Attenuation of a virulent A eromonas hydrophila with novobiocin and pathogenic characterization of the novobiocin‐resistant strain

Aim To determine whether novobiocin resistance strategy could be used to attenuate a virulent A eromonas hydrophila AH 11 P strain and to characterize the growth and pathogenic differences between the novobiocin‐resistant strain and its virulent parent strain AH 11 P . Methods and Results A novobiocin‐resistant strain AH 11 NOVO was obtained from a virulent A er. hydrophila strain AH 11 P through selection of resistance to novobiocin. AH 11 NOVO was found to be avirulent to channel catfish ( I ctalurus punctatus ), whereas AH 11 P was virulent. When AH 11 NOVO vaccinated channel catfish were challenged with AH 11 P at 14 days postvaccination, relative per cent of survival of vaccinated fish was 100%. The cell proliferation rate of AH 11 NOVO was found to be significantly ( P  < 0·05) less than that of AH 11 P . In vitro motility assay revealed that AH 11 NOVO was nonmotile, whereas AH 11 P was motile. AH 11 NOVO had significantly ( P  < 0·05) lower in vitro chemotactic response to catfish mucus than that of AH 11 P . Although the ability of AH 11 NOVO to attach catfish gill cells was similar to that of AH 11 P , the ability of AH 11 NOVO to invade catfish gill cells was significantly ( P  < 0·05) lower than that of AH 11 P . Conclusions The novobiocin‐resistant AH 11 NOVO is attenuated and different from its parent AH 11 P in pathogenicity. Significance and Impact of the Study The significantly lower chemotactic response and invasion ability of AH 11 NOVO compared with that of its virulent parent strain AH 11 P might shed light on the pathogenesis of A er. hydrophila .