Antimicrobial susceptibility patterns and R plasmid-mediated resistance of the fish pathogen Yersinia ruckeri

Fifty strains of Yersinia ruckeri, the causative agent of enteric redmouth disease of salmonid fish, were tested for susceptibility to 23 antimicrobial agents by using an agar dilution procedure. The MICs were generally uniform for all serological varieties. Two of the 50 strains carried a 36-megadalton plasmid which determined resistance to tetracyclines and sulfonamides and was transferable to both Escherichia coli and Y. ruckeri recipients. The serovars did differ in their response to polymyxin B. Strains of serovars II, III, and V were highly resistant (MICs of 128 to 512 micrograms/ml), whereas most serovar I strains were susceptible to less than or equal to 2.0 micrograms/ml. Of 33 serovar I strains, 6 were highly resistant to polymyxin B, which is a characteristic that may divide serovar I (Hagerman) strains into two distinct subgroups.