Abstract
BackgroundGeneralist bacterial pathogens, with the ability for environmental survival and growth, often face variable conditions during their outside-host period. Abiotic factors (such as nutrient deprivation) act as selection pressures for bacterial characteristics, but their effect on virulence is not entirely understood. “Sit and wait” hypothesis expects that long outside-host survival selects for increased virulence, but maintaining virulence in the absence of hosts is generally expected to be costly if active investments are needed. We analysed how long term starvation influences bacterial population structure and virulence of an environmentally transmitting fish pathogen Flavobacterium columnare.ResultsF. columnare populations in distilled water and in lake water were monitored for 5 months. During the experiment, the population structure of F. columnare diversified by rough and soft colony morphotypes appearing among the ancestral rhizoid ones. After 5 months starvation in lake water, the virulence of the starved and ancestral bacterial isolates was tested. The starved rhizoid isolates had significantly higher virulence than the ancestral rhizoid, whereas the virulence of the rough isolates was low.ConclusionsWe suggest that F. columnare population diversification is an adaptation to tolerate unpredictable environment, but may also have other biological significance. Maintaining and increasing virulence ensures efficient invasion into the host especially under circumstances when the host density is low or the outside-host period is long. Changing from rhizoid into a rough morphotype has trade-offs in making bacteria less virulent and unable to exploit the host, but may ensure bacterial survival under unpredictable conditions. Our study gives an example how abiotic selection can diversify virulence of environmentally transmitting bacterial pathogen.
Highlights
Generalist bacterial pathogens, with the ability for environmental survival and growth, often face variable conditions during their outside-host period
In the real world the complexity of host-pathogen relationships extends beyond traditional theories, because not all bacterial pathogens suffer from transmission-virulence trade-off [1,2], but are able to survive and reproduce in the environment outside the host
Analyzing the selection pressures that have impact on virulence of opportunistic pathogens is important, because these pathogens can maintain infective populations in the environment that act as sources of disease epidemics
Summary
Generalist bacterial pathogens, with the ability for environmental survival and growth, often face variable conditions during their outside-host period. In contrast to obligate pathogens capable for replicating only within the host, these opportunistic pathogens need to adjust their behavior to maximize both the Pathogenic bacteria with outside-host growth and transmission are subjected to abiotic stress. In aquatic environment nutrient deprivation can be a major selection pressure for bacteria, causing changes in bacterial cell structure and life cycle strategies. Growing bacteria face competition, that may prevent their ability to invade host populations and cause epidemics [11]. Analyzing the selection pressures that have impact on virulence of opportunistic pathogens is important, because these pathogens can maintain infective populations in the environment that act as sources of disease epidemics
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