The bumble bee gut microbiota: ecological dynamics, functional roles and environmental responses

Bumble bees ( Bombus spp.) are important pollinators in natural and agricultural ecosystems, but many species are increasingly exposed to interacting pressures including habitat alteration, climate change, pesticides, nutritional stress, and pathogens. The gut microbiota has emerged as a potentially important component of bumble bee biology, with evidence linking host-associated microorganisms to nutrition, immunity, and pathogen resistance. Bumble bees harbor a relatively simple and specialized gut community dominated by recurrent bacterial lineages including Snodgrassella , Gilliamella , Bombiscardovia , Bifidobacterium , and Lactobacillaceae, although substantial variation occurs at the strain and community levels. This review critically examines current knowledge of bumble bee gut microbiota composition, transmission, functional significance, and responses to environmental conditions. Particular attention is given to the strength of evidence supporting proposed microbial functions and to the distinction between taxonomic associations, predicted metabolic potential, and experimentally demonstrated effects on the host. Current evidence provides comparatively strong experimental support for microbiota-mediated resistance to intestinal pathogens and immune modulation, together with genomic and culture-based evidence for microbial specialization in carbohydrate metabolism. By contrast, proposed contributions to behavioral regulation, caste differentiation, detoxification, and host responses to environmental stressors remain less firmly established. Diet, captive rearing, pesticides, pollutants, habitat, and climatic conditions can alter gut microbial communities, but the functional consequences of these compositional changes remain poorly resolved. Recent advances in strain-resolved metagenomics, functional omics, and experimental microbiota manipulation provide new opportunities to address these limitations. Establishing when microbiota variation represents functional change rather than taxonomic restructuring alone will be essential for defining the contribution of gut microorganisms to bumble bee biology and for evaluating their potential relevance to pollinator health and management.