Protective role and in vitro activity of fractions extracted from Lactococcus garvieae, the lactococcosis agent in rainbow trout (O. mykiss).

Effective protocols of vaccination against lactococcosis in rainbow trout (O. mykiss) are still under investigation and the methods currently employed are based on “autovaccines” that are injected intraperitoneally to fish (Bercovier et al., 1997; Prearo, 2006). These vaccines allow a protection for 8 months, when integrated with adjuvants (Ravelo et al, 2006). The bacterial antigenic components involved in the protection are only partially considered by the literature. This investigation evaluated the effect of some fractions of L. garvieae (strain B05/3) in the development of a protective immune response to the infection. Extra cellular products (ECPs), bacterial whole cells (WCs) and membrane antigens (MAs) were injected intraperitoneum to 90g rainbow trouts. Fish were subsequently submitted to an intraperitoneal challenge with L. garvieae (2.6×105 cfu/individual). The relative percentages of survival (RPS) were 95% for WCs, 35% for ECPs and 33% for MAs. These results suggest that WCs provided the best protection, but also ECPs and MAs were effective. Samples of serum collected from immunized and control fish were analysed by immunoblotting against the SDS-PAGE/Western Blotting protein profile of each bacterial fraction. The control and immunized fish sera contained immunoglobulins able to bind aspecifically the proteins having a molecular weight of 23, 48 and 102 kDa respectively. Similar findings were previously reported by Barnes et al., 2002. Moreover the respiratory burst of leukocytes isolated from rainbow trout head kidney was measured by a luminol-based microtitre plate chemiluminescence assay after stimulation with the L. garvieae extracts. The tests were performed using 20 healthy non-immunized fish, by incubating the cells and the stimulants in presence and absence of autologous serum. Preliminary results suggest an evident individual variability in the response and that the release of reactive oxygen species is strongly affected by the serum addiction. This indicates an important role of antibodies and complement in promoting the leukocyte response to different antigens (Barnes et al., 2002). The specific stimulation ability of each fraction (WC, ECPs, MA) will be discussed.