Potential threats of microplastics and pathogenic bacteria to the immune system of the mussels Mytilus galloprovincialis.

As one of the main components of marine pollution, microplastics (MPs) inevitably enter the mussel aquaculture environment. At the same time, pathogenic bacteria, especially pathogens such as Vibrio, can cause illness outbreaks, leading to large-scale death of mussels. The potential harm of MPs and pathogenic bacteria to bivalve remains unclear. This study designed two experiments (1) mussels (Mytilus galloprovincialis) were exposed to 100 particles/L or 1,000 particles/L polymethyl methacrylate (PMMA, 17.01 ± 6.74 µm) MPs and 1 × 107 CFU/mL Vibrio parahaemolyticus at the same time (14 days), and (2) mussels were exposed to 100 particles/L or 1,000 particles/L MPs for a long time (30 days) and then exposed to 1 × 107 CFU/mL V. parahaemolyticus to explore the effects of these two stresses on the mussel immune system. The results showed that after the combined exposure of V. parahaemolyticus and MPs, the lysosomal membrane stability of hemocytes decreased, lysozyme activity was inhibited, and hemocytes were induced to produce more lectins and defensins to fight pathogenic invasion. Long-term exposure to MPs caused a large amount of energy consumption in mussels, inhibited most of the functions of humoral immunity, increased the risk of mussel infection with pathogenic bacteria, and negatively affected mussel condition factor, the number of hemocytes, and the number of byssuses. Mussels may allocate more energy to deal with MPs and pathogenic bacterial infections rather than for growth. Above all, MPs exposure can affect mussel immune function or reduce its stress resistance, which in turn has an impact on mollusk farming.

Understanding the role of Francisella halioticida in mussel mortalities in France: an integrative approach.

Since 2014, mass mortalities of mussels Mytilus spp. have occurred in production areas on the Atlantic coast of France. The aetiology of these outbreaks remained unknown until the bacterium Francisella halioticida was detected in some mussel mortality cases. This retrospective study was conducted to assess the association between F. halioticida and these mussel mortalities. Mussel batches (n = 45) from the Atlantic coast and English Channel were selected from archived individual samples (n = 863) collected either during or outside of mortality events between 2014 and 2017. All mussels were analysed by real-time PCR assays targeting F. halioticida; in addition, 185 were analysed using histological analysis and 178 by 16S rRNA metabarcoding. F. halioticida DNA was detected by real-time PCR and 16S rRNA metabarcoding in 282 and 34 mussels, respectively. Among these individuals, 82% (real-time PCR analysis) and 76% (16S rRNA metabarcoding analysis) were sampled during a mortality event. Histological analyses showed that moribund individuals had lesions mainly characterized by necrosis, haemocyte infiltration and granulomas. Risk factor analysis showed that mussel batches with more than 20% of PCR-positive individuals were more likely to have been sampled during a mortality event, and positive 16S rRNA metabarcoding batches increased the strength of the association with mortality by 11.6 times. The role of F. halioticida in mussel mortalities was determined by reviewing the available evidence. To this end, a causation criteria grid, tailored to marine diseases and molecular pathogen detection tools, allowed more evidence to be gathered on the causal role of this bacterium in mussel mortalities.

Bacterial c-di-GMP signaling gene affects mussel larval metamorphosis through outer membrane vesicles and lipopolysaccharides.

Biofilms serve as crucial cues for settlement and metamorphosis in marine invertebrates. Within bacterial systems, c-di-GMP functions as a pivotal signaling molecule regulating both biofilm formation and dispersion. However, the molecular mechanism of how c-di-GMP modulates biofilm-induced larval metamorphosis remains elusive. Our study reveals that the deletion of a c-di-GMP related gene in Pseudoalteromonas marina led to an increase in the level of bacterial c-di-GMP by knockout technique, and the mutant strain had an enhanced ability to produce more outer membrane vesicles (OMVs) and lipopolysaccharides (LPS). The mutant biofilms had higher induction activity for larval metamorphosis in mussels Mytilus coruscus, and OMVs play a major role in the induction activity. We further explored the function of LPS in OMVs. Extracted LPS induced high larval metamorphosis rate, and LPS content were subject to c-di-GMP and LPS-biosynthesis gene. Thus, we postulate that the impact of c-di-GMP on biofilm-induced metamorphosis is mediated through OMVs and LPS.

