Functional and molecular responses in Mytilus edulis hemocytes exposed to bacteria, Vibrio splendidus

This study aims at examining the morphological, functional and molecular responses of Mytilus edulis hemocytes exposed to different strains of Gram-negative bacteria Vibrio splendidus (a virulent strain V. splendidus LGP32, V. splendidus LGP32 Δvsm without metalloprotease and an environmental type strain V. splendidus 7SHRW) at a 1:3 ratio for 2, 4, and 6 h. Our data showed that hemocytes could have a discriminative capacity towards microorganisms. Both V. splendidus LGP32 strains had an effect on hemocyte adhesion, phagocytosis abilities and oxidative burst, whereas the environmental strain 7SHRW induced weak and delayed hemocyte responses. At a molecular level, differential levels of candidate transcripts were measured in M. edulis hemocytes exposed to V. splendidus LGP32-GFP and 7SHRW. Mainly, a down-regulation of defensin was recorded in hemocytes exposed to V. splendidus LGP32. A significant up-regulation of lysozyme and proteasome 26S was observed at 2 h followed by a down-regulation at 4 and 6 h of exposure to the LGP32 strain. Similarly, SOD and GPx genes were up-regulated 2 h post-exposure to LGP32 strain and their expressions decreased after 4 and 6 h post-exposure. Further analysis is however needed in a near future to relate the transcript level variations with the physiological process.

► Differential responses of Mytilus edulis hemocytes exposed to Vibrio splendidus strains.
► Vibrio splendidus LGP32 strains induce stronger effect compared to 7SHRW strain.
► Immune transcripts are regulated during the first hours of exposure.
► The observed responses are not linked to metalloproteinase.