Mutualistic bacteria lead to asymmetric competition between two viral pathogens
Multicellular organisms simultaneously host a multitude of mutualistic, commensal, and pathogenic microbes, and there is the potential for interactions between cohabiting microbes to affect host immunity and health. Coinfections of one host with multiple pathogens are widespread yet understudied. Most studies of pathogen coinfection ignore the presence of commensal or mutualistic microbes, which have the potential to shift competitive outcomes between competing pathogen strains or species. In order to understand the interactions between coinfecting mutualistic and pathogenic microbes, we examined the implications of single infections and coinfections of two viruses of common beans, bean common mosaic virus (BCMV) and clover yellow vein virus(ClYVV), in hosts with and without the presence of nitrogen-fixing rhizobia bacteria. We hypothesized that the presence of rhizobia would shift competitive outcomes between the two viruses, and that viral coinfections would cause a synergistic reduction in nodule numbers and the nitrogen that hosts receive from their bacterial mutualists.
We found that virus coinfection has little effect on viral concentrations within the host in plants growing without rhizobia. In the presence of rhizobia, however, ClYVV concentrations are significantly higher in coinfected hosts than in singly infected hosts. Hosts colonized by rhizobia are different from uncolonized hosts in at least two ways that could affect viral competition. They exhibit increased percent tissue nitrogen as well as a significant increase in the concentration of salicylic acid, a plant hormone that is related to plant immune responses against pathogens. Viral coinfection, in turn, results in a synergistic negative effect on both root nodule numbers and the percentage of plant nitrogen derived from the atmosphere. Consequently, seed production by coinfected hosts increases less as a result of rhizobia colonization in comparison to any other treatment. In conclusion, the presence of a mutualist microbe can lead to asymmetric competition between coinfecting pathogens, while coinfecting microbes have a greater than additive negative effect on the microbial mutualist.