Multidrug resistant Vibrio spp. identified from mussels farmed for human consumption in Central Italy.

AIMS: This study investigated phenotypic and genotypic antimicrobial resistance profiles of Vibrio strains identified from Mytilus galloprovincialis farmed for human consumption in the Adriatic Sea Central Italy. METHODS AND RESULTS: A total of 475 mussels (M. galloprovincialis) were involved in the present study, and culture-dependent microbiological methods permitted to identify a total of 50 Vibrio strains that were tested for antibiotic susceptibility followed by the genetic determinant detections. Antibiograms showed resistance against ampicillin (36.0%), amoxicillin-clavulanic acid (30.0%), gentamycin (14.0%), and imipenem (18.0%). Biomolecular assays amplified a total of 264 antibiotic resistance genes harbored by both susceptible and resistant Vibrio species. Among resistance genes, aacC2 (62.0%) and aadA (58.0%) for aminoglycosides, blaTEM (54.0%) for beta-lactams, qnrS (24.0%) for quinolones, tetD (66.0%) for tetracyclines, and vanB (60.0%) for glycopeptides were mainly amplified by PCR assays. CONCLUSIONS: Vibrio genus is involved in the antibiotic resistance phenomenon diffusion in the aquatic environments, as demonstrated by the harboring of many genetic determinants representing a kind of genetic "dark world".

Impact of epizootics on mussel farms: Insights into microbiota composition of Mytilus species.

Outbreaks of marine mussel mortality on French farms could have different aetiologies. One of them implies Vibrio splendidus strains. Beyond the involvement of this pathogen, there is considerable evidence that diseases often result from interactions between several microbes and the host. In this study, we explored the bacterial communities associated with mussel species and the surrounding water collected from a mussel farm affected by mortalities. The microbiota of Mytilus edulis, Mytilus galloprovincialis and their hybrids displayed an abnormal abundance of Proteobacteria, in particular the genera Vibrio, Cobetia and Arcobacter. Despite the dysbiosis, the Mediterranean mussel showed a different microbiota profile with a higher richness and presence of the phylum Bacteroidetes. Bipartite network analyses at the level of bacteria families confirmed this finding and showed that the microbiomes of M. edulis and the hybrids tended to cluster together. In addition, injection of mussels with the virulent V. splendidus induced less mortality rate in M. galloprovincialis compared to the other Mytilus sp. suggesting a better resistance of the Mediterranean mussel to infection. Our findings point to a probable aetiology of pathobiome-mediated disease in mussels. To fully understand this phenomenon, more knowledge is needed on the roles of pathobiotic systems and their development during disease establishment.

Two-Component System Response Regulator ompR Regulates Mussel Settlement through Exopolysaccharides.

The outer membrane protein (OMP) is a kind of biofilm matrix component that widely exists in Gram-negative bacteria. However, the mechanism of OMP involved in the settlement of molluscs is still unclear. In this study, the mussel Mytilus coruscus was selected as a model to explore the function of ompR, a two-component system response regulator, on Pseudoalteromonas marina biofilm-forming capacity and the mussel settlement. The motility of the ΔompR strain was increased, the biofilm-forming capacity was decreased, and the inducing activity of the ΔompR biofilms in plantigrades decreased significantly (p < 0.05). The extracellular α-polysaccharide and ß-polysaccharide of the ΔompR strain decreased by 57.27% and 62.63%, respectively. The inactivation of the ompR gene decreased the ompW gene expression and had no impact on envZ expression or c-di-GMP levels. Adding recombinant OmpW protein caused the recovery of biofilm-inducing activities, accompanied by the upregulation of exopolysaccharides. The findings deepen the understanding of the regulatory mechanism of bacterial two-component systems and the settlement of benthic animals.

Pathology, epidemiology, and phylogeny of mussel egg disease due to the microsporidian Steinhausia mytilovum (Field, 1924) in the Mediterranean mussel (Mytilus galloprovincialis).

