Vibrio tapetis-like strain isolated from introduced Manila clams Ruditapes philippinarum showing symptoms of brown ring disease in Norway

Autophagic processes in Mytilus galloprovincialis hemocytes: Effects of Vibrio tapetis

Autophagy is a highly conserved and regulated catabolic process involved in maintaining cell homeostasis in response to different stressors. The autophagic machinery is also used as an innate immune mechanism against microbial infection. In invertebrates, that lack acquired immunity, autophagy may thus play a key role in the protection against potential pathogens. In aquatic molluscs, evidence has been provided for induction of autophagy by starvation and different environmental stressors; however, no information is available on autophagic pathways in the immune cells, the hemocytes. In this work, the autophagic processes were investigated in the hemocytes of the marine bivalve, the mussel Mytilus galloprovincialis. The effects of classical inducers/inhibitors of mammalian autophagy were first tested. Rapamycin induced a decrease in lysosomal membrane stability-LMS that was prevented by the autophagy inhibitor Wortmannin. Increased MDC fluorescence and expression of LC3-II were also observed. Moreover, responses to in vitro challenge with the bivalve pathogen Vibrio tapetis were evaluated. Mussel hemocytes were unable to activate the immune response towards V. tapetis; however, bacterial challenge induced a moderate decrease in LMS, corresponding to lysosomal activation but no cytotoxicity; the effect was prevented by Wortmannin. TEM observations showed that V. tapetis resulted in rapid formation of autophagosomes and autolysosomes. Accordingly, increased LC3-II expression, decreased levels of phosphorylated mTor and of p62 were observed. The results represent the first evidence for autophagic processes in bivalve hemocytes in response to bacterial challenge, and underline the protective role of autophagy towards potential pathogenic vibrios.

Vibrio tapetis Displays an Original Type IV Secretion System in Strains Pathogenic for Bivalve Molluscs

The Brown Ring Disease (BRD) caused high mortality rates since 1986 in the Manila clam Venerupis philippinarum introduced and cultured in Western Europe from the 1970s. The causative agent of BRD is a Gram-Negative bacterium, Vibrio tapetis, which is also pathogenic to fish. Here we report the first assembly of the complete genome of V. tapetis CECT4600^T, together with the genome sequences of 16 additional strains isolated across a broad host and geographic range. Our extensive genome dataset allowed us to describe the pathogen pan- and core genomes and to identify putative virulence factors. The V. tapetis core genome consists of 3,352 genes, including multiple potential virulence factors represented by haemolysins, transcriptional regulators, Type I restriction modification system, GGDEF domain proteins, several conjugative plasmids, and a Type IV secretion system. Future research on the coevolutionary arms race between V. tapetis virulence factors and host resistance mechanisms will improve our understanding of how pathogenicity develops in this emerging pathogen.

Ancient DNA analysis identifies marine mollusc shells as new metagenomic archives of the past

Marine mollusc shells enclose a wealth of information on coastal organisms and their environment. Their life history traits as well as (palaeo-) environmental conditions, including temperature, food availability, salinity and pollution, can be traced through the analysis of their shell (micro-) structure and biogeochemical composition. Adding to this list, the DNA entrapped in shell carbonate biominerals potentially offers a novel and complementary proxy both for reconstructing palaeoenvironments and tracking mollusc evolutionary trajectories. Here, we assess this potential by applying DNA extraction, high-throughput shotgun DNA sequencing and metagenomic analyses to marine mollusc shells spanning the last ~7,000 years. We report successful DNA extraction from shells, including a variety of ancient specimens, and find that DNA recovery is highly dependent on their biomineral structure, carbonate layer preservation and disease state. We demonstrate positive taxonomic identification of mollusc species using a combination of mitochondrial DNA genomes, barcodes, genome-scale data and metagenomic approaches. We also find shell biominerals to contain a diversity of microbial DNA from the marine environment. Finally, we reconstruct genomic sequences of organisms closely related to the Vibrio tapetis bacteria from Manila clam shells previously diagnosed with Brown Ring Disease. Our results reveal marine mollusc shells as novel genetic archives of the past, which opens new perspectives in ancient DNA research, with the potential to reconstruct the evolutionary history of molluscs, microbial communities and pathogens in the face of environmental changes. Other future applications include conservation of endangered mollusc species and aquaculture management.

