Fungal mycelia and bacterial thiamine establish a mutualistic growth mechanism

Exclusivity in physical spaces and nutrients is a prerequisite for survival of organisms, but a few species have been able to develop mutually beneficial strategies that allow them to co-habit. Here, we discovered a mutualistic mechanism between filamentous fungus, Aspergillus nidulans, and bacterium, Bacillus subtilis. The bacterial cells co-cultured with the fungus traveled along mycelia using their flagella and dispersed farther with the expansion of fungal colony, indicating that the fungal mycelia supply space for bacteria to migrate, disperse, and proliferate. Transcriptomic, genetic, molecular mass, and imaging analyses demonstrated that the bacteria reached the mycelial edge and supplied thiamine to the growing hyphae, which led to a promotion of hyphal growth. The thiamine transfer from bacteria to the thiamine non-auxotrophic fungus was directly demonstrated by stable isotope labeling. The simultaneous spatial and metabolic interactions demonstrated in this study reveal a mutualism that facilitates the communicating fungal and bacterial species to obtain an environmental niche and nutrient, respectively.