Anti-Bacillus megaterium activity was measured in unfractionated plasma withdrawn from three common US East Coast bivalve molluscs: an oyster Crassostrea virginica and the mussels Geukensia demissa and Mytilus edulis. The activities of the plasma samples from these bivalves were also measured against a C. virginica pathogen Perkinsus marinus. Strong anti-B. megaterium activity was measured in plasma from C. virginica and M. edulis, but was not detected in G. demissa. Bactericidal activity was found in hemocyte extracts from all bivalves in this study, suggesting a cellular origin of cytotoxic humoral factors. Peptides (<<space>10<space>kDa) were separated from the plasma samples by ultrafiltration; weak antibacterial peptide activity was quantified in C. virginica peptides, but not in peptides from the mussels. In the case of P. marinus, plasma from M. edulis or G. demissa was strongly cidal as compared to plasma from C. virginica. This difference in activity probably reflects the low pathogenicity of this oyster parasite for the mussel species tested. In summary, the bactericidal activity of plasma proteins from these bivalves showed considerable interspecies variation and did not necessarily correlate directly with antiprotistan activity. When present, antibacterial and antiprotistan activities seemed to be associated with plasma proteins rather than<space><<space>10-kDa plasma peptides, with the possible exception of C. virginica anti-B. megaterium activity and the occasionally expressed anti-P. marinus activity of M. edulis peptides. The precise identity of the plasma protein(s) responsible for the antimicrobial activities measured have yet to be determined, but it is likely that agents other than, or in addition to, lysozyme play significant roles in the process.
Nous avons mesure l'activite anti-Bacillus megaterium du plasma entier de trois mollusques bivalves communs sur la cote est des Etats-Unis<space>: l'huitre Crassostrea virginica et les moules Geukensia demissa et Mytilus edulis. L'activite des echantillons de plasma de ces bivalves a aussi ete mesuree vis-a-vis du protiste Perkinsus marinus, pathogene pour C. virginica. Une forte activite anti-B. megaterium est mesuree dans le plasma de C. virginica et M. edulis, mais n'est pas decelee chez G. demissa. L'activite bactericide est observee dans des extraits d'hemocytes chez ces trois bivalves, ce qui supposerait une origine cellulaire des facteurs humoraux cytotoxiques. Des peptides (<<space>10<space>kDa) ont ete separes des echantillons de plasma par ultrafiltration<space>; une faible activite antibacterienne de cette fraction peptidique a ete mesuree chez C. virginica, mais pas chez les moules. Dans le cas de P. marinus, le plasma de M. edulis ou de G. demissa montre une forte activite antiprotiste, compare au plasma de C. virginica. Cette difference au niveau de l'activite reflete probablement la faible pathogenicite de ce parasite de l'huitre pour les especes de moules testees. En resume, l'activite bactericide des proteines du plasma de ces bivalves montre une variation considerable suivant les especes et n'est pas necessairement et directement correlee avec l'activite antiprotiste. Lorsque ces activites antiprotistes et antibacteriennes sont presentes, elles semblent etre associees aux proteines du plasma, plutot qu'aux peptides (<<space>10<space>kDa), a l'exception peut-etre de l'activite anti-B. megaterium de C. virginica et de l'activite anti-P. marinus occasionnelle de M. edulis. L'identite precise de la proteine ou des proteines du plasma responsable(s) des activites antimicrobiennes mesurees reste encore a determiner, mais il est probable que des agents autres que le lysozyme, ou supplementaires au lysozyme, jouent un role significatif dans le processus.