Responses of Mytilus galloprovincialis hemocytes to environmental strains of Vibrio parahaemolyticus, Vibrio alginolyticus, Vibrio vulnificus

Effects of Vibrio vulnificus and Microcystis aeruginosa co-exposures on microplastic accumulation and depuration in the Eastern Oyster (Crassostrea virginica)

Microplastics are ubiquitous in the aquatic environment, and bivalves such as the Eastern oyster (Crassostrea virginica) can accumulate these particles directly from the water column. Bivalves are concurrently exposed to pathogenic and toxin-producing bacteria, including Vibrio spp. and Microcystis spp., which have been shown to adversely impact filtration rates. Exposure to these bacteria could thus affect oysters' ability to accumulate and depurate microplastics. As climate change creates conditions that favor Vibrio spp. and Microcystis spp. growth in estuaries, it is increasingly important to understand how these co-occurring biotic stressors influence microplastic contamination in bivalves. The objective of this study was to examine how co-exposures to Vibrio vulnificus and Microcystis aeruginosa influence microplastic accumulation and depuration in Eastern oysters. Oysters were exposed to nylon microplastics (5000 particles L-1) and either V. vulnificus, M. aeruginosa, or both species (104 colony-forming units or cells mL-1, respectively) and sampled over time up to 96 h. Following exposure, remaining oysters were allowed to depurate in clean seawater and sampled over time for up to 96 h. Microplastic concentrations in oysters were quantified and compared among treatments, and rate constants for uptake (ku) and depuration (kd) were calculated using nonlinear regression and two-compartment kinetic models. Overall, microplastic concentrations in oysters exposed to V. vulnificus (X‾ = 2.885 ± 0.350 (SE) particles g-1 w.w.) and V. vulnificus with M. aeruginosa (X‾ = 3.089 ± 0.481 particles g-1 w.w.) were higher than oysters exposed to M. aeruginosa (X‾ = 1.540 ± 0.235 particles g-1 w.w.) and to microplastics alone (X‾ = 1.599 ± 0.208 particles g-1 w.w.). Characterizing microplastic accumulation and depuration in oysters co-exposed to these biotic stressors is an important first step in understanding how contaminant loads in bivalves can change. With this research, the efficacy of depuration for commonly-consumed seafood species can be estimated.

Pathological and oxidative stress responses of Mytilus galloprovincialis to Vibrio mediterranei infection: An in vivo challenge

Since the identification of Vibrio mediterranei as a causative agent in mass mortalities of pen shells across the Mediterranean, elucidating its pathogenicity, virulence, and interactions with other bivalves has gained importance. While the cellular and immune responses of bivalves to various Vibrio species have been extensively studied, the infectious characteristics of this Vibrio species, particularly in the context of pen shell outbreaks, remain unclear for other bivalves. Therefore, to evaluate its pathogenicity, we investigated the histological and oxidative effects on the Mediterranean mussel (Mytilus galloprovincialis), a key species in aquaculture. Two distinct infection setups were established: one involving the inoculation of seawater with the bacterial isolate and another involving direct injection of the bacteria into the mussels. After a 24-h exposure period, histological evaluations were conducted on the mantle, gill, and digestive gland tissues of the mussels. Additionally, measurements of superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase (GST), and lipid peroxidation levels were performed in the gill and digestive gland tissues. Oxidative responses were significantly elevated in both infection setups compared to the control group, with the directly injected samples exhibiting the highest oxidative responses (p < 0.05). Histological findings indicated that tissue-specific responses to host-pathogen interactions were consistent under both infection conditions. Notable observations included intense hemocytic infiltration in tissues, epithelial hyperplasia, and vacuolization in the gills, as well as focal necrotic areas in the digestive gland. The findings of this study indicate that V. mediterranei, a relatively novel pathogen, can provoke significant acute immune responses and tissue-level reactions in M. galloprovincialis, a species that is both widely distributed and vital to the food chain. These insights into the potential susceptibility of mussels underscore the need for further comprehensive research and inform the development of effective management strategies.

Responses of Mytilus galloprovincialis to challenge with environmental isolates of the potential emerging pathogen Malaciobacter marinus

Bacteria of the Arcobacter-like spp. represent emerging foodborne zoonotic pathogens in humans and animals. Their increasing presence in seafood, suggesting higher occurrence in seawater due to marine pollution, is raising some environmental concern. Although Arcobacter is frequently detected in diseased oysters and stressed bivalve species, no data are available so far on its potential pathogenicity or interactions with the immune system of the bivalve host. In this work, responses to challenge with two strains of Malaciobacter marinus IRTA-19-131 and IRTA-19-132, R1 and R2), isolated from adult Crassostrea gigas during a mortality event in 2019 in Spain, were investigated in the mussel Mytilus galloprovincialis. In vivo experiments were performed in larvae (48 h post-fertilization), and in adult mussels at 24 h post-injection, in order to evaluate the pathogenicity for early developmental stages, and the hemolymph immune responses, respectively. Both R1 and R2 were moderately pathogenic to early larvae, with significant decreases in the development of normal D-veligers from 10⁴ and 10³ CFU/mL, respectively. In adults, both strains decreased hemocyte lysosomal membrane stability (LMS), and stimulated extracellular defense responses (ROS production and lysozyme activity). The interactions between mussel hemocytes and M. marinus were investigated in in vitro short-term experiments (30-90 min) using the R1 strain (10⁶-10⁸ CFU/mL). R1 decreased LMS and induced lysosomal enlargement, but not cell detachment or death, and stimulated extracellular ROS production and lysozyme release, confirming in vivo data. Moreover, lysosomal internalization and degradation of bacteria were observed, together with changes in levels of activated mTor and LC3, indicating phagocytic activity. Overall, the results indicate the activation of both extracellular and intracellular immune defenses against M. marinus R1. Accordingly, these responses resulted in a significant hemolymph bactericidal activity, with a large contribution of hemolymph serum. The results represent the first data on the potential pathogenicity of Arcobacter isolated from a shellfish mortality to bivalve larvae and adults, and on their interactions with the immune system of the host.

Gender Differences in Hemocyte Immune Parameters of Hong Kong Oyster Crassostrea hongkongensis During Immune Stress

Gender differences in individual immune responses to external stimuli have been elucidated in many invertebrates. However, it is unclear if gender differences do exist in the Hong Kong oyster Crassostrea hongkongensis, one of the most valuable marine species cultivated along the coast of South China. To clarify this, we stimulated post-spawning adult C. hongkongensis with Vibrio harveyi and lipopolysaccharide (LPS). Gender-based differences in some essential functional parameters of hemocytes were studied via flow cytometry. Obvious gender-, subpopulation-, and immune-specific alterations were found in the hemocyte immune parameters of C. hongkongensis. Three hemocyte subpopulations were identified: granulocytes, semi-granulocytes, and agranulocytes. Granulocytes, the chief phagocytes and major producers of esterase, reactive oxygen species, and nitric oxide, were the main immunocompetent hemocytes. Immune parameter alterations were notable in the accumulation of granulocyte esterase activities, lysosomal masses, nitric oxide levels, and granulocyte numbers in male oysters. These results suggest that post-spawning-phase male oysters possess a more powerful immune response than females. Gender and subpopulation differences in bivalve immune parameters should be considered in the future analysis of immune parameters when studying the impact of pathogenic or environmental factors.

Innate immune responses and metabolic alterations of mud crab (Scylla paramamosain) in response to Vibrio parahaemolyticus infection

Vibrio parahaemolyticus is one of the major pathogens caused diseases in cultured mud crab (Scylla paramamosain). Mud crabs lack an adaptive immune system, their defenses depend almost on innate immunity. Evaluation of the molecular responses of mud crabs to pathogens is essential for control of disease occurrence in farmed animals. In this study, the impacts of V. parahaemolyticus on immunity-related genes and metabolites in mud crabs of different groups (PG, SG and MG refer to controlled, survival and moribund groups, respectively) were investigated. Our results revealed that V. parahaemolyticus infection stimulated significant expressions of immune-related genes (prophenoloxidase, alpha 2-macroglobulin, lysosomal-associated membrane protein, Rab5, C-type lectin B and anti-lipopolysaccharide factor 5) in the MG within 72 h post-infection. The ATP content was significantly reduced in all tissues except muscle of moribund mud crabs. A total of 668 metabolites (including 190 down-regulated and 145 up-regulated) were identified and assigned to 77 pathways in both SG and MG. Metabolites involved in the saturated fatty acid are up-regulated, whereas unsaturated fatty acid and amino acid metabolisms are down-regulated in the immune system of mud crabs during the bacterial infection in MG. Furthermore, a reduction of hemocyte number and an increase of microbial abundance was found in MG. Our results demonstrated that V. parahaemolyticus induced death of mud crabs through reducing the metabolites associate with energy biosynthesis and innate immune system (i.e. proliferation of hemocyte and melanization), resulting in decrease of ATP in different tissues and failed to clearance of pathogens, respectively. The findings of this study provide a basic information of the responses of mud crab on bacterial infection, which is essential for prevention and control of diseases in mud crab aquaculture.

Responses of Mytilus galloprovincialis to challenge with the emerging marine pathogen Vibrio coralliilyticus

Vibrio coralliilyticus has emerged as a coral pathogen of concern throughout the Indo-Pacific reef. The interest towards understanding its ecology and pathogenic potential has increased since V. coralliilyticus was shown to be strongly virulent also for other species; in particular, it represents a serious threat for bivalve aquaculture, being one of the most important emerging pathogen responsible for oyster larval mortalities worldwide. V. coralliilyticus has a tightly regulated temperature-dependent virulence and it has been related to mass mortalities events of benthic invertebrates also in the temperate northwestern Mediterranean Sea. However, no data are available on the effects of V. coralliilyticus in the mussel Mytilus galloprovincialis, the most abundant aquacultured species in this area. In this work, responses of M. galloprovincialis to challenge with V. coralliilyticus (ATCC BAA-450) were investigated. In vitro, short term responses of mussel hemocytes were evaluated in terms of lysosomal membrane stability, bactericidal activity, lysozyme release, ROS and NO production, and ultrastructural changes, evaluated by TEM. In vivo, hemolymph parameters were measured in mussels challenged with V. coralliilyticus at 24h p.i. Moreover, the effects of V. coralliilyticus on mussel early embryo development (at 48 hpf) were evaluated. The results show that both in vitro and in vivo, mussels were unable to activate immune response towards V. coralliilyticus, and that challenge mainly induced lysosomal stress in the hemocytes. Moreover, V. coralliilyticus showed a strong and concentration-dependent embryotoxicity. Overall, the results indicate that, although M. galloprovincialis is considered a resistant species to vibrio infections, the emerging pathogen V. coralliilyticus can represent a potential threat to mussel aquaculture.

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.

Polyvalent protective immunogens identified from outer membrane proteins of Vibrio parahaemolyticus and their induced innate immune response

Vaccines are the most economic, efficient and environment-friendly agents in protecting host against bacterial infection. In aquaculture, polyvalent vaccines targeting more than one bacterial specie are highly demanded due to the presence of various types of bacterial pathogens in farming environment. Here eighteen genes encoding outer membrane proteins of Vibrio parahaemolyticus were cloned and expressed. The expressed recombinant proteins were used for antiserum preparation. Passive and active immune protection of the antiserum and recombinant proteins was investigated in the zebrafish model. Two recombinant proteins, VP1667 and VP2369, showed effective immune protection against at least two genera of bacteria, Vibrio (V. parahaemolyticus and V. alginolyticus), Pseudomonas (P. fluorescens) or/and Aeromonas (A. hydrophila), and thereby are potential polyvalent vaccine candidates to defend against bacterial infection in fish farming. Furthermore, the mechanisms for the two polyvalent vaccines in triggering immune response were explored. Antiserum to VP1667 or VP2369 was not cross-reacted with P. fluorescens and A. hydrophila, whereas both recombinant proteins induced significant innate immune response. Comparatively, VP1667 stimulates stronger lymphokine and monokine, and VP2369 induces stronger humoral immune response, while both produce similar NF-κB, COX-2, TLR-1 and TLR-3 expression. Our results identify two polyvalent vaccines and demonstrate characteristics features of their cross-protection at the content of the innate immune response.