Microsporidia are well known fungal pathogens of aquatic animals. However, the taxonomy of microsporidia is generally poorly resolved, which has consequently constrained our understanding of their pathology and epidemiology in marine animals. To date, microsporidia have been reported in both bivalves and gastropods, and microsporidia from mollusks have been classified in different genera. Despite ongoing work to better describe these genera, including detailed microscopic and ultrastructural images, so far we lack information on microsporidian phylogeny and pathogenicity of species within these genera. Here we investigate the microsporidian parasite Steinhausia mytilovum associated with the mussel, Mytilus galloprovincialis, in natural beds and farms along coast of southern Italy. A survey of M. galloprovincialis was conducted in 13 mussel farms and one natural bed between 2009 and 2020. We found the presence of S. mytilovum in 10 of the investigated farms, with a prevalence ranging between 14 and 100% of female mussels, depending on the population and season in which they were sampled. The parasite developed in the oocytes within a sporophorous vesicle (SV) where it produced 1-3 spores per cell, both in the cytoplasm and in the nucleus. Stenhausia mytilovum elicited an infiltrative (24.8%) or a strong capsular inflammatory response (43.4%) at gonadal follicles and surrounding vesicular connective tissue, in some cases accompanied by gonadal atresia (24.8%), leading to loss of gonadal architecture. In 7% of cases no reaction was observed. Ultrastructural observations revealed a mitochondrial re-organization to interact with all the phases of parasite development; the mitochondria were arranged outside the parasitophorous vesicle (PV) or directly interacting with the spore inside vesicle. There are five taxonomic clades of microsporidians as identified by SSU ribosomal gene sequence data. Maximum likelihood analysis assigned S. mytilovum within the Clade IV, defined as the Class Terresporidia, with closest genetic relationship (83.6% identity) to an undetermined invertebrate ovarian microsporidian. The constant presence, prevalence, and severity of S. mytilovum in coastline populations of M. galloprovincialis populations in southern Italy may indirectly reflect immunocompetence at both individual and population levels.

Changes in extractive components and bacterial flora in live mussels Mytilus galloprovincialis during storage at different temperatures.

To estimate the quality of mussels during storage, the mortality, succinate dehydrogenase (SDH) activity, extractive components, viable bacterial count (VBC), and bacterial flora of live mussels were investigated. The hierarchical cluster analysis, based on extractive components and VBC, taste active value (TAV), and equivalent umami concentration (EUC), suggested that metabolite composition, bacterial, and taste changing patterns of samples stored at 5 and 10°C differed from those stored at 0°C. The mortality of mussels stored at 5 and 10°C was lower than those at 0°C. The gills of live mussels stored at 0°C for more than 7 days exhibited significantly lower SDH activity than those stored at 5 and 10°C. There was no significant difference in EUC among the samples stored at different temperatures, but a significantly higher TAV of Ala and succinic acid was observed in live mussels after 12 days of storage at 5 and 10°C than in those stored at 0°C. Next-generation sequencing analysis showed that samples stored at 5 and 10°C lost bacterial diversity, and their bacterial flora changed compared to that before storage. Considering these results, the most suitable storage condition to maintain high quality for live mussels is 5°C for less than 7 days.

Host Species and Environment Shape the Gut Microbiota of Cohabiting Marine Bivalves.

Pacific oysters (Crassostrea gigas) and Mediterranean mussels (Mytilus galloprovincialis) are commercially important marine bivalves that frequently coexist and have overlapping feeding ecologies. Like other invertebrates, their gut microbiota is thought to play an important role in supporting their health and nutrition. Yet, little is known regarding the role of the host and environment in driving these communities. Here, bacterial assemblages were surveyed from seawater and gut aspirates of farmed C. gigas and co-occurring wild M. galloprovincialis in summer and winter using Illumina 16S rRNA gene sequencing. Unlike seawater, which was dominated by Pseudomonadata, bivalve samples largely consisted of Mycoplasmatota (Mollicutes) and accounted for >50% of the total OTU abundance. Despite large numbers of common (core) bacterial taxa, bivalve-specific species (OTUs) were also evident and predominantly associated with Mycoplasmataceae (notably Mycoplasma). An increase in diversity (though with varied taxonomic evenness) was observed in winter for both bivalves and was associated with changes in the abundance of core and bivalve-specific taxa, including several representing host-associated and environmental (free-living or particle-diet associated) organisms. Our findings highlight the contribution of the environment and the host in defining the composition of the gut microbiota in cohabiting, intergeneric bivalve populations.

