Intensive aquaculture selects for increased virulence and interference competition in bacteria

Abstract

Although increased disease severity driven by intensive farming practices is problematic in food production, the role of evolutionary change in disease is not well understood in these environments. Experiments on parasite evolution are traditionally conducted using laboratory models, often unrelated to economically important systems. We compared how the virulence, growth and competitive ability of a globally important fish pathogen, Flavobacterium columnare, change under intensive aquaculture. We characterized bacterial isolates from disease outbreaks at fish farms during 2003-2010, and compared F. columnare populations in inlet water and outlet water of a fish farm during the 2010 outbreak. Our data suggest that the farming environment may select for bacterial strains that have high virulence at both long and short time scales, and it seems that these strains have also evolved increased ability for interference competition. Our results are consistent with the suggestion that selection pressures at fish farms can cause rapid changes in pathogen populations, which are likely to have long-lasting evolutionary effects on pathogen virulence. A better understanding of these evolutionary effects will be vital in prevention and control of disease outbreaks to secure food production.

Keywords: Flavobacterium columnare; aquaculture; evolution; fish farming; pathogen; virulence.

Figure 1.

Virulence of Flavobacterium columnare in experimentally infected zebra fish (Danio rerio). (a) The mean longevity (±s.e., open squares, left axis) and survival percentage (filled circles, right axis) of the infected zebra fish (n = 10, in each bacterial exposure) after exposure to bacterial strains isolated in 2003–2010. (b) Survival percentage of zebra fish (n = 14 in each bacterial exposure) after exposure to bacterial strains isolated from inlet water (B355–B407) and outlet water (B399–B397) of a fish farm (filled circles for individual strains, open circles for mean survival ± s.e.).

Figure 2.

Changes in (arcsin-transformed) interaction index of the competing pairs of Flavobacterium columnare strains on limited resources in relation to the mean isolation time of the pair of isolates. Lower index indicates higher competition. Circle indicates the mean of the two replicates of the tested F. columnare strain pairs.