The honeybee microbiota and its impact on health and disease

Abstract

Honeybees (Apis mellifera) are key pollinators that support global agriculture and are long-established models for developmental and behavioural research. Recently, they have emerged as models for studying gut microbial communities. Earlier research established that hindguts of adult worker bees harbour a conserved set of host-restricted bacterial species, each showing extensive strain variation. These bacteria can be cultured axenically and introduced to gnotobiotic hosts, and some have basic genetic tools available. In this Review, we explore the most recent research showing how the microbiota establishes itself in the gut and impacts bee biology and health. Microbiota members occupy specific niches within the gut where they interact with each other and the host. They engage in cross-feeding and antagonistic interactions, which likely contribute to the stability of the community and prevent pathogen invasion. An intact gut microbiota provides protection against diverse pathogens and parasites and contributes to the processing of refractory components of the pollen coat and dietary toxins. Absence or disruption of the microbiota results in altered expression of genes that underlie immunity, metabolism, behaviour and development. In the field, such disruption by agrochemicals may negatively impact bees. These findings demonstrate a key developmental and protective role of the microbiota, with broad implications for bee health.

Fig. 1.. Microbial dynamics and spatial organization in the honeybee gut.

a) Characteristic bacterial communities colonize the distal region of a typical worker honeybee gut (pylorus, ileum, and rectum), based on fluorescence in situ hybridization, localization of fluorescently marked strains, 16S rRNA amplicon sequencing and quantitative PCR studies^,,. b) In the pylorus, Frischella perrara activates the host immune system, including humoral immunity (Toll and Imd pathways that lead to the production of antimicrobial peptides) and cellular immunity (melanization cascade that leads to the scab-like phenotype observed in this tissue). Gilliamella apis is involved in the recycling of waste nitrogen and some degradation and fermentation of polysaccharides present in pollen. c) In the ileum, Snodgrassella alvi and Gilliamella apicola form a stable biofilm, activate the host immune system^,, and are potentially involved in cross-feeding with one another and the host^,. G. apicola produces enzymes for digestion and fermentation of pollen wall components. S. alvi can use host-derived organic acids to independently colonize the bee gut, but it is unclear whether bees can use bacteria-derived organic acids. d) In the rectum, Lactobacillus spp. and Bifidobacterium spp. are the most abundant bacteria and are involved in digestion and host immune system activation^,,. A distinctive bacteriophage community is associated with specific members of the microbiota^–. Gut microbiota members possess extensive mechanisms for antagonism, such as T6SS and bacteriocins, and these may play a role in community dynamics and pathogen protection. Question marks indicate unconfirmed processes, such as microbial interactions between and within S. alvi and G. apicola strains and absorption of bacteria-derived organic acids by bees. Black arrows indicate activation of host immunity pathways and blue arrows indicate metabolism (degradation, uptake, and/or utilization). Complexities of bee gut morphology are not depicted in this diagram.

Fig. 2.. The roles of the honeybee gut microbiota in pathogen protection.

Members of the microbiota protect honeybees from prokaryotic and eukaryotic pathogens. Protection may arise from activation of bee innate immune pathway, as in the case of protection by Snodgrassella alvi against Serratia marcescens and potentially Escherichia coli, and Lactobacillus spp. against Hafnia alvei and potentially trypanosomatids. Protection can occur also from production of antimicrobial molecules (for example, Bombella apis protection against Aspergillus flavus; Apilactobacillus kunkeei protection against Paenibacillus larvae and Melissococcus plutonius^,), or from formation of a stable biofilm that forms a physical barrier on the gut wall. An intact microbiota provides greater protection. The top left shows a worker honeybee with a simplified image of the gut. The top right shows a piece of frame comb from a hive, in which cells have different contents, including larvae (brown), pollen (yellow), and nectar (orange). Solid arrows indicate activation of specific immunity pathways, solid lines indicate inhibition of specific pathogens, and dashed lines indicate potential inhibition of specific pathogens. Complexities of bee gut morphology are not depicted in this diagram.

Fig. 3.. The roles of the honeybee gut microbiota in development and behaviour.

An intact microbiota is associated with increased expression of genes for vitellogenin and insulin signalling pathway, olfactory functions and behavioural shifts, neural development and synaptic transmission, and increased abundance of amino acids, glycerophospholipids, hormones and short-chain fatty acids in the gut, haemolymph and/or brain tissues^,,,,. These metabolites are associated with gut physiology, oxygen concentrations, pH, redox potential, and with regulation of developmental and behavioural genes, olfactory learning, and social interactions^,,,. AcCoA, acetyl coenzyme A; SCFA, short-chain fatty acid. Curved solid arrows indicate microbial metabolism, straight solid arrows indicate host uptake of microbial-derived byproducts, and dashed arrows indicate movement of microbial-derived metabolites within the honeybee body. Colours of the bacterial taxa correspond to those in Figure 1.

Fig. 4.. The roles of the honeybee gut microbiota in digestion and detoxification.

a) Easy-to-digest components from nectar (for example, some polysaccharides and simple sugars) and pollen (for example, amino acids, lipids, vitamins) are absorbed or metabolized by bee enzymes in the midgut. Metabolism of polysaccharides from the pollen coat releases several simple sugars that may be metabolized (for example, fructose and glucose) or not (for example, arabinose, galactose, mannose, and rhamnose) by bee enzymes. b) Hard-to-digest components, including refractory polysaccharides, toxic sugars^, and plant secondary metabolites like flavonoid and cyanogenic glycosides^,,, are primarily metabolized by specific strains of major members of the native microbiota (for example, Gilliamella spp., Bifidobacterium spp., Bombilactobacillus spp., and Lactobacillus spp.) in the ileum and rectum, through the production of pectin lyases and glycoside hydrolases^,. Solid arrows indicate production and release of digestive enzymes, and dashed arrows indicate microbial metabolism of toxic sugars and plant secondary metabolites.

Fig. 5.. Beekeeping and agricultural practices affecting honeybee gut communities.

In beekeeping, the overuse of antibiotics and acaricides for the treatment of larval infections and mite infestation, respectively, can negatively impact the abundance and composition of beneficial bacteria in the adult worker bee microbiota, with consequences for bee health, such as increased susceptibility to infections and higher mortality rates^,. Similarly, the indiscriminate use of fungicides, herbicides, and insecticides in agriculture, can negatively impact the adult worker bee microbiota, but effects are highly variable depending on the compounds involved and exposure level^,,. From left to right, pie charts illustrate the relative abundance of bee gut bacteria under normal conditions, and under exposure to tetracycline, glyphosate and imidacloprid.