Molecular characterisation of Flavobacterium columnare associated with columnaris disease in freshwater fishes with emphasis on virulence and antibiotic resistance pattern

Abstract

Flavobacterium columnare is a major aquaculture pathogen that causes columnaris disease. However, only very limited studies have been conducted on its virulence and drug resistance pattern. Three pathogenic bacterial isolates of F. columnare (CIFRI-RCM8, CFCCSL66 and CFCCO41) were obtained from diseased fish (Labeo rohita, L. catla and Clarias magur) of culture farms in Odisha and West Bengal, India. Clinical signs such as tail rot, skin ulcer and gill discolouration, as well as soft ulceration across the entire body, irrespective of the fish size were noticed. The virulence of the isolates was confirmed by a recursive in vitro experiment. The pathological changes in infected L. rohita were studied to confirm the pathogenicity. Considerable difference in virulence was noticed when fishes were challenged with different F. columnare strains. Among the isolates, CIFRI-RCM8 was the most virulent, whereas CFCCO41 was the least virulent one. All the isolates were found to be multidrug-resistant showing resistance to cefuroxime, ampicillin, cephalothin, amoxyclav, cefalexin, ceftazidime, carbenicillin, trimethoprim, clindamycin, ticarcillin and penicillin-G. The study suggested that F. columnare is one of the most important etiological agents of disease in freshwater fishes for which appropriate management protocols needs to be developed.