Rich resource environment of fish farms facilitates phenotypic variation and virulence in an opportunistic fish pathogen

Phenotypic variation allows adaptation of opportunistic pathogens to variable conditions in the outside-host environment with strong effects on their epidemiology and pathogenicity in hosts. Here we found that the isolates of an opportunistic fish pathogen Flavobacterium columnare from fish farming environment had higher phenotypic variation between two morphotypes in growth, as compared to the isolates from the natural water environment. The rough morphotypes had higher growth rate than the rhizoid morphotypes especially in the higher resource concentrations and in the higher temperature, but only if the isolate was originating from the fish farms. Rhizoid morphotype was more virulent than the rough type regardless of their origin. However, the virulence of the rough type increased sharply with the size of the fish, and the bacterial isolates from the gills of diseased fish were rhizoid type, indicating a reversal of the rough morphotype into rhizoid in contact with the fish. The high growth rate of the rough morphotype combined with the morphotype reversibility could increase the probability of columnaris epidemics at fish farms. Our findings suggest that intensive farming imposes different selection pressures on bacterial survival in the outside-host environment and its transmission compared to the natural water environment.