Pollution Indicators and HAB-Associated Halophilic Bacteria Alongside Harmful Cyanobacteria in the Largest Mussel Cultivation Area in Greece.

Taking into consideration the essential contribution of Mytilus galloprovincialis farming, it is of rising importance to add knowledge regarding bacterial species occurrence in water samples from aquaculture zones from the point of view of both the organism and public health. In the present study, we investigated the bacterial community existing in water samples from six Mytilus galloprovincialis aquaculture areas in the Thermaikos gulf, northern Greece, that may provoke toxicity in aquatic organisms and humans and may indicate environmental pollution in mussel production as well as algal blooms. Bacterial species were identified molecularly by sequencing of a partial 16s rRNA segment and were analyzed phylogenetically for the confirmation of the bacterial taxonomy. The results obtained revealed the presence of four bacterial genera (Halomonas sp., Planococcus sp., Sulfitobacter sp., and Synechocystis sp.). Members of the Halomonas and Sulfitobacter genera have been isolated from highly polluted sites, Planococcus bacteria have been identified in samples derived directly from plastic debris, and Synechocystis bacteria are in line with microcystin detection. In this context, the monitoring of the bacteria community in mussel aquaculture water samples from the Thermaikos gulf, the largest mussel cultivation area in Greece, represents an indicator of water pollution, microplastics presence, algal blooms, and toxin presence.

Impact of different enzymes on biofilm formation and mussel settlement.

Enzymes have been known to impact the biofilm forming capacity. However, how the enzymes mediate the biofilm formation and macrofouling remains little known. Here, we investigated the effects of the three kinds of proteases, four kinds of glycosidases and one kind of lipase on the detachment of biofilms of Shewanella marisflavi ECSMB14101, identified biofilm total proteins response to enzyme treatments, and then tested the effects of biofilms treated with enzymes on the settlement of the mussel Mytilus coruscus plantigrades. The results showed that the cell density of bacteria in biofilms formed at different initial bacterial density were noticeably reduced after treating with all tested enzymes, and Neutrase and α-Amylase exhibited best removing efficiency of > 90%. Bacterial total proteins in S. marisflavi biofilm noticeably reduced or disappeared after treated by Alcalase. For the settlements of the mussel M. coruscus plantigrades, inducing capacities of S. marisflavi biofilm were noticeably suppressed and downregulation was > 75% at the initial density of 5 × 106 cells/cm2. Thus, the tested enzymes could effectively remove the adhered bacterial cell, inhibit the biofilm formation and finally suppress the mussel settlement. Our findings extend novel knowledge to developing eco-friendly approach to control micro- and macro-fouling.

Detection of Antimicrobial and Heavy-Metal Resistance Genes in Aeromonas spp. Isolated from Hard-Shelled Mussel (Mytilus Coruscus).

Hard-shelled mussel (Mytilus coruscus) is a popular seafood in South Korea because of its delicacy and high nutritional value. Our study aimed to identify antimicrobial and heavy-metal resistance determinants in Aeromonas isolates from marketed hard-shelled mussel in South Korea. A total of 33 Aeromonas species were isolated, and antimicrobial disk diffusion test was done to observe antimicrobial resistance patterns. In addition, broth microdilution test was performed to determine resistance to heavy-metals. PCR amplification was done to detect resistance genes. High resistance to amoxicillin (100.0%), ampicillin (93.9%), rifampicin (78.8%), and cephalothin (48.5%) was observed where least resistance to other antimicrobials was also detected. In addition, the isolates showed high resistance to cadmium (Cd) (57.6%), and 42.4% and 27.3% were resistant to chromium (Cr) and copper (Cu). The occurrence of antimicrobial resistance genes, such as blaTEM, blaSHV, blaCTX-M, tetB, tetE, and intI1 genes, was observed in 9 (27.3%), 8 (24.2%), 8 (24.2%), 6 (18.2%), 5 (15.2%), and 9 (27.3%) isolates, respectively. Also, heavy-metal resistance genes, czcA, copA, and merA were detected in 17 (51.5%), 11 (33.3%), and 7 (21.2%) of the isolates, respectively. The results suggest that mussels are a reservoir of multidrug and heavy-metal-resistant Aeromonas spp.

