Researchers discovered that cultures of a fungal endophyte found in perennial ryegrass contain complex bacterial communities dominated by Paenibacillus bacteria. These bacterial cells live on the surface of fungal filaments and regulate each other’s populations through antagonistic interactions, with one strain (E300) acting as a keystone species that controls the community structure. Despite these dynamic bacterial changes, the fungal host’s growth remained unaffected, suggesting a balanced symbiotic relationship important for the grass’s agricultural performance.