A Multidimensional Framework for Understanding Microbial Mutualisms

Microbial mutualisms are still commonly described as obligate or facultative, but this dichotomy is insufficient to describe the complexity of host–microbe interactions. A multi-dimensional conceptual framework that accounts for asymmetry in dependency and taxonomic membership flexibility is introduced. This framework allows the classification of mutualisms by how they are structured and for whom the interaction is necessary. The dimensions include dependency considered independently for both organisms, and both the global/potential and individual/actual microbiome diversities. Examples are given primarily from the field of insect–microbe interactions, showing how past frameworks cannot fully encompass the diversity of symbioses. This new framework integrates symbiont community complexity and highlights functional redundancy, which is useful for understanding host plasticity, vulnerability, invasiveness, and resilience.