In vitro infection of a cell line from Ictalurus nebulosus with Piscirickettsia salmonis

P. salmonis, the aetiologic agent of salmonid rickettsial septicaemia (SRS), affects several species of salmonids. Previous reports using the appearance of cytopathic effect (CPE) as the criterion for susceptibility, showed that P. salmonis (ATCC strain) can be grown in vitro in some cells lines derived from salmonid fish, but not in BB cells from brown bullhead (I. nebulosus) and BF-2 cells from bluegill (Lepomis macrochirus). The growth of P. salmonis (ATCC strain VR 1361) in a cell line previously believed to be non-permissive for this organism is described. CPE was first detected in chinook salmon embryo (CHSE-214) and epithelioma papulosum cyprini (EPC) cell lines at 6 days after infection (dpi). In contrast, using BB cell line, CPE was first detected 45 dpi and the monolayer completed CPE by 78 dpi. Electron microscopic examination of BB cells 78 dpi showed free, intracytoplasmic and extracellular localization of the agent. P. salmonis was also observed within membrane-bounded vacuoles in BB cells, similar to that described in CHSE 214 cells. Contrary to earlier reports, results from this study show that the BB cell line, is susceptible to P. salmonis infection. The delayed onset of CPE in BB cells in comparison to other permissive cell lines suggests that BB cells are not ideal hosts for P. salmonis. Interestingly, however, these results demonstrate that P. salmonis can infect non-salmonid cell lines, and raises the possibility that non-salmonid fish may be involved in the persistence and transmission of SRS in the natural environment..