Comparison of western blot, direct fluorescent antibody and drop-plate culture methods for the detection of Renibacterium salmoninarum in Atlantic salmon ( Salmo salar L.)

A western blot method was evaluated and compared to the commonly used direct fluorescent antibody technique (DFAT) for rapid detection of Renibacterium salmoninarum in tissues from field samples of Atlantic salmon ( Salmo salar L. ). The drop-plate culture technique (DPCT) was used to confirm the presence of cultivatable R. salmoninarum cells in tissues examined by western blot and DFAT. Tissue homogenates were from kidney, spleen, liver and intestine, removed from a random fish sample obtained in the spring from a cultured population experiencing a bacterial kidney disease (BKD) infection. Kidney tissue provided the highest probability of detecting R. salmoninarum by all three methods. Detection of the major 57 kDa R. salmoninarum soluble antigen (SA) by western blot was significantly more sensitive and reliable than detection of R. salmoninarum cell-like fluorescent-positive objects (fpo s) by the DFAT in tissue homogenates from actively infected fish. The western blot results correlated well with those of the DPCT. However, DPCT results demonstrated that a significant number of false negative reactions were generated by the DFAT (approximately 40 to 50%) which were not consistently correlated with low numbers of colony-forming units (cfu's) g −1 of kidney tissue. The status of surviving fish was examined the following winter and indicated that the fish had recovered from the infection. All tissues from sampled fish were negative for R. salmoninarum by western blot and the DPCT. However, kidney tissue from 2 of 20 fish sampled contained fpo's by the DFAT indicating a potential presence of dead (or at least inactive) bacteria remaining in the tissues of some of the fish. In the absence of any apparent active infection, a modified western blot procedure was used to detect seropositive fish which indicated that at least one third of the serum samples from surviving fish contained immunoglobulin reacting moderately to strongly with R. salmoninarum SA's. These results are discussed in terms of the potential use of rapid western blot methods as alternatives to the equivocal FAT methods for monitoring the BKD status of cultured fish populations from the field.