Vibrio ulleungensis sp. nov., isolated from Mytilus coruscus.

A Gram-stain-negative, rod-shaped, motile via polar flagellum, facultatively aerobic, light-yellow, bacterium (designated 188UL20-2T) was isolated from a mussel sample of Mytilus coruscus collected on Ulleung Island, Ulleung-gun, Gyeongsangbuk-do, Republic of Korea. On the basis of 16S rRNA gene sequencing results, strain 188UL20-2T clustered with species of the genus Vibrio and appeared closely related to Vibrio marisflavi DSM 23086T (96.59%), Vibrio variabilis DSM 26147T (96.57%), Vibrio penaeicida DSM 14398T (96.37%) and Vibrio litoralis DSM 17657T (95.97%). The average nucleotide identity and digital DNA-DNA hybridization values between strain 188UL20-2T and its closest related strain were 71.3 and 16.4%, indicating that 188UL20-2T represents a novel species of the genus Vibrio. Growth occurred at 18-37 °C on MA medium in the presence of 1-4% NaCl (w/v) and at pH 5.0-10.0. The DNA G+C content of the genomic DNA was 45.4 mol%, and ubiquinone-8 (Q-8) was the major respiratory quinone. The major cellular fatty acids (>5%) were C16:1 ω6c and/or C16:1 ω7c (summed feature 3), C18:1 ω7c and/or C18:1 ω6c (summed feature 8), C16:0, C16:0 iso, C14:0, C14:0 iso and C12:0. The polar lipids consisted of phosphatidylglycerol, phosphatidylethanolamine, two unidentified phospholipids, unidentified aminophospholipid, unidentified glycolipid and seven unidentified lipids. Physiological and biochemical characteristics indicated that strain 188UL20-2T represents a novel species of the genus Vibrio, for which the name Vibrio ulleungensis sp. nov. is proposed. The type strain is 188UL20-2T (=KACC 22258T=LMG 32202T).

Tetrodotoxins (TTXs) and Vibrio alginolyticus in Mussels from Central Adriatic Sea (Italy): Are They Closely Related?

Tetrodotoxins (TTXs), potent neurotoxins, have become an increasing concern in Europe in recent decades, especially because of their presence in mollusks. The European Food Safety Authority published a Scientific Opinion setting a recommended threshold for TTX in mollusks of 44 µg equivalent kg-1 and calling all member states to contribute to an effort to gather data in order to produce a more exhaustive risk assessment. The objective of this work was to assess TTX levels in wild and farmed mussels (Mytilus galloprovincialis) harvested in 2018-2019 along the coastal area of the Marche region in the Central Adriatic Sea (Italy). The presence of Vibrio spp. carrying the non-ribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) genes, which are suspected to be involved in TTX biosynthesis, was also investigated. Out of 158 mussel samples analyzed by hydrophilic interaction liquid chromatography coupled with tandem mass spectrometry (HILIC-MS/MS), 11 (7%) contained the toxins at detectable levels (8-26 µg kg-1) and 3 (2%) contained levels above the EFSA safety threshold (61-76 µg kg-1). Contaminated mussels were all harvested from natural beds in spring or summer. Of the 2019 samples, 70% of them contained V. alginolyticus strains with the NRPS and/or PKS genes. None of the strains containing NRPS and/or PKS genes showed detectable levels of TTXs. TTXs in mussels are not yet a threat in the Marche region nor in Europe, but further investigations are surely needed.

Development of a semi-quantitative PCR assay for the detection of Francisella halioticida and its application to field samples.

