Phage-driven loss of virulence in a fish pathogenic bacterium

PLoS One. 2012 Dec 20;7(12):e53157. doi: 10.1371/journal.pone.0053157. Epub 2012 Dec 31.


Parasites provide a selective pressure during the evolution of their hosts, and mediate a range of effects on ecological communities. Due to their short generation time, host-parasite interactions may also drive the virulence of opportunistic bacteria. This is especially relevant in systems where high densities of hosts and parasites on different trophic levels (e.g. vertebrate hosts, their bacterial pathogens, and virus parasitizing bacteria) co-exist. In farmed salmonid fingerlings, Flavobacterium columnare is an emerging pathogen, and phage that infect F. columnare have been isolated. However, the impact of these phage on their host bacterium is not well understood. To study this, four strains of F. columnare were exposed to three isolates of lytic phage and the development of phage resistance and changes in colony morphology were monitored. Using zebrafish (Danio rerio) as a model system, the ancestral rhizoid morphotypes were associated with a 25-100% mortality rate, whereas phage-resistant rough morphotypes that lost their virulence and gliding motility (which are key characteristics of the ancestral types), did not affect zebrafish survival. Both morphotypes maintained their colony morphologies over ten serial passages in liquid culture, except for the low-virulence strain, Os06, which changed morphology with each passage. To our knowledge, this is the first report of the effects of phage-host interactions in a commercially important fish pathogen where phage resistance directly correlates with a decline in bacterial virulence. These results suggest that phage can cause phenotypic changes in F. columnare outside the fish host, and antagonistic interactions between bacterial pathogens and their parasitic phage can favor low bacterial virulence under natural conditions. Furthermore, these results suggest that phage-based therapies can provide a disease management strategy for columnaris disease in aquaculture.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Aquaculture
  • Bacteriophages* / genetics
  • Bacteriophages* / pathogenicity
  • Biological Evolution
  • Fish Diseases / genetics
  • Fish Diseases / microbiology
  • Fishes / microbiology
  • Flavobacteriaceae Infections / genetics
  • Flavobacteriaceae Infections / microbiology
  • Flavobacterium / pathogenicity
  • Flavobacterium / virology
  • Host-Parasite Interactions*
  • Salmon / microbiology
  • Salmon / virology
  • Virulence / genetics*
  • Zebrafish* / microbiology
  • Zebrafish* / virology

Grant support

This work was supported by Finnish Centre of Excellence Program of the Academy of Finland 2006–2011 CoE in Virus Research (#1129648), and CoE in Biological Interactions 2012–2017(#252411, Johanna Mappes), Academy of Finland grants 127500 (L.R.S.) and 125572 (J.L.), and a grant from the Maj and Tor Nessling Foundation (J.K.H.B.). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.