A Bacterial Symbiont Protects Honey Bees from Fungal Disease.

Fungal pathogens, among other stressors, negatively impact the productivity and population size of honey bees, one of our most important pollinators (1, 2), in particular their brood (larvae and pupae) (3, 4). Understanding the factors that influence disease incidence and prevalence in brood may help us improve colony health and productivity. Here, we examined the capacity of a honey bee-associated bacterium, Bombella apis, to suppress the growth of fungal pathogens and ultimately protect bee brood from infection. Our results showed that strains of inhibit the growth of two insect fungal pathogens, Beauveria bassiana and Aspergillus flavus, . This phenotype was recapitulated ; bee broods supplemented with were significantly less likely to be infected by A. flavus. Additionally, the presence of reduced sporulation of A. flavus in the few bees that were infected. Analyses of biosynthetic gene clusters across strains suggest antifungal candidates, including a type 1 polyketide, terpene, and aryl polyene. Secreted metabolites from alone were sufficient to suppress fungal growth, supporting the hypothesis that fungal inhibition is mediated by an antifungal metabolite. Together, these data suggest that can suppress fungal infections in bee brood via secretion of an antifungal metabolite. Fungi can play critical roles in host microbiomes (5-7), yet bacterial-fungal interactions are understudied. For insects, fungi are the leading cause of disease (5, 8). In particular, populations of the European honey bee (Apis mellifera), an agriculturally and economically critical species, have declined in part due to fungal pathogens. The presence and prevalence of fungal pathogens in honey bees have far-reaching consequences, endangering other species and threatening food security (1, 2, 9). Our research highlights how a bacterial symbiont protects bee brood from fungal infection. Further mechanistic work could lead to the development of new antifungal treatments.