heavy metals and pathogenic bacteria. TOHK cell line showed high virus susceptibility, such as grouper nervous necrosis virus (GNNV) and Singapore grouper iridovirus (SGIV). Then, TOHK cell line could be used for the study of viral pathogenesis and the development of antiviral strategies.

Establishment and characterization of a mid-kidney cell line derived from golden pompano Trachinotus ovatus, a new cell model for virus pathogenesis and toxicology studies

Golden pompano Trachinotus ovatus, a popularly cultured and commercially important marine fish worldwide, has been recognized as a promising candidate for mariculture. However, outbreaks of infectious bacterial or viral diseases and environmental deterioration have led to great economic losses in T. ovatus aquaculture recently. In our research, we established a new mid-kidney cell line, designated as TOK, from golden pompano, T. ovatus. The optimized growth temperature and working concentration of fetal bovine serum (FBS) were 28°C and 10-20%, respectively. Foreign genes could express well in TOK cells. The modal number of TOK cells was 54. The TOK cells were susceptive to Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV), and the virus could propagate in cells. Propagation was verified by qRT-PCR, and virions were observed under electron microscopy. Cytotoxicity analysis revealed that TOK cells were sensitive to different concentrations of extracellular products (ECPs) from Vibrio alginolyticus and V. anguillarum. Moreover, heavy metals (Cd, Cu, and Hg) also showed dose-dependent cytotoxicity to the TOK cell line. We established a mid-kidney cell line from T. ovatus which could be applied to cytotoxicity assays of heavy metals. The newly established TOK cell line possesses great application potential in genetic manipulation, virus-host interaction studies, and toxicity assays of bacterial extracellular products and heavy metals.

Evidence for Cleavage of the Metalloprotease Vsm from Vibrio splendidus Strain JZ6 by an M20 Peptidase (PepT-like Protein) at Low Temperature

Metalloprotease Vsm is a major extracellular virulence factor of Vibrio splendidus. The toxicity of Vsm from V. splendidus strain JZ6 has been characterized, and production of this virulence factor proved to be temperature-regulated. The present study provides evidence that two forms (JZE1 and JZE2) of Vsm protein exist in extracellular products (ECPs) of strain JZ6, and a significant conversion of these two forms was detected by SDS-PAGE and immunoblotting analyses of samples obtained from cells grown at 4, 10, 16, 20, 24, and 28°C. Mass spectroscopy confirmed that JZE1 was composed only of the peptidase_M4 domain of Vsm, and JZE2 contained both the PepSY domain and the peptidase_M4 domain. An M20 peptidase T-like protein (PepTL) was screened from the transcriptome data of strain JZ6, which was considered as a crucial molecule to produce the active Vsm (JZE1) by cleavage of the propeptide. Similar to that of Vsm, PepTL mRNA accumulation was highest at 4°C (836.82-fold of that at 28°C), decreased with increasing of temperature and reached its lowest level at 28°C. Deletion of the gene encoding the PepTL resulted in a mutant strain that did not produce the JZE1 cleavage product. The peptidase activity of PepTL recombinant protein (rPepTL) was confirmed by cleaving the Vsm in ECPs with an in vitro degradation reaction. These results demonstrate that PepTL participates in activating Vsm in strain JZ6 by proteolytic cleavage at low temperature.