Specific inhibition of chemiluminescent activity by pathogenic vibrios in hemocytes of two marine bivalves: Pecten maximus and Crassostrea gigas.

Hemocytes from two adult bivalves, Pecten maximus and Crassostrea gigas, were exposed to 12 different bacterial strains including 2 Alteromonas spp. (U1 and T413), 2 type strains of vibrios (V. anguillarum ATCC 19264 and V. alginolyticus ATCC 17749), 1 vibrio (S322) pathogenic to C. gigas larvae, 2 vibrios (V110 and V117) virulent to Ostrea edulis larvae, and 5 different strains of a same Vibrio sp. (group A496) isolated from moribund P. maximus larvae. After 1.5 h contact with bacteria, zymosan particles were added to the hemocytes and the chemiluminescent (CL) activity of the cells was measured over 6 h. Analysis of CL activity after bacterial inoculation indicated that most of the strains could initiate the respiratory burst. However, the intensity of CL was not related to the virulence of the bacteria. In contrast the CL activity after stimulation by zymosan was modulated by the previous exposure of bacteria. This second CL response may depend on the virulent characteristics of the bacteria. As evidence, the strain S322 completely inhibited the CL activity of oyster hemocytes, whereas in the scallop hemocytes the CL activity was only moderately repressed. Inversely, the strain A496 was very effective in disrupting the CL activity in scallop hemocytes and reduced the CL activity to 30% in oyster hemocytes. V. anguillarum completely inhibited scallop and oyster hemocytes, whereas the strain U1 decreased the CL activity only by 20%. Finally, the measurement of CL activity allowed to partially elucidate the mechanism of infection as well as to determine some characteristics of bacterial virulence.