BBSRC-NSF/BIO: Investigating microbial predation as a driver of endosymbiosis and phagocyte evasion

Oregon State University, Corvallis OR

Investigators

Abstract

Soil microbes compete. As a result they evolved tools to resist their enemies. However, there is also evidence of collaboration between microbes (endosymbiosis), where bacteria live inside fungal cells. We have shown that, together, a bacterial endosymbiont and its fungal host create a holobiont that can make a powerful toxin that blocks soil-dwelling amoebae from engulfing the fungus. This endosymbiotic bacterium also changes how the fungus controls its own gene expression to fight different kinds of stress. This project examines how frequently fungi and endosymbionts evade amoebae; the implications for fungal housing of endosymbionts; and whether these symbioses lead to co-evolution. To answer these questions, we will look at bacterial-fungal partnerships across a wide range of species, looking for differences and commonalities in their shared genomes. We will also record reactions of wild-type and mutant fungal-bacterial endosymbionts with amoebae in the lab to identify the different strategies they can take to evade amoebae engulfment. Finally, we will examine one endosymbiont pair in depth to understand the mechanisms that allow these partnerships to exist. Amoebae are very similar to the cells in the human immune system that are the first line of defense against infection. Thus, this study can help our understanding of immunity. Additional broader impacts of this proposal include developing fungal biological education modules and teaching resources in the K-6 classrooms, develop mushroom grow kits for children, and participation in science podcasts. Identifying how environmental fungi developed traits to evade immune cells is critical to understanding the causes of opportunistic fungal infections in nature and in humans. Mucoromycota are primarily soil-associated fungi, some of which are also opportunistic mammalian pathogens. We recently identified an endosymbiosis between Betaproteobacteria Ralstonia (bacterium) and Mucoromycota Rhizopus (fungus), a symbiosis that blocks engulfment and killing of the fungus by the soil-dwelling amoeba Dictyostelium and also confers virulence in animals. Endosymbioses between Betaproteobacteria and Mucoromycota are environmentally ubiquitous. Endosymbionts are also observed in approximately half of clinical Rhizopus isolates, where phagocyte-related deficiencies are a major predictor of susceptibility. We hypothesize that interactions between bacterium-fungus holobionts and soil amoeba drive their evolutionary trajectories and opportunistic virulence in mammals. We will use phenotypic, genomic, and molecular tools to dissect the holobiont-phagocyte interaction and investigate host-pathogen interactions at two levels: 1) interactions between bacteria and their fungal hosts, and 2) the effect of endosymbionts on phagocyte evasion and opportunistic virulence. We will take both an unbiased approach to survey bacterial-fungal-amoebal interactions across the genus and a directed approach to investigate the molecular mechanisms for specific bacterial-fungal isolates alone and with amoeba. These mechanistic studies will be coupled to comparative genomics analyses to reveal how evolutionary pressures exerted by amoebae drive endosymbiosis and immune evasion. This collaborative US/UK project is supported by the US National Science Foundation and the UK Biotechnology and Biological Sciences Research Council. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.