Microbial Diseases of Bivalve Mollusks: Infections, Immunology and Antimicrobial Defense

A variety of bivalve mollusks (phylum Mollusca, class Bivalvia) constitute a prominent commodity in fisheries and aquacultures, but are also crucial in order to preserve our ecosystem’s complexity and function. Bivalve mollusks, such as clams, mussels, oysters and scallops, are relevant bred species, and their global farming maintains a high incremental annual growth rate, representing a considerable proportion of the overall fishery activities. Bivalve mollusks are filter feeders; therefore by filtering a great quantity of water, they may bioaccumulate in their tissues a high number of microorganisms that can be considered infectious for humans and higher vertebrates. Moreover, since some pathogens are also able to infect bivalve mollusks, they are a threat for the entire mollusk farming industry. In consideration of the leading role in aquaculture and the growing financial importance of bivalve farming, much interest has been recently devoted to investigate the pathogenesis of infectious diseases of these mollusks in order to be prepared for public health emergencies and to avoid dreadful income losses. Several bacterial and viral pathogens will be described herein. Despite the minor complexity of the organization of the immune system of bivalves, compared to mammalian immune systems, a precise description of the different mechanisms that induce its activation and functioning is still missing. In the present review, a substantial consideration will be devoted in outlining the immune responses of bivalves and their repertoire of immune cells. Finally, we will focus on the description of antimicrobial peptides that have been identified and characterized in bivalve mollusks. Their structural and antimicrobial features are also of great interest for the biotechnology sector as antimicrobial templates to combat the increasing antibiotic-resistance of different pathogenic bacteria that plague the human population all over the world.

Diseases of north European wrasse (Labridae) and possible interactions with cohabited farmed salmon, Salmo salar L

There have been several reported studies of wrasse health but none of these has shown transmission of wrasse diseases when stocked with farmed Atlantic salmon. Most of the studies have focussed on bacterial and parasite issues, including treatment of bacterial diseases with antibiotics and vaccination of wrasse. Classical and atypical furunculosis have been reported in wrasse following stress, and wrasse have been susceptible to vibrio infection. Further study is required on the vaccination of wrasse for furunculosis with latent carrier status to maximize survival. There are studies on viral diseases such as infectious pancreatic necrosis, infectious salmon anaemia and pancreas disease and although these did not give any undue concern for salmon health, there is also scope for further study in this area. Resident parasite communities of wrasse are largely host-specific and do not appear to be a threat to salmon. Given that wrasse have not, to date, been a vector of disease in salmon, attention should be placed on maintaining best practice in cohabiting wrasse with salmon. Other issues that should be addressed are good welfare of wrasse in pens and identifying measures of this, the identification of losses of wrasse in pens, being alert to potential emerging diseases through health screening of mortalities and assessing the risks associated with carrying forward wrasse from one salmon production cycle to the next. Issues of exploitation by fishing on wild wrasse stocks and improved biosecurity may be addressed by the increased movement by the industry to the stocking of farmed wrasse.

A novel molluscan sigma-like glutathione S-transferase from Manila clam, Ruditapes philippinarum: cloning, characterization and transcriptional profiling

Glutathione S-transferases (GSTs) are versatile enzymes, act as primary intracellular detoxifiers and contribute to a broad range of physiological processes including cellular defense. In this study, a full-length cDNA representing a novel sigma-like GST was identified from Manila clam, Ruditapes philippinarum (RpGSTσ). RpGSTσ (884 bp) was found to possess an open reading frame of 609 bp. The encoded polypeptide (203 amino acids) had a predicted molecular mass of 23.21 kDa and an isoelectric point of 7.64. Sequence analysis revealed two conserved GST domain profiles in N- and C-termini. Alignment studies revealed that the identity between deduced peptides of RpGSTσ and known GSTσ members was relatively low (<35%), except a previously identified Manila clam GSTσ isoform (87.2%). Phylogenetic analysis indicated that RpGSTσ clustered together with molluscan GSTσ homologs, which were closely related to insect GSTσs. The RpGSTσ was subsequently cloned and expressed as recombinant protein, in order to characterize its biological activity. The recombinant RpGSTσ exhibited characteristic glutathione conjugating catalytic activity toward 1-chloro-2,4-dinitrobenzene, 3,4-dichloronitrobenzene and ethacrynic acid. It had an optimal pH and temperature of 8.0 and 35 °C, respectively. Expression profiles under normal conditions and in response to lipopolysaccharide-, poly I:C- and Vibrio tapetis-challenges were also investigated. RpGSTσ demonstrated a differential tissue distribution with robust transcription in gills of normal animals. We explored potential association of GSTσ in cellular defense during bacterial infection and found that in challenged clams, RpGSTσ gene was significantly induced in internal and external tissues, in conjunction with manganese- as well as copper-zinc superoxide dismutase (MnSOD and CuZnSOD) genes. Moreover, the induction was remarkably higher in hemocytes than in gill. Collectively, our findings suggested that RpGSTσ could play a significant role in cellular defense against oxidative stress caused by bacteria, in conjunction with other antioxidant enzymes, such as SODs.

