Abstract
BackgroundIn nature, organisms are commonly coinfected by two or more parasite strains, which has been shown to influence disease virulence. Yet, the effects of coinfections of environmental opportunistic pathogens on disease outcome are still poorly known, although as host-generalists they are highly likely to participate in coinfections. We asked whether coinfection with conspecific opportunistic strains leads to changes in virulence, and if these changes are associated with bacterial growth or interference competition. We infected zebra fish (Danio rerio) with three geographically and/or temporally distant environmental opportunist Flavobacterium columnare strains in single and in coinfection. Growth of the strains was studied in single and in co-cultures in liquid medium, and interference competition (growth-inhibiting ability) on agar.ResultsThe individual strains differed in their virulence, growth and ability for interference competition. Number of coinfecting strains significantly influenced the virulence of infection, with three-strain coinfection differing from the two-strain and single infections. Differences in virulence seemed to associate with the identity of the coinfecting bacterial strains, and their pairwise interactions. This indicates that benefits of competitive ability (production of growth-inhibiting compounds) for virulence are highest when multiple strains co-occur, whereas the high virulence in coinfection may be independent from in vitro bacterial growth.ConclusionsIntraspecific competition can lead to plastic increase in virulence, likely caused by faster utilization of host resources stimulated by the competitive interactions between the strains. However, disease outcome depends both on the characteristics of individual strains and their interactions. Our results highlight the importance of strain interactions in disease dynamics in environments where various pathogen genotypes co-occur.
Highlights
In nature, organisms are commonly coinfected by two or more parasite strains, which has been shown to influence disease virulence
High relatedness can lead to low virulence if the cooperation of pathogens leads to prudent host exploitation [20] or to high virulence if the Kinnula et al BMC Evolutionary Biology (2017) 17:77 cooperation leads to faster growth [21]
Virulence in zebra fish hosts We found a significant interaction between bacterial dose and strain identity on their effect on the host mortality risk (p < 0.001, Fig. 1, Tables 1 and 2)
Summary
Organisms are commonly coinfected by two or more parasite strains, which has been shown to influence disease virulence. While recent studies have demonstrated that a single host is often infected by a multitude of pathogen strains or species [1,2,3,4,5,6,7,8,9,10,11], empirical investigations of the effects of coinfections on disease dynamics and virulence (i.e., damage caused to the host) are still limited (but see e.g., [11,12,13]), and their importance for many diseases is still unknown [14]. In the latter case the pathogen strains experience a cost, when the energy is allocated to toxin production instead of growth or virulence
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