Incubation time for the cultivation of Renibacterium salmoninarum from Atlantic salmon, Salmo salar L., broodfish.
Cultivation plates for the primary isolation of R. salmoninarum were incubated for 12 or more weeks over 3.5 years. The incubation time for samples of two groups of Atlantic salmon broodfish on a selective agar medium is reported. Samples turned positive over a long time-span, but the longest incubation time recorded for growth of the bacterium was 19 weeks. In one group, 79% of all positive samples from fish with macroscopic signs of bacterial kidney disease were positive after 6 weeks of incubation and the highest incidence of positive samples occurred during the sixth week. From covertly infected fish within the same group, 42% of samples were positive after 6 weeks of incubation, and the highest incidence of positive samples occurred during the sixth and ninth weeks of incubation. The growth pattern of the bacterium at primary isolation was characterized by a prolonged lag phase followed by a relatively rapid growth in one week from microscopic to macroscopic size of colonies.