The current study describes the development and application of a TaqMan® real-time PCR assay for the detection of the bacterium Francisella halioticida. Previously, detection of F. halioticida is relied on bacterial culture and conventional PCR; however, the real-time PCR provides many advantages because it is faster, less labour-intensive and reduces the risk of cross-contamination. DNA samples from mussels collected in April 2020 from seven sites in northern Brittany (France) were tested using the newly developed real-time PCR assay. The objective was to screen for the presence of F. halioticida during spring mortality events. The bacterium was detected in 71.4% of the samples tested and was present at all sites except for Saint-Brieuc and Mont-Saint-Michel, two sites which were not concerned by mortality at the time of sampling. Less than a month later, Saint-Brieuc was affected by unusual mortalities and F. halioticida was detected in almost all mussels (81.25%). The findings from this study provide further evidence indicating that F. halioticida may be contributing to mussel mortalities; however, a direct causal relationship has not yet been established. The real-time PCR assay developed in this study allows for rapid, specific and sensitive detection of F. halioticida which should prove useful for future studies concerning the involvement of this bacterium with shellfish mortalities.

Integrated analysis of miRNAome and transcriptome reveals miRNA-mRNA network regulation in Vibrio alginolyticus infected thick shell mussel Mytilus coruscus.

The thick shell mussel Mytilus coruscus has developed into a model species for studying the interaction between molluscs and environmental stimuli. Herein, integrated analysis of miRNAome and transcriptome was performed to reveal miRNA-mRNA network regulation in Vibrio alginolyticus infected M. coruscus. There have detected some histological abnormalities in digestive gland and gills of V. alginolyticus challenged mussels, ascertaining the effective irritation by the present bacterial strain. A total of 265 novel miRNAs were finally predicted, of which 26 were differentially expressed miRNAs (DEMs). Additionally, 667 differentially expressed genes (DEGs) were detected, which may be potentially associated with innate immune response to V. alginolyticus infection. A regulatory network linked to 22 important pathways and 16 DEMs and 34 OGs was constructed. Some traditional immune-related signaling pathways such as toll-like receptor signaling pathway (TLR) signaling pathway, transforming growth factor-beta (TGF-beta) signaling pathway, peroxisome, phagosome, lysosome, mammalian target of rapamyoin (mTOR) signaling pathway were linked to specific miRNAs and genes in this network. Further, interactional relationship between certain miRNAs and TLR pathway was dissected, which the results predicted that a number of TLRs and TLR-associated signaling genes including TLR1, TLR2, TLR4, TLR6, IRAK1, TRAF6, MAPK, and IL-17 were negatively regulated by novel_miR_11, novel_miR_145, novel_miR_196, novel_miR_5, novel_miR_163 and novel_miR_217 in the TLR pathway. Additionally, interactional relationship between novel_miR_145 and TLR2 was validated by laboratory experiment. The integrated analysis of mRNA and microRNA deep sequencing data exhibited a sophisticated miRNA-mRNA regulation network in M. coruscus in response to V. alginolyticus challenge, which shed a new light on the underlying mechanism of molluscan confronting bacterial infection.

Deep-sea bacteria trigger settlement and metamorphosis of the mussel Mytilus coruscus larvae.

Bacteria from coast seawaters are widely known to induce larval recruitment of many invertebrates. However, whether and how deep-sea bacteria, that play crucial roles in the ecological and biogeochemical cycles, promote larval recruitment remains little known. Here, the interaction between deep-sea bacterial biofilms (BFs) and Mytilus coruscus larvae was tested. All these nine deep-sea bacterial isolates triggered planktonic-sessile transition, and the highest percentage of post-larvae was observed in Virgibacillus sp. 1 BF. Except for Pseudomonas sp. 3, Pseudoalteromonas sp. 32 and Bacillus sp. 13, other BF cell  densities were significantly related to their corresponding inductive efficiency. The deep-sea Virgibacillus sp. 1 BF's cue that triggers planktonic-sessile transition was uncovered. Treating Virgibacillus sp. 1 BFs through physic-chemical approaches reduced inducing impact and cell survival. The conditioned water collaborated with formalin-fixed Virgibacillus sp. 1 BF hoisted planktonic-sessile transition efficiency in comparison to each one alone. Thus, two signals derived from deep-sea bacteria trigger planktonic-sessile transition in M. coruscus. This finding firstly demonstrates that deep-sea bacteria has good potential for application in the mussel seed production and provides novel insight to clarify the bacteria-mussel interaction.

