The quest to manipulate microbiomes has intensified, but many microbial communities have proven to be recalcitrant to sustained change. Developing model communities amenable to genetic dissection will underpin successful strategies for shaping microbiomes by advancing an understanding of community interactions. We developed a model community with representatives from three dominant rhizosphere taxa, the Firmicutes, Proteobacteria, and Bacteroidetes. We chose Bacillus cereus as a model rhizosphere firmicute and characterized 20 other candidates, including “hitchhikers” that coisolated with B. cereus from the rhizosphere. Pairwise analysis produced a hierarchical interstrain-competition network. We chose two hitchhikers, Pseudomonas koreensis from the top tier of the competition network and Flavobacterium johnsoniae from the bottom of the network, to represent the Proteobacteria and Bacteroidetes, respectively. The model community has several emergent properties, induction of dendritic expansion of B. cereus colonies by either of the other members, and production of more robust biofilms by the three members together than individually. Moreover, P. koreensis produces a novel family of alkaloid antibiotics that inhibit growth of F. johnsoniae, and production is inhibited by B. cereus. We designate this community THOR, because the members are the hitchhikers of the rhizosphere. The genetic, genomic, and biochemical tools available for dissection of THOR provide the means to achieve a new level of understanding of microbial community behavior. IMPORTANCE The manipulation and engineering of microbiomes could lead to improved human health, environmental sustainability, and agricultural productivity. However, microbiomes have proven difficult to alter in predictable ways, and their emergent properties are poorly understood. The history of biology has demonstrated the power of model systems to understand complex problems such as gene expression or development. Therefore, a defined and genetically tractable model community would be useful to dissect microbiome assembly, maintenance, and processes. We have developed a tractable model rhizosphere microbiome, designated THOR, containing Pseudomonas koreensis, Flavobacterium johnsoniae, and Bacillus cereus, which represent three dominant phyla in the rhizosphere, as well as in soil and the mammalian gut. The model community demonstrates emergent properties, and the members are amenable to genetic dissection. We propose that THOR will be a useful model for investigations of community-level interactions.

23!The quest to manipulate microbiomes has intensified, but many microbial communities have 24! proven recalcitrant to sustained change. Developing model communities amenable to genetic 25! dissection will underpin successful strategies for shaping microbiomes by advancing 26! understanding of community interactions. We developed a model community with 27! representatives from three dominant rhizosphere taxa: the Firmicutes, Proteobacteria, and 28! Bacteroidetes. We chose Bacillus cereus as a model rhizosphere Firmicute and characterized 29! twenty other candidates, including "hitchhikers" that co-isolated with B. cereus from the 30! rhizosphere. Pairwise analysis produced a hierarchical interstrain-competition network. We 31! chose two hitchhikers ! Pseudomonas koreensis from the top tier of the competition network 32! and Flavobacterium johnsoniae from the bottom of the network to represent the Proteobacteria 33! and Bacteroidetes, respectively. The model community has several emergent properties-34! induction of dendritic expansion of B. cereus colonies by either of the other members and 35! production of more robust biofilms by the three members together than individually. Moreover, 36! P. koreensis produces a novel family of alkaloid antibiotics that inhibit growth of F. johnsoniae, 37! and production is inhibited by B. cereus. We designate this community THOR, because the 38! members are the hitchhikers of the rhizosphere. The genetic, genomic, and biochemical tools 39! available for dissection of THOR provide the means to achieve a new level of understanding of 40! microbial community behavior. 41! 42! IMPORTANCE 43! The manipulation and engineering of microbiomes could lead to improved human health, 44! environmental sustainability, and agricultural productivity. However, microbiomes have proven 45! ! 3! difficult to alter in predictable ways and their emergent properties are poorly understood. The 46! history of biology has demonstrated the power of model systems to understand complex 47! problems such as gene expression or development. Therefore, a defined and genetically tractable 48! model community would be useful to dissect microbiome assembly, maintenance, and processes. 49! We have developed a tractable model rhizosphere microbiome, designated THOR, containing 50! Pseudomonas koreensis, Flavobacterium johnsoniae, and Bacillus cereus, which represent three 51! dominant phyla in the rhizosphere, as well as in soil and the mammalian gut. The model 52! community demonstrates emergent properties and the members are amenable to genetic 53! dissection. We propose that THOR will be a useful model for investigations of community-level 54! interactions. 55! 56!