Isolation, identification and sensitivity to antimicrobial agents of columnaris disease pathogen from channel catfish (Ictalurus punctatus)

A strain bacterium (YZ130310) was isolated from the rotted skin of the channel catfish with columnaris disease. Artificial infection test showed that YZ130310 could lead to 100%mortality of fish in the experimental group with the same signs as the natural infected fish and was the causative pathogen associated with the disease. The bacterial colony was yellow, irregular shape with undulated margin in Shieh solid medium. High-power optical microscope observation revealed that YZ130310 was gram negative and exhibits rodshaped morphology. Physiological and biochemical analysis showed that it had the characteristics of the oxidase, catalase and nitrate reduction positive; hydrolyzation of casein and gelatin; non -hydrolyzation of cellulos, chitin, tyrosine, aesculin and starch; non -fermentation of indole and glucose. The analysis on 16S rRNA gene sequence displayed that YZ130310 had the highest homology in the 16S rRNA nucleotide level with Flavobacterium columnare strain ATCC 49512 with the value 99.6%. Phylogenetic analysis was constructed based on the 16S rRNA gene of F. columnare in the GenBank database and revealed that the strain YZ130310 clustered with F.columnare (AY095342). According to the morphological features, physiological and biochemical characteristics and 16S rRNA gene homology comparison of the bacteria, YZ130310 strain was identified as F. columnare. The sensitivity test using 16 antimicrobial agents showed that strain YZ130310 was sensitive to 7 agents including cefoperazone, was intermediately sensitive to amikacin and ofloxacin, was resistant to 7 agents including cefalexin.