Characterization of four Flavobacterium columnare (Flexibacter columnaris) strains isolated from tropical fish

Four Flavobacterium columnare strains (AJS 1-4) were isolated from black mollies (Poecilia sphenops) and platies (Xiphophorus maculatus), showing white spots on the back, head and skin ulcers. The isolates developed characteristic rhizoid yellow pigmented colonies on Shieh agar and typical growth in Shieh broth. They were Gram-negative, filamentous bacteria exhibiting flexing movements. When compared to F. columnare strains isolated from temperate fish, it was noted that the four strains originating from tropical aquarium fish are more capable of growing at higher temperatures, the opposite being true for the strains isolated from temperate fish. Biochemical characterization and agglutination tests proved that the isolated strains could be classified as F. columnare. Low minimal inhibitory concentration (MIC) values were found for chloramphenicol, erythromycin, furazolidone, kanamycin, lincomycin, nalidixic acid, oxytetracycline and streptomycin. MIC values were high for colistin, sulfamethoxazole and neomycin. Pathogenicity studies were performed on black mollies. When these animals were submersed in an infective solution of the F. columnare strains, a marked difference in virulence was noted among the four isolated strains, strain AJS 1 being the most virulent one and strain AJS 4 being of low virulence.