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The antibacterial properties of the indigenous microflora of rainbow trout (Oncorhynchus mykiss Walbaum) and the potential use of inhibitory bacteria as fish probiotics were investigated. A total of 1018 bacteria and yeasts were isolated on tryptone soy agar (TSA) from skin, gills and intestine. Forty-five of these inhibited growth of the fish pathogenic bacterium Vibrio anguillarum in a well diffusion assay. The antagonism was most prominent among Pseudomonas spp., as 28 (66%) of the antagonistic bacteria belonged to this genus, despite constituting only 15% of the total tested flora. As pseudomonads are typically siderophore producers, chrome azurol S (CAS) agar was used as a semi-selective medium for isolation of antagonistic bacteria. On this medium, 75% of the iron-chelating strains were inhibitory to V. anguillarum. Eight strains out of a subset of 11 antagonists caused a 3-6 log unit reduction in the density of V. anguillarum [measured by polymerase chain reaction (PCR) detection in a most probable number (MPN) regimen] in a broth co-culture assay. Survival of rainbow trout infected with vibriosis was improved 13-43% by six out of nine antagonistic strains tested in vivo. All disease-protecting strains were pseudomonads, isolated from CAS plates, whereas two Carnobacterium spp. that were antagonistic in in vitro well diffusion assays did not alter the accumulated mortality of rainbow trout. The addition of live bacterial cultures to fish-rearing water may thus improve survival of the fish; however, in vitro antagonism could not completely predict an in vivo effect. Further studies on the underlying mechanism of activity are required to design appropriate selection criteria for fish probiotic bacteria.

To study immunological and immunogenetical parameters related to resistance against viral haemorrhagic septicaemia (VHS), attempts to make gynogenetic strains of rainbow trout selected for high and low resistance to VHS were initiated in 1988. The first gynogenetic generation of inbreeding resulted in the more resistant offspring E8 and the low resistance offspring K3; the K3 offspring having the same high mortality as the susceptible reference strain of outbred trout in infection trials. A second gynogenetic generation derived from the E8 strain resulted in some low resistance offspring, and two gynogenetic families in which all, or nearly all, fish survived challenge with VHS virus. In this study, an attempt to associate the distribution of different MHC class II genotypes with low and high resistance gynogenetic offspring was performed. Two different MHC haplotypes could be distinguished, and in both low and high resistance families all three genotypes were found, which could be explained by the fact that the mother fish carried the heterozygous genotype. Although no significant differences in MHC II genotypes were found between the high and low resistance offspring, a significantly different distribution of haplotypes in the low resistance offspring was observed, that could not be explained by a one- or two-locus model.

213 Monoclonal antibodies (mAbs) raised against leucocyte surface antigens from human and 11 animal species were analyzed for reactivities against leucocytes from human and 15 different animal species. We found 77 mAbs (36%) to cross-react. Altogether, 217 cross reactions were registered out of 3195 possible combinations (7%). Most of the cross reacting mAbs had integrin or MHC class II specificities. This study defined cross reactions on the following markers: CD1a, 1c, 2, 4, 5, 8, 9, 11a, 11b, 14, 18, 20, 21, 23, 29, 31, 41, 43, 44, 45, 45R, 46, 49, 61, 62L, TCR gamma/delta, BCR, Thy-1, MHC class I and MHC class II, Swine-WC7 and Cattle-WC1. In order to characterize the molecular weight (MW) of the corresponding cross reacting antigens, selected mAbs were used to immunoprecipitate the antigens. The MW's of the analyzed precipitated antigens were in good agreement with the MWs of the homologous antigens. The followed strategy was found to be efficient and economical in defining new leucocyte antigen reactive mAbs.