Higher resource level promotes virulence in an environmentally transmitted bacterial fish pathogen

Abstract

Diseases have become a primary constraint to sustainable aquaculture, but remarkably little attention has been paid to a broad class of pathogens: the opportunists. Opportunists often persist in the environment outside the host, and their pathogenic features are influenced by changes in the environment. To test how environmental nutrient levels influence virulence, we used strains of Flavobacterium columnare, an environmentally transmitted fish pathogen, to infect rainbow trout and zebra fish in two different nutrient concentrations. To separate the effects of dose and nutrients, we used three infective doses and studied the growth of bacteria in vitro. High nutrient concentration promoted both the virulence and the outside‐host growth of the pathogen, most notably in a low‐virulence strain. The increase in virulence could not be exhaustively explained by the increased dose under higher nutrient supply, suggesting virulence factor activation. In aquaculture settings, accumulation of organic material in rearing units can locally increase water nutrient concentration and therefore increase disease risk as a response to elevated bacterial density and virulence factor activation. Our results highlight the role of increased nutrients in outside‐host environment as a selective agent for higher virulence and faster evolutionary rate in opportunistic pathogens.

A microbial perspective on the life‐history evolution of marine invertebrate larvae: If, where and when to feed

“…Moreover, in high nutrient environments, some bacteria express virulence factors and other pathogenic characteristics (e.g. Brown & Williams, 1985;Kinnela et al, 2017;McKenny & Allison, 1995).…”

Parasite-mediated anorexia increases or decreases virulence evolution, depending on dietary context

“…Next, we examined how parasite-mediated anorexia could alter the evolution of virulence by considering cases where within-host resource conditions strongly affect parasite evolution and disease dynamics as shown, for example, in malaria [21] and bacteria in zooplankton and managed fish populations [22,23]. Again, we derived the invasion fitness of the mutant parasite ( r m ) using the next generation theorem, which is the expected lifetime transmission from a host infected with a rare mutant: where S * is now the number of hosts left uninfected by the resident parasite.…”

Influence of native catfish mucus on Flavobacterium columnare growth and proteolytic activity

“…We recently demonstrated that different genetic groups of F. columnare utilized autoclaved tilapia mucus as a nutrient source (Shoemaker & LaFrentz, 2015a) and suggested the potential for increased virulence upon growth on and/or in the presence of dead fish (Shoemaker & LaFrentz, 2015b). The presence of dead fish and/ or secreted mucus could be considered a nutrient source, and nutrient availability was suggested to drive F. columnare virulence due to increased bacterial numbers (i.e., infectious dose) and/or virulence factor activation (Kinnula, Mappes, Valkonen, Pulkkinen, & Sundberg, 2016). Although mucus and/or mucin is the first barrier to F. columnare colonization and subsequent pathogenesis, few studies have examined the ability of F. columnare to utilize or degrade native fish mucus.…”