Recent research shows that the hyphae of arbuscular mycorrhizal fungi is inhabited by very specific bacteria, in particular, bacteria able to mineralize phosphorous, a crucial nutrient in the symbioses between those fungi and 80% of land plants. Furthermore, there seems to be core microbiome of such bacteria across vastly different sites. We suggest that the plant host plus some mycorrhizal fungi plus some bacteria living in the hyphae of such fungi might constitute a unit of natural selection.