Role of capsule in serotypic differences and complement fixation by Lactococcus garvieae

Abstract

Seventeen geographically distinct isolates of Lactococcus garvieae, isolated from diseased fish, were compared serologically using antiserum raised against the various isolates in rainbow trout. Sera raised against a capsule deficient isolate did not agglutinate capsulated isolates, regardless of origin. In contrast, all antisera raised against capsulated isolates cross reacted strongly with non-capsulated isolates. Antisera raised against capsulated Japanese isolates cross reacted with other capsulated Japanese isolates including isolates from geographically distinct prefectures within Japan (Ehime and Oita). However, antisera against these virulent capsulated isolates did not cross react with European capsulated isolates. Antisera raised against European capsulated isolates cross reacted with other European isolates, regardless of origin within Europe (UK, Italy, Spain), but did not cross-react with Japanese capsulated isolates. Agglutination assays performed with a range of fifteen lectins revealed differences in surface carbohydrate structure: capsule deficient isolates agglutinated with concanavalin A, Ricinis communis agglutinin, Pisum sativum agglutinin, Lens culinaris agglutinin, wheat germ agglutinin and succinylated wheat germ agglutinin. European capsulated isolates agglutinated with concanavalin A only. The Japanese capsulated isolates were not agglutinated by any of the lectins used in this study. Representative isolates from each group (Japanese capsulated and non-capsulated, European capsulated and non-capsulated) were investigated for their ability to fix complement. Non-capsulated isolates fixed complement regardless of origin, and antibody did not markedly enhance complement fixation. In contrast, the capsulated isolates were less efficient at fixing complement, but complement fixation was markedly increased by homologous antibody.