Toxicity of bacteria towards haemocytes of Mytilus edulis

Haemocytes of Mytilus edulis, allowed to attach to the plastic surface of a tissue culture plate in the presence of haemolymph, were observed by time- lapse video recording. When bacteria were added at concentrations of 10 or 50 bacteria per haemocyte, certain bacterial strains caused rounding of the cells within 2-3 h. Haemolymph was necessary for the rounding to occur; if bacteria were added in sterile seawater there was no significant differenc e in the number of rounded cells between control and bacteria-treated cultu res for up to 4 h. The haemolymph factor required for this activity was act ive at 1/64 dilution in seawater, was sensitive to trypsin treatment, and a ctivity was halved on heating at 56 or 100 degrees C for 30 min. For the mo st toxic bacteria tested, Vibrio alginolyticus NCMB 1339 and Vibrio anguill arum A7, haemocyte cell rounding appeared to be induced by a very small num ber of bacterial cells. Bacteria-free culture supernatant of V. anguillarum 2981 induced rounding of haemocytes in a dose-dependent manner, with 50% o f cells being rounded at a dilution of approximately 1/500 of the culture s upernatant. In a survey of 226 bacterial isolates, those isolated from inci dents of disease in a bivalve hatchery were significantly more toxic toward s haemocytes than bacteria isolated from hatcheries without disease or from turbot hatcheries. (C) 1999 Ifremer/Cnrs/Inra/Ird/Cemagref/Editions scient ifiques et medicales Elsevier SAS.