Abstract
Healthy oysters are inhabited by abundant microbial communities that vary with environmental conditions and coexist with immunocompetent cells in the circulatory system. In Crassostrea gigas oysters, the antimicrobial response, which is believed to control pathogens and commensals, relies on potent oxygen-dependent reactions and on antimicrobial peptides/proteins (AMPs) produced at low concentrations by epithelial cells and/or circulating hemocytes. In non-diseased oysters, hemocytes express basal levels of defensins (Cg-Defs) and proline-rich peptides (Cg-Prps). When the bacterial load dramatically increases in oyster tissues, both AMP families are driven to sites of infection by major hemocyte movements, together with bactericidal permeability/increasing proteins (Cg-BPIs) and given forms of big defensins (Cg-BigDef), whose expression in hemocytes is induced by infection. Co-localization of AMPs at sites of infection could be determinant in limiting invasion as synergies take place between peptide families, a phenomenon which is potentiated by the considerable diversity of AMP sequences. Besides, diversity occurs at the level of oyster AMP mechanisms of action, which range from membrane lysis for Cg-BPI to inhibition of metabolic pathways for Cg-Defs. The combination of such different mechanisms of action may account for the synergistic activities observed and compensate for the low peptide concentrations in C. gigas cells and tissues. To overcome the oyster antimicrobial response, oyster pathogens have developed subtle mechanisms of resistance and evasion. Thus, some Vibrio strains pathogenic for oysters are equipped with AMP-sensing systems that trigger resistance. More generally, the known oyster pathogenic vibrios have evolved strategies to evade intracellular killing through phagocytosis and the associated oxidative burst.
Highlights
The Pacific oyster Crassostrea gigas is a marine invertebrate belonging to the family Ostreidae (Mollusca, Bivalvia) with a worldwide distribution from Japan to occidental countries in Europe and America (Guo et al, 2008)
This abundance in non-diseased animals could be relevant of a protective function and raises several questions on oyster biology, immunity, and homeostasis: Does an oyster-specific microbial community exist? How does the oyster microflora coexist with immunocompetent cells in the circulatory system? How can the oyster immune system discriminate between pathogens and commensals? In invertebrates, pathogen recognition, and disease control are mediated by an innate immune system, in which reactive oxygen species (ROS) and antimicrobial peptides play a key defense function (Lemaitre and Hoffmann, 2007)
In support of such a role are the major selective pressures that have been shown to shape antimicrobial peptides/proteins (AMPs) sequences at positions required for an efficient mechanism of action
Summary
The Pacific oyster Crassostrea gigas is a marine invertebrate belonging to the family Ostreidae (Mollusca, Bivalvia) with a worldwide distribution from Japan to occidental countries in Europe and America (Guo et al, 2008). It is still difficult to determine how far the increase of the Vibrio load in marine coastal water contributes to animal disease Environmental factors such as temperature modulate the virulence status of bacteria (Vezzulli et al, 2010) and the physiological status of the animal species cultured in these systems, in particular oysters, whose reproductive status is dependent on an elevated sea water temperature. OYSTER COMMENSAL MICROFLORA AND HOMEOSTASIS Animals including marine invertebrates carry numerous species of bacteria and it is becoming increasingly clear that most of these microorganisms constitute a host-specific community. Oysters need high volumes of water for their nutrition and respiration They are in continuous contact with abundant microorganisms from their marine environment, which include both pathogens and commensals. The ability of Vibrio species to persist within oyster tissues is responsible for their high concentration within this host that can act as an environmental reservoir of human pathogens (Pruzzo et al, 2005)
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