Hemocytes released in seawater act as Trojan horses for spreading of bacterial infections in mussels.

Global warming has been associated with increased episodes of mass mortality events in invertebrates, most notably in bivalves. Although the spread of pathogens is one of multiple factors that contribute to such mass mortality events, we don't fully understand the pathophysiological consequences of sea warming on invertebrates. In this work, we show that in temperature stress conditions, circulating hemocytes in mussels leave the hemolymph to gain access to the intervalvar fluid before being released in seawater. External hemocytes can survive for several hours in seawater before entering other mussels. When infected by bacteria, externally-infected hemocytes can enter naive mussels and promote bacterial dissemination in the host. These results reveal the existence of a new opportunistic mechanism used by pathogens to disseminate in marine ecosystems. Such mechanisms may explain how thermal anomalies triggered by global warming can favor episodic mass mortality observed in recent years in marine ecosystem.

Vibrio parahaemolyticus control in mussels by a Halobacteriovorax isolated from the Adriatic sea, Italy.

This study evaluated the application of a Halobacteriovorax isolated from water of the Adriatic Sea (Italy) in controlling V. parahaemolyticus in mussels (Mytilus galloprovincialis). Two 72 h laboratory-scale V. parahaemolyticus decontamination experiments of mussels were performed. The test microcosm of experiment 1 was prepared using predator/prey free mussels experimentally contaminated with Halobacteriovorax/V. parahaemolyticus at a ratio of 103 PFU/105 CFU per ml, while that of experiment 2 using mussels naturally harbouring Halobacteriovorax that were experimentally contaminated with 105 CFU per ml of V. parahaemolyticus. For experiment 1, was also tested a control microcosm only contaminated with 105 CFU per ml of V. parahaemolyticus.. Double layer agar plating and pour plate techniques were used to enumerate Halobacteriovorax and V. parahaemolyticus, respectively. 16 S rRNA analysis was used to identify Halobacteriovorax. For both experiments in the test microcosm the concentration of prey remained at the same level as that experimentally added, i.e. 5 log for the entire analysis period. In experiment 1, V. parahaemolyticus counts in mussels were significantly lower in the test microcosm than the control with the maximum difference of 2.2 log at 24 h. Results demonstrate that Halobacteriovorax can modulate V. parahaemolyticus level in the mussels. The public impact of V. parahaemolyticus in bivalves is relevant and current decontamination processes are not always effective. Halobacteriovorax is a suitable candidate in the development of a biological approach to the purification of V. parahaemolyticus in mussels.

Immune-responsive gene 1 (IRG1) and dimethyl itaconate are involved in the mussel immune response.

Immune-responsive gene 1 (irg1) is a gene that is well-conserved among different taxa and is highly expressed in the mussel Mytilus galloprovincialis at the constitutive level. The expression of this gene increases after a bacterial infection, primarily in haemocytes. irg1 catalyses the production of itaconic acid from cis-aconitic acid in the Krebs cycle. Recently, itaconate has been revealed as an immune metabolite involved in macrophage polarization. In this work, we studied the effects of exogenous dimethyl itaconate (DI) on mussels in vitro and in vivo at relevant previously described endogenous concentrations and in mussels infected with Vibrio splendidus. DI did not have adverse effects on the haemocytes viability, apoptotic cells, proliferation and phagocytic activity; however, haemocyte size, velocity and accumulated distance were decreased. The antibacterial activity of DI in vitro and in vivo was observed with high concentrations of DI, that is, 30 and 50 mM, respectively. Furthermore, DI inhibited total ROS, increased mitochondrial ROS and modulated antioxidant genes, such as SOD and CAT, related to Nrf2 activation. In this research, we have demonstrated some important pathways in haemocytes in which itaconate can be involved after its production in a bacterial infection.