Mitochondrial thioredoxin-2 from Manila clam (Ruditapes philippinarum) is a potent antioxidant enzyme involved in antibacterial response

Thioredoxin (TRx) is a ubiquitous protein involved in the regulation of multiple biological processes. The TRx-2 isoform is exclusively expressed in mitochondria, where it contributes to mitochondrial redox state maintenance. In the present study, a novel thioredoxin-2 gene was identified in the Manila clam, Ruditapes philippinarum. The full-length sequence of RpTRx-2 (1561 bp) consists of a 498 bp coding region encoding a 166 amino acid protein. The N-terminal region of RpTRx-2 harbors a mitochondrial localization signal (56 amino acids), while the C-terminal portion contains the characteristic (89)WCGPC(93) catalytic active site. Phylogenetic analysis revealed that RpTRx-2 is closest to its ortholog from abalone. The broad distribution pattern of RpTRx-2 mRNA in healthy animal tissues implicates a generally significant function in normal clam physiology. The transcription level of RpTRx-2, however, is highest in hemocytes. Lipopolysaccharide and Vibrio tapetis bacterium caused up-regulation of the RpTrx-2 transcript levels in gill and hemocytes. Interestingly, clam manganese superoxide dismutase (MnSOD) mRNA levels in hemocytes elicited a corresponding response to these immune challenges. RpTRx-2 was recombinantly expressed in Escherichia coli BL21 (DE3) and used in insulin disulfide reduction assay as well as metal-catalyzed oxidation assay to elucidate its antioxidant property by reducing substrate and protecting super-coiled DNA from oxidative damage through free radical scavenging, respectively. Collectively, our data indicated that RpTRx-2, a mitochondrial TRx-2 family member, is an antioxidant enzyme that may be involved in antibacterial defense of clams.

First molluscan theta-class Glutathione S-Transferase: identification, cloning, characterization and transcriptional analysis post immune challenges

Glutathione S-Transferases (GSTs) are multifunctional cytosolic isoenzymes, distinctly known as phase II detoxification enzymes. GSTs play a significant role in cellular defense against toxicity and have been identified in nearly all organisms studied to date, from bacteria to mammals. In this study, we have identified a full-length cDNA of the theta class GST from Ruditapes philippinarum (RpGSTθ), an important commercial edible molluscan species. RpGSTθ was cloned and the recombinant protein expressed, in order to study its biochemical characteristics and determine its physiological activities. The cDNA comprised an ORF of 693 bp, encoding 231 amino acids with a predicted molecular mass of 27 kDa and an isoelectric point of 8.2. Sequence analysis revealed that RpGSTθ possessed characteristic conserved domains of the GST_N family, Class Theta subfamily (PSSM: cd03050) and GST_C_family Super family (PSSM: cl02776). Phylogenetic analysis showed that RpGSTθ evolutionarily linked with other theta class homologues. The recombinant protein was expressed in Escherichia coli BL21(DE3) cells and the purified enzyme showed high activity with GST substrates like CDNB and 4-NBC. Glutathione dependent peroxidase activity of GST, investigated with cumene hydroperoxide as substrate affirmed the antioxidant property of rRpGSTθ. By quantitative PCR, RpGSTθ was found to be ubiquitously expressed in all tissues examined, with the highest levels occurring in gills, mantle, and hemocytes. Since GSTs may act as detoxification enzymes to mediate immune defense, the effects of pathogen associated molecular pattern, lipopolysaccharide and intact Vibrio tapetis bacteria challenge on RpGSTθ gene transcription were studied. Furthermore, the RpGSTθ expression changes induced by immune challenges were similar to those of the antioxidant defense enzyme manganese superoxide dismutase (RpMnSOD). To our knowledge, RpGSTθ is the first molluscan theta class GST reported, and its immune-related role in Manila clam may provide insights into potential therapeutic targets for protecting this important aquaculture species.

Cytosolic thioredoxin from Ruditapes philippinarum: molecular cloning, characterization, expression and DNA protection activity of the recombinant protein

Thioredoxin (TRx) is a small redox protein that plays significant roles in protection against oxidative stress and in cell homeostasis by maintaining oxidized proteins in a reduced state. Here, we describe the isolation and characterization of a full-length TRx cDNA sequence from manila clam, Ruditapes philippinarum and named it as RpTRx. The full length sequence consists of 1416 bp with an open reading frame of 318 bp encoding for 106 amino acids. RpTRx protein harbors evolutionarily-conserved TRx active site (32)WCGPC(36). Phylogenetic analysis revealed a close proximity of RpTRx with the orthologue in Japanese scallop, Chlamys farreri. RpTRx was found to be constitutively expressed in hemocyte, gill, mantle, foot and siphon indicating a general role in physiological processes in various tissues. With regard to a potential role in immune responses, the RpTRx mRNA was found to be up-regulated in hemocytes after bacterial (Vibrio tapetis) and lipopolysaccharide (LPS) challenge at 3h post-infection (p.i.); a wavering increase was observed up to 96 h p.i. for LPS challenge and 48 h p.i. for bacterial challenge. Thus, RpTRx may function as an intracellular antioxidant to protect the cells against ROS induced by LPS and bacterial challenges. Indeed, when recombinant RpTRx protein (rRpTRx) was over-expressed in Escherichiacoli Rosetta gami(TM) (DE3) cells, it was able to scavenge free radicals and protect super-coiled DNA from oxidative damage induced by a metal-ion catalyzed oxidation reaction. In summary, RpTRx plays an essential role in cellular defense and maintenance of homeostasis in the manila clam.

Evidence for the role of horizontal transfer in generating pVT1, a large mosaic conjugative plasmid from the clam pathogen, Vibrio tapetis

The marine bacterium Vibrio tapetis is the causative agent of the brown ring disease, which affects the clam Ruditapes philippinarum and causes heavy economic losses in North of Europe and in Eastern Asia. Further characterization of V. tapetis isolates showed that all the investigated strains harbored at least one large plasmid. We determined the sequence of the 82,266 bp plasmid pVT1 from the CECT4600(T) reference strain and analyzed its genetic content. pVT1 is a mosaic plasmid closely related to several conjugative plasmids isolated from Vibrio vulnificus strains and was shown to be itself conjugative in Vibrios. In addition, it contains DNA regions that have similarity with several other plasmids from marine bacteria (Vibrio sp., Shewanella sp., Listonella anguillarum and Photobacterium profundum). pVT1 contains a number of mobile elements, including twelve Insertion Sequences or inactivated IS genes and an RS1 phage element related to the CTXphi phage of V. cholerae. The genetic organization of pVT1 underscores an important role of horizontal gene transfer through conjugative plasmid shuffling and transposition events in the acquisition of new genetic resources and in generating the pVT1 modular organization. In addition, pVT1 presents a copy number of 9, relatively high for a conjugative plasmid, and appears to belong to a new type of replicon, which may be specific to Vibrionaceae and Shewanelleacae.

Interactive effects of metal contamination and pathogenic organisms on the introduced marine bivalve Ruditapes philippinarum in European populations

In natural environment, marine organisms are concomitantly exposed to pollutants and multiple disease agents resulting in detrimental interactions. The present study evaluated interactive effects of metal contamination (cadmium) and pathogenic organisms (trematode parasites Himasthla elongata and pathogenic bacteria Vibrio tapetis) singularly and in combination on the bivalve Ruditapes philippinarum, an introduced species to Europe, under laboratory controlled conditions. After 7 days, metal bioaccumulation and pathogen load were analyzed as well as metallothionein (MT) response and hemocyte concentrations and activities. Results showed that infection by opportunistic pathogens affects metal accumulation, leading to maximal Cd accumulation in co-infected clams. Among stressors only V. tapetis induced significant effects on immune parameters whereas a particular interaction "trematode-bacteria" was shown on MT responses. Despite low trematode infection in agreement with the resistant status of R. philippinarum to these macroparasites, significant interaction with bacteria and metal occurred. Such results highlight the necessity of taking pathogens into account in ecotoxicological studies.

Interactive effects of metal contamination and pathogenic organisms on the marine bivalve Cerastoderma edule

The present study evaluated the interactive effects of cadmium contamination and pathogenic organisms (trematodes Himasthla elongata and bacteria Vibrio tapetis) singularly and in combination during 7 days on the bivalve Cerastoderma edule. Some defense-related activities were analyzed such as genetic expression, metallothionein and immune responses. Trematode metacercarial infection, similar whatever the treatment, induced the strongest responses of immune parameters. Particularly, the interaction between cadmium and parasite exposures induced unusual responses on gene expression and immune responses. No effect of bacterial challenge appeared on bivalve responses, nevertheless a strong mortality of V. tapetis infected cockles occurred between 7 and 14 days. Cadmium bioaccumulation was significantly modulated by both pathogenic organisms. Furthermore, an antagonistic effect of trematodes and bacteria was shown on metal bioaccumulation of co-infected cockles. These results highlighted the importance of considering the multiplicity of perturbation sources in coastal ecosystems to assess the health status of organisms.

Diversity and pathogenecity of Vibrio species in cultured bivalve molluscs

Shellfish production is seriously affected by bacterial pathogens that cause high losses in hatcheries and in the aquaculture sector. A number of Vibrio species are considered important pathogens and have provoked severe mortality outbreaks. The pathologies caused by vibrios in bivalves have been described since the 1960s; however, over recent years, successive episodes of high mortality have been recorded due to these microorganisms. The present work provides an updated overview of the different studies performed in relation with the diversity of Vibrio spp. associated to bivalves. Special attention is given to the main Vibrio diseases and implicated species affecting the different life stages of cultured bivalves.