Development in vitro and pathogenicity of an erythromycin-resistant strain of Renibacterium salmoninarium, the causative agent of bacterial kidney disease in salmonids

Abstract

This study was prompted by concern that the widespread use of erythromycin (EY) for prophylactic treatment of R. salmoninarum (Rs) infection could lead to the development of resistant strains. From a British Columbia strain of Rs sensitive to EY (minimum inhibitory concentration of about 0.25 g EY/ml) a highly resistant culture was developed in vitro by inoculating cells growing in the presence of 0.1 g EY/ml onto media containing increasingly higher concentrations of antibiotic, up to 500 g EY/ml. The high level of resistance was maintained for at least 6 passages on EY-free medium but declined on further passaging. The resistant strain was of lower virulence for sockeye salmon Oncorhynchus nerka than the parent strain. Attenuation of the EY-resistant strain appeared to be attributable more to serial passage on culture media than to an effect of the antibiotic. Implications of these findings are discussed in relation to fish culture practices.