The susceptibility of Irish-grown and Galician-grown Manila clams, Ruditapes philippinarum, to Vibrio tapetis and Brown Ring Disease

Mitochondria are essential for antibacterial extracellular trap formation mediated by zymosan in hemocytes of Ruditapes philippinarum

The formation of extracellular traps (ETs) is an important innate immune mechanism that serves to combat different invading pathogens. In this study, zymosan significantly induced the formation of ETs in the hemocytes of Ruditapes philippinarum, and this effect was accompanied by translocation of the mitochondria to the cell surface. Zymosan stimulation clearly induced an increase in intracellular ROS and MPO production and an overexpression of ROS-related genes (PI3K, AKT and HIF). In response to the ROS burst, the mitochondrial membrane potential decreased, and the mitochondrial permeability transition pore opened. Conversely, mitochondrial superoxide inhibitor (Mito-TEMPO) significantly inhibited the formation of ETs, suggesting that mitochondrial ROS were necessary for the formation of ETs. In addition, we found that zymosan-induced ETs showed antibacterial activities against gram-negative and gram-positive bacteria, such as Vibrio anguillarum, Vibrio harveyi, Escherichia coli and Micrococcus luteus. Taken together, these findings elucidated a new antibacterial approach for R. philippinarum and highlighted the role of mitochondria in the formation of zymosan-induced ETs.

AHL-lactonase expression in three marine emerging pathogenic Vibrio spp. reduces virulence and mortality in brine shrimp (Artemia salina) and Manila clam (Venerupis philippinarum)

Bacterial infectious diseases produced by Vibrio are the main cause of economic losses in aquaculture. During recent years it has been shown that the expression of virulence genes in some Vibrio species is controlled by a population-density dependent gene-expression mechanism known as quorum sensing (QS), which is mediated by the diffusion of signal molecules such as N-acylhomoserine lactones (AHLs). QS disruption, especially the enzymatic degradation of signalling molecules, known as quorum quenching (QQ), is one of the novel therapeutic strategies for the treatment of bacterial infections. In this study, we present the detection of AHLs in 34 marine Vibrionaceae strains. Three aquaculture-related pathogenic Vibrio strains, V. mediterranei VibC-Oc-097, V. owensii VibC-Oc-106 and V. coralliilyticus VibC-Oc-193 were selected for further studies based on their virulence and high production of AHLs. This is the first report where the signal molecules have been characterized in these emerging marine pathogens and correlated to the expression of virulence factors. Moreover, the results of AHL inactivation in the three selected strains have been confirmed in vivo against brine shrimps (Artemia salina) and Manila clams (Venerupis philippinarum). This research contributes to the development of future therapies based on AHL disruption, the most promising alternatives for fighting infectious diseases in aquaculture.

Disentangling the population structure and evolution of the clam pathogen Vibrio tapetis

Vibrio tapetis is a fastidious slow-growing microorganism that causes the Brown Ring Disease in clams. Recently, two subspecies for this bacterial pathogen have been proposed. We have developed a multilocus sequence typing scheme and performed evolutionary studies of V. tapetis population using the great majority of isolates of V. tapetis obtained worldwide until now (30 isolates). V. tapetis constitutes a high polymorphic population, showing low diversity indexes and some genetic discontinuity among the isolates. Mutation events are more frequent than recombination, although both are approximately equally important for genetic diversification. In fact, the divergence between subspecies occurred exclusively by mutation but the diversity observed among isolates of the same subspecies appeared to be generated mostly by recombination. Between the subspecies, genetic distance is very high and almost no recurrent gene flow exists. This pathogen displays a non-clonal population structure with an ancient spatial segregation population and some degree of geographical isolation, followed by a population expansion, at least for V. tapetis subsp. tapetis. A database from this study was created and hosted on publmlst.org ( http://pubmlst.org/vtapetis/ ).

Uptake and retention of Vibrio parahaemolyticus in a cohabitating population of Ruditapes decussatus and Ruditapes philippinarum under experimental conditions

The presence of Vibrio parahaemolyticus in bivalve mollusc is an important cause of foodborne illnesses, and their levels are influenced by environmental changes, such as temperature and salinity. Clams are common species in estuaries and are used in environmental monitoring programmes. Present study compared the uptake and retention of nonpathogenic V. parahaemolyticus by two species of clam (Ruditapes decussatus and R. philippinarum), cohabitating in a closed system. Results showed no significant differences were found between both species of clams. Bacterial levels are following a similar trend with values between 3.48 and 3.70 log CFU/g for R. decussatus and between 3.15 and 3.49 log CFU/g for R. philippinarum. So, in the absence of water renewal, high and stable levels of V. parahaemolyticus were observed in cultured clams after exposure. Changes in physical parameters should be taken into account to design surveillance programmes in bivalves, and sampling should focus on species that have faster filtration rates at that water temperature since they potentially represent the worst-case scenario.

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.

DjlA, a membrane-anchored DnaJ-like protein, is required for cytotoxicity of clam pathogen Vibrio tapetis to hemocytes

DjlA is an inner membrane cochaperone belonging to the DnaJ family, which has been shown to be involved in Legionella sp. pathogenesis. In this study, we explored the role of this protein in the physiology and virulence of Vibrio tapetis, the etiological agent of brown ring disease (BRD) in Manila clam (Ruditapes philippinarum). Analysis of the djlA locus in V. tapetis revealed a putative organization in an operon with a downstream gene that we designated duf924(Vt), which encodes a conserved protein with an unknown function and has homologues in bacteria and eukaryotes. djlA mutants displayed a reduced growth rate and showed an important loss of cytotoxic activity against R. philippinarum hemocytes in vitro, which could be restored by extrachromosomal expression of wild-type djlA(Vt) but not duf924(Vt). These results are in keeping with the potential importance of DjlA for bacterial pathogenicity and open new perspectives for understanding the mechanism of action of this protein in the novel V. tapetis-R. philippinarum interaction model.