The microbiome and gene expression of honey bee workers are affected by a diet containing pollen substitutes

A Review of Diet and Foraged Pollen Interactions with the Honeybee Gut Microbiome

The honeybee Apis mellifera is a globally vital pollinator for flowering plants and crops, but it is currently facing mounting threats to survival due to habitat anthropization, emerging pathogens, and climate change. Over the past decade, increasing research efforts to understand and combat these challenges have led to an exploration of the honeybee gut microbiome-a relatively simple and highly conserved community of commensals which has a range of effects on the host. Researchers have now unravelled the main functional roles of this microbiome which include innate immune system stimulation, metabolism of dietary compounds, and mediation of host development and behaviour. Key amongst these is its role in aiding nutrition through the metabolism of complex carbohydrates and by degradation of otherwise indigestible pollen compounds. Increasingly, research is indicating that a diverse and high-quality pollen diet is key to maintaining healthy colonies and a stable microbiome. However, colonies can struggle to meet these dietary needs, particularly if they are located in anthropized ecosystems. Disruptions to honeybee diets or a reduction in the availability of diverse foraging options can significantly alter the composition of the microbiome, shifting it towards an abnormal state that leaves the honeybee more vulnerable to infection. Seasonal changes, primarily the overwintering period, also induce shifts in microbiome composition and are periods of time when a colony is particularly vulnerable to pathogenic infection. A comprehensive understanding of the effect these variables have on both microbiome composition and colony health is key to tackling the unprecedented environmental challenges that honeybees now face. This review summarises recent research which has elucidated the functional role of the gut microbiome in metabolism and how the composition of this bacterial community can alter due to seasonal change, anthropized landscapes, and dietary shifts. Finally, we also discuss recent studies investigating the effect that dietary supplementation has on the gut microbiome and the application of probiotic candidates for improving colony resilience and strength.

Core symbionts, age at inoculation and diet affect colonization of the bumblebee gut by a common bacterial pathogen

Microbes shape the health of bumblebees, an important group of pollinators, including species of conservation concern. Most microbial research on bumblebees has focused on eukaryotic and viral pathogens or the core gut microbiome, a community of host-specialized bacterial symbionts that helps protect hosts against eukaryotic pathogens. Bumblebees also harbour a third class of microbes: non-core gut bacteria, which are non-host specific and vary among individuals. Understanding their functional role and how they interact with core symbionts is important for bumblebee ecology and management. We surveyed non-core bacteria in wild bumblebee workers (Bombus impatiens) and conducted laboratory experiments with gnotobiotic B. impatiens to examine factors shaping colonization by a focal non-core bacterium (Serratia marcescens) and its consequences for bee health. Non-core bacteria, including Serratia, frequently occur at high abundance in wild bumblebees, with roughly half of individuals harbouring at least 10% non-core gut bacteria. Experiments showed that Serratia marcescens better colonizes the gut when bees are inoculated early (within 1 day of adult emergence) and the core gut microbiome is disrupted. A mixed wildflower pollen diet facilitated the highest level of infection compared with two monofloral pollen treatments. We also provide evidence that Serratia is pathogenic: exposing bees with disrupted gut microbiomes to Serratia strongly reduced lifespan and, as a result, also reduced total reproduction. These results have three important implications: first, non-core bacteria are widespread in wild bumblebees, and some species are opportunistic pathogens. Second, the core gut microbiome plays a crucial role in protecting against these pathogens. Third, the timing of inoculation relative to bee age, as well as diet, is a key factor controlling bacterial pathogen colonization of the gut. Overall, these findings suggest that gut bacterial health could be an important target for monitoring and managing bumblebee health.

Honey bee immune response to trace concentrations of clothianidin goes beyond the macronutrients found in artificial diets

Honey bees (Apis mellifera) often encounter a variety of stressors in their environment, including poor nutrition and pesticides. These stressors interact and can be exacerbated in large-scale agroecosystems. We investigated how diets varying in macronutrient ratios can affect nurse bee susceptibility to pesticide stressors. Nurse bees were fed trace concentrations of clothianidin (CLO), a neonicotinoid insecticide known to have sublethal and lethal effects on honey bees, after newly emerged bees were given diets varying in proteins and lipids, a natural pollen diet, or sucrose solution diet. Bees given pollen had improved longevity, physiology, enzyme activity, and gene expression related to pesticide detoxification. The artificial diets helped improve bee health and physiology but did little to promote bee detoxification enzymes and genes. There was no effect of the trace CLO treatments on its own, but there was an interactive effect between our higher CLO treatment and poor nutrition on bee longevity and vitellogenin expression. Our results suggest that (1) exposure to even trace concentrations of CLO can interact with poor nutrition to undermine adult bee health and (2) macronutrients in artificial diets can help promote bee physiology, but other nutrients in pollen, such as potentially phytochemicals, are more directly linked honey bee tolerance to pesticide stress.

Purified fibers in chemically defined synthetic diets destabilize the gut microbiome of an omnivorous insect model

The macronutrient composition of a host's diet shapes its gut microbial community, with dietary fiber in particular escaping host digestion to serve as a potent carbon source for gut microbiota. Despite widespread recognition of fiber's importance to microbiome health, nutritional research often fails to differentiate hyper-processed fibers from cell-matrix-derived intrinsic fibers, limiting our understanding of how individual polysaccharides influence the gut community. We use the American cockroach (Periplaneta americana) as a model system to dissect the response of complex gut microbial communities to dietary modifications that are difficult to test in traditional host models. Here, we designed synthetic diets from lab-grade, purified ingredients to identify how the cockroach microbiome responds to six different carbohydrates (chitin, methylcellulose, microcrystalline cellulose, pectin, starch, and xylan) in otherwise balanced diets. We show via 16S rRNA gene profiling that these synthetic diets reduce bacterial diversity and alter the phylogenetic composition of cockroach gut microbiota in a fiber-dependent manner, regardless of the vitamin and protein content of the diet. Comparisons with cockroaches fed whole-food diets reveal that synthetic diets induce blooms in common cockroach-associated taxa and subsequently fragment previously stable microbial correlation networks. Our research leverages an unconventional microbiome model system and customizable lab-grade artificial diets to shed light on how purified polysaccharides, as opposed to nutritionally complex intrinsic fibers, exert substantial influence over a normally stable gut community.

Diet affects reproductive development and microbiota composition in honey bees

BACKGROUND

Gut microbes are important to the health and fitness of many animals. Many factors have been shown to affect gut microbial communities including diet, lifestyle, and age. Most animals have very complex physiologies, lifestyles, and microbiomes, making it virtually impossible to disentangle what factors have the largest impact on microbiota composition. Honeybees are an excellent model to study host-microbe interactions due to their relatively simple gut microbiota, experimental tractability, and eusociality. Worker honey bees have distinct gut microbiota from their queen mothers despite being close genetic relatives and living in the same environment. Queens and workers differ in numerous ways including development, physiology, pheromone production, diet, and behavior. In the prolonged absence of a queen or Queen Mandibular Pheromones (QMP), some but not all workers will develop ovaries and become "queen-like". Using this inducible developmental change, we aimed to determine if diet and/or reproductive development impacts the gut microbiota of honey bee workers.

RESULTS

Microbiota-depleted newly emerged workers were inoculated with a mixture of queen and worker gut homogenates and reared under four conditions varying in diet and pheromone exposure. Three weeks post-emergence, workers were evaluated for ovary development and their gut microbiota communities were characterized. The proportion of workers with developed ovaries was increased in the absence of QMP but also when fed a queen diet (royal jelly). Overall, we found that diet, rather than reproductive development or pheromone exposure, led to more "queen-like" microbiota in workers. However, we revealed that diet alone cannot explain the microbiota composition of workers.

CONCLUSION

The hypothesis that reproductive development explains microbiota differences between queens and workers was rejected. We found evidence that diet is one of the main drivers of differences between the gut microbial community compositions of queens and workers but cannot fully explain the distinct microbiota of queens. Thus, we predict that behavioral and other physiological differences dictate microbiota composition in workers and queens. Our findings not only contribute to our understanding of the factors affecting the honey bee microbiota, which is important for bee health, but also illustrate the versatility and benefits of utilizing honeybees as a model system to study host-microbe interactions.

Purified fibers in chemically defined synthetic diets destabilize the gut microbiome of an omnivorous insect model

The macronutrient composition of a host's diet shapes its gut microbial community, with dietary fiber in particular escaping host digestion to serve as a potent carbon source for gut microbiota. Despite widespread recognition of fiber's importance to microbiome health, nutritional research often fails to differentiate hyper-processed fibers from cell-matrix derived intrinsic fibers, limiting our understanding of how individual polysaccharides influence the gut community. We use the American cockroach (Periplaneta americana) as a model system to dissect the response of complex gut microbial communities to diet modifications that are impossible to test in traditional host models. Here, we designed synthetic diets from lab-grade, purified ingredients to identify how the cockroach microbiome responds to six different carbohydrates (chitin, methylcellulose, microcrystalline cellulose, pectin, starch, xylan) in otherwise balanced diets. We show via 16S rRNA gene profiling that these synthetic diets reduce bacterial diversity and alter the phylogenetic composition of cockroach gut microbiota in a fiber-dependent manner, regardless of the vitamin and protein content of the diet. Comparisons with cockroaches fed whole-food diets reveal that synthetic diets induce blooms in common cockroach-associated taxa and subsequently fragment previously stable microbial correlation networks. Our research leverages an unconventional microbiome model system and customizable lab-grade artificial diets to shed light on how purified polysaccharides, as opposed to nutritionally complex intrinsic fibers, exert substantial influence over a normally stable gut community.

Temporospatial dynamics and host specificity of honeybee gut bacteria

Honeybees are important pollinators worldwide, with their gut microbiota playing a crucial role in maintaining their health. The gut bacteria of honeybees consist of primarily five core lineages that are spread through social interactions. Previous studies have provided a basic understanding of the composition and function of the honeybee gut microbiota, with recent advancements focusing on analyzing diversity at the strain level and changes in bacterial functional genes. Research on honeybee gut microbiota across different regions globally has provided insights into microbial ecology. Additionally, recent findings have shed light on the mechanisms of host specificity of honeybee gut bacteria. This review explores the temporospatial dynamics in honeybee gut microbiota, discussing the reasons and mechanisms behind these fluctuations. This synopsis provides insights into host-microbe interactions and is invaluable for honeybee health.

Improving bee feed recipes to safeguard honeybee colonies during times of food scarcity

In beekeeping, when natural nectar or pollen sources become limited, it is crucial to provide supplemental bee feed to maintain the viability of the bee colony. This study was conducted during the autumn food shortage season, during which bees were fed with different proportions of modified bee feed. We identified an optimal bee diet by evaluating honeybee longevity, food consumption, body weight, and gut microbe distribution, with natural pollen serving as a control diet. The results indicated that bees preferred a mixture of 65% defatted soy flour, 20% corn protein powder, 13% wheat germ flour, 2% yeast powder, and a 50% sucrose solution. This bee food recipe significantly increased the longevity, feed consumption, and body weight of bees. The group fed the natural pollen diet exhibited a greater abundance of essential intestinal bacteria. The bee diets used in this study contained higher protein levels and lower concentrations of unsaturated fatty acids and vitamins than did the diets stored within the colonies. Therefore, we propose that incorporating both bee feed and natural pollen in beekeeping practices will achieve more balanced nutritional intake.

Evaluation of Functional Properties of Some Lactic Acid Bacteria Strains for Probiotic Applications in Apiculture

This study evaluates the suitability of three lactic acid bacteria (LAB) strains-Lactiplantibacillus plantarum, Lactobacillus acidophilus, and Apilactobacillus kunkeei-for use as probiotics in apiculture. Given the decline in bee populations due to pathogens and environmental stressors, sustainable alternatives to conventional treatments are necessary. This study aimed to assess the potential of these LAB strains in a probiotic formulation for bees through various in vitro tests, including co-culture interactions, biofilm formation, auto-aggregation, antioxidant activity, antimicrobial activity, antibiotic susceptibility, and resistance to high osmotic concentrations. This study aimed to assess both the individual effects of the strains and their combined effects, referred to as the LAB mix. Results indicated no mutual antagonistic activity among the LAB strains, demonstrating their compatibility with multi-strain probiotic formulations. The LAB strains showed significant survival rates under high osmotic stress and simulated gastrointestinal conditions. The LAB mix displayed enhanced biofilm formation, antioxidant activity, and antimicrobial efficacy against different bacterial strains. These findings suggest that a probiotic formulation containing these LAB strains could be used for a probiotic formulation, offering a promising approach to mitigating the negative effects of pathogens. Future research should focus on in vivo studies to validate the efficacy of these probiotic bacteria in improving bee health.

The Bee Gut Microbiota: Bridging Infective Agents Potential in the One Health Context

The bee gut microbiota plays an important role in the services the bees pay to the environment, humans and animals. Alongside, gut-associated microorganisms are vehiculated between apparently remote habitats, promoting microbial heterogeneity of the visited microcosms and the transfer of the microbial genetic elements. To date, no metaproteomics studies dealing with the functional bee microbiota are available. Here, we employ a metaproteomics approach to explore a fraction of the bacterial, fungal, and unicellular parasites inhabiting the bee gut. The bacterial community portrays a dynamic composition, accounting for specimens of human and animal concern. Their functional features highlight the vehiculation of virulence and antimicrobial resistance traits. The fungal and unicellular parasite fractions include environment- and animal-related specimens, whose metabolic activities support the spatial spreading of functional features. Host proteome depicts the major bee physiological activities, supporting the metaproteomics strategy for the simultaneous study of multiple microbial specimens and their host-crosstalks. Altogether, the present study provides a better definition of the structure and function of the bee gut microbiota, highlighting its impact in a variety of strategies aimed at improving/overcoming several current hot topic issues such as antimicrobial resistance, environmental pollution and the promotion of environmental health.

The honeybee microbiota and its impact on health and disease

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.

Gustatory Responsiveness of Honey Bees Colonized with a Defined or Conventional Gut Microbiota

Gut microbes have many beneficial functions for host animals, such as food digestion and development of the immune system. An increasing number of studies report that gut bacteria also affect host neural function and behavior. The sucrose responsiveness of the western honey bee Apis mellifera, which harbors a characteristic gut microbiota, was recently reported to be increased by the presence of gut microbes. However, this responsiveness may vary depending on the experimental design, as animal behavior may be modulated by physiological states and environmental conditions. To evaluate the robustness of the effects of the gut microbiota on host gustatory responsiveness, we herein examined the sucrose responsiveness of honey bees colonized with a defined bacterial community or a conventional gut microbiota extracted from a field-collected bee. Although colonization was experimentally verified, sucrose responsiveness did not significantly differ among treatments after the 2- or 5-h starvation period. We concluded that the sucrose responsiveness of A. mellifera is not always affected by its gut microbiota. Therefore, host physiological conditions and environmental factors need to be considered when evaluating the impact of the gut microbiota on host neural function and behavior.