Dominance of Fructose-Associated Fructobacillus in the Gut Microbiome of Bumblebees (Bombus terrestris) Inhabiting Natural Forest Meadows

Bee-Associated Beneficial Microbes-Importance for Bees and for Humans

Bees are one of the best-known and, at the same time, perhaps the most enigmatic insects on our planet, known for their organization and social structure, being essential for the pollination of agricultural crops and several other plants, playing an essential role in food production and the balance of ecosystems, being associated with the production of high-value-added inputs, and a unique universe in relation to bees' microbiota. In this review, we summarize information regarding on different varieties of bees, with emphasis on their specificity related to microbial variations. Noteworthy are fructophilic bacteria, a lesser-known bacterial group, which use fructose fermentation as their main source of energy, with some strains being closely related to bees' health status. The beneficial properties of fructophilic bacteria may be extendable to humans and other animals as probiotics. In addition, their biotechnological potential may ease the development of new-generation antimicrobials with applications in biopreservation. The concept of "One Health" brings together fundamental and applied research with the aim of clarifying that the connections between the different components of ecosystems must be considered part of a mega-structure, with bees being an iconic example in that the healthy functionality of their microbiota is directly and indirectly related to agricultural production, bee health, quality of bee products, and the functional prosperity for humans and other animals. In fact, good health of bees is clearly related to the stable functionality of ecosystems and indirectly relates to humans' wellbeing, a concept of the "One Health".

A comparative genomic analysis of Fructobacillus evanidus sp. nov. from bumble bees

The increase in studies on bee microbiomes is prompted by concerns over global pollinator declines. Bumble bees host core and non-core microbiota which may contribute to increased lifetime fitness. The presence of Fructobacillus in the gut microbiomes of bumble bee workers, or the replacement of core symbionts with Fructobacillus bacteria, has been considered a marker of dysbiosis. A phylogenomic analysis and functional genomic characterization of the genomes of 21 Fructobacillus isolates from bumble bees demonstrated that they represented four species, i.e. Fructobacillus cardui, Fructobacillus fructosus, Fructobacillus tropaeoli, and the novel species Fructobacillus evanidus sp. nov. Our results confirmed and substantiated the presence of two phylogenetically and functionally distinct Fructobacillus species clades that differ in genome size, percentage G + C content, the number of coding DNA sequences and metabolic characteristics. Clade 1 and clade 2 species differed in amino acid and, to a lesser extent, in carbohydrate metabolism, with F. evanidus and F. tropaeoli genomes featuring a higher number of complete metabolic pathways. While Fructobacillus genomes encoded genes that allow adhesion, biofilm formation, antibacterial activity and detoxification, other bacteria isolated from the bumble bee gut appeared better equipped to co-exist with the bumble bee host. The isolation and identification of multiple Fructobacillus species from several bumble bee gut samples in the present study also argued against a specific partnership between Fructobacillus species and their bumble bee hosts.

Bumble bee microbiota shows temporal succession and increase of lactic acid bacteria when exposed to outdoor environments

Question

The large earth bumble bee (Bombus terrestris) maintains a social core gut-microbiota, similar as known from the honey bee, which plays an important role for host health and resistance. Experiments under laboratory conditions with commercial hives are limited to vertically transmitted microbes and neglect influences of environmental factors or external acquisition of microbes. Various environmental and landscape-level factors may have an impact on the gut-microbiota of pollinating insects, with consequences for pollinator health and fitness in agroecosystems. Still, it is not fully clear whether access to different flower diversities will have a significant influence on the bumble bee microbiota. Here, we tested in a semi-field experiment if the bumble bee microbiota changes over time when exposed to different flower diversities within outdoor flight cages. We used commercial hives to distinguish between vertically and horizontally transmitted bacteria, respectively from the nest environment or the exposed outside environment.

Result

The sequential sampling of foraging workers over a period of 35 days indicated a temporal progression of the bumble bee microbiota when placed outside. The microbiota increased in diversity and changed in composition and variability over time. We observed a major increase in relative abundance of the families Lactobacillaceae, Bifidobacteriaceae and Weeksellaceae. In contrast, major core-taxa like Snodgrassella and Gilliamella declined in their relative abundance over time. The genus Lactobacillus showed a high diversity and strain specific turnover, so that only specific ASVs showed an increase over time, while others had a more erratic occurrence pattern. Exposure to different flower diversities had no significant influence on the progression of the bumble bee microbiota.

Conclusion

The bumble bee microbiota showed a dynamic temporal succession with distinct compositional changes and diversification over time when placed outdoor. The exposure of bumble bees to environmental conditions, or environmental microbes, increases dissimilarity and changes the gut-community composition. This shows the importance of environmental influences on the temporal dynamic and progression of the bumble bee microbiota.

A case study of the diet-microbiota-parasite interplay in bumble bees

AIMS

Diets and parasites influence the gut bacterial symbionts of bumble bees, but potential interactive effects remain overlooked. The main objective of this study was to assess the isolated and interactive effects of sunflower pollen, its phenolamides, and the widespread trypanosomatid Crithidia sp. on the gut bacterial symbionts of Bombus terrestris males.

METHODS AND RESULTS

Bumble bee males emerged in microcolonies fed on either (i) willow pollen (control), (ii) sunflower pollen, or (iii) willow pollen spiked with phenolamide extracts from sunflower pollen. These microcolonies were infected by Crithidia sp. or were pathogen-free. Using 16S rRNA amplicon sequencing (V3-V4 region), we observed a significant alteration of the beta diversity but not of the alpha diversity in the gut microbial communities of males fed on sunflower pollen compared to males fed on control pollen. Similarly, infection by the gut parasite Crithidia sp. altered the beta diversity but not the alpha diversity in the gut microbial communities of males, irrespective of the diet. By contrast, we did not observe any significant alteration of the beta or alpha diversity in the gut microbial communities of males fed on phenolamide-enriched pollen compared to males fed on control pollen. Changes in the beta diversity indicate significant dissimilarities of the bacterial taxa between the treatment groups, while the lack of difference in alpha diversity demonstrates no significant changes within each treatment group.

CONCLUSIONS

Bumble bees harbour consistent gut microbiota worldwide, but our results suggest that the gut bacterial communities of bumble bees are somewhat shaped by their diets and gut parasites as well as by the interaction of these two factors. This study confirms that bumble bees are suitable biological surrogates to assess the effect of diet and parasite infections on gut microbial communities.

Studies on the novel effects of electron beam treated pollen on colony reproductive output in commercially-reared bumblebees (Bombus terrestris) for mass pollination applications

Commercially-reared bumblebees provide an important pollinator service that helps support food production and security. The deployment of an appropriate non-thermal disinfection technology for the bulk treatment of pollen collected from honeybees for the feeding of commercial bumblebees is important in order to mitigate against complex diseases and unwanted pathogen spillover to native bees. High level disinfection of pollen was achieved using an electron (e)-beam dose of 100 kGy that corresponded to 78 % loss of cellular viability of bee pathogens before feeding to bumblebees as measured by the novel in vitro use of flow cytometry (FCM). Novel findings showed that e-beam treated-pollen that was fed to bumblebees produced fewer females, gynes and exhibited an absence of males when compared to control bumblebee colonies that were fed untreated commercial pollen. A similar trend emerged in bumblebee colony reproductive outputs when using membrane filtered washed pollen. Proteomic analysis of bumblebees from individual colonies fed with treated-pollen revealed a differential abundance of proteins associated with stress, immunity and metabolism when compared to the untreated pollen control group. Microbiome analysis of the bumblebee gut content revealed differences in microbiota between treated and untreated pollen in bumblebee colony studies. This novel study evaluated the impact of industrial e-beam treated-pollen on complex bee disease mitigation where physically treated-pollen fed to bumblebees was shown to substantially affect colony reproductive outputs.

Assessing potential impact of gut microbiome disruptions on the environmental stress resilience of indoor-reared Bombus terrestris

Bumblebees are crucial for both natural ecosystems and agriculture, but their decline in distribution and abundance over the past decade is alarming. The global importance of bumblebees in natural ecosystems and agricultural food production cannot be overstated. However, the reported decline over the past decade has led to a surge of interest in understanding and addressing bumblebee population decline. Hence, we aimed to detect disruptions in the gut microbiome of male and worker bumblebees reared indoor and outdoor to assess potential resilience to environmental stress. Using the Illumina MiSeq platform for 16s rRNA amplicon sequencing, we analyzed the gut microbiome of male and worker bees that were raised indoors (designated as the IM and IW group) and those that were raised outdoors (also designated as the OM and OW group). Our results show presence of core bacteria Neisseriaceae, Orbaceae, Lactobacillaceae and Bifidobacteriaceae from indoor reared worker bees. However, a higher abundance of Bifidobacterium and absence of Fructobacillus from indoor reared worker bees was also observed. Indoor-reared male bees had lower diversity and fewer observed OTUs compared to outdoor-reared male bees. Additionally, the relative abundance of Actinobacteriota, Bacteroidota, and Firmicutes was significantly lower in indoor-reared males, while Proteobacteria was significantly increased. Despite this, we did not observe any dysbiosis in the gut microbiota of indoor-reared bumblebees when comparing the role of the gut symbionts among the groups. These results suggest that indoor-reared Bombus terrestris may be resilient to environmental stress when used as outdoor pollinators.

The wild solitary bees Andrena vaga, Anthophora plumipes, Colletes cunicularius, and Osmia cornuta microbiota are host specific and dominated by endosymbionts and environmental microorganisms

We characterized the microbial communities of the crop, midgut, hindgut, and ovaries of the wild solitary bees Andrena vaga, Anthophora plumipes, Colletes cunicularius, and Osmia cornuta through 16S rRNA gene and ITS2 amplicon sequencing and a large-scale isolation campaign. The bacterial communities of these bees were dominated by endosymbionts of the genera Wolbachia and Spiroplasma. Bacterial and yeast genera representing the remaining predominant taxa were linked to an environmental origin. While only a single sampling site was examined for Andrena vaga, Anthophora plumipes, and Colletes cunicularius, and two sampling sites for Osmia cornuta, the microbiota appeared to be host specific: bacterial, but not fungal, communities generally differed between the analyzed bee species, gut compartments and ovaries. This may suggest a selective process determined by floral and host traits. Many of the gut symbionts identified in the present study are characterized by metabolic versatility. Whether they exert similar functionalities within the bee gut and thus functional redundancy remains to be elucidated.

Untargeted Lipidomics Analysis Unravels the Different Metabolites in the Fat Body of Mated Bumblebee (Bombus terrestris) Queens

The fat body has important functions in energy, fertility, and immunity. In female insects, mating stimulates physiological, behavioral, and gene expression changes. However, it remains unclear whether the metabolites in the fat body are affected after the bumblebee (Bombus terrestris) queen mates. Here, the ultrastructure and lipid metabolites in fat body of mated queens were compared with those of virgins. The fat body weight of mated bumblebee queens was significantly increased, and the adipocytes were filled with lipid droplets. Using LC-MS/MS-based untargeted lipidomics, 949 and 748 differential metabolites were identified in the fat body of virgin and mated bumblebee queens, respectively, in positive and negative ion modes. Most lipid metabolites were decreased, especially some biomembrane components. In order to explore the relationship between the structures of lipid droplets and metabolite accumulation, transmission electron microscopy and fluorescence microscopy were used to observe the fat body ultrastructure. The size/area of lipid droplets was larger, and the fusion of lipid droplets was increased in the mated queen's fat body. These enlarged lipid droplets may store more energy and nutrients. The observed differences in lipid metabolites in the fat body of queens contribute to understanding the regulatory network of bumblebees post mating.

Microbiome assembly and maintenance across the lifespan of bumble bee workers

How a host's microbiome changes over its lifespan can influence development and ageing. As these temporal patterns have only been described in detail for a handful of hosts, an important next step is to compare microbiome succession more broadly and investigate why it varies. Here we characterize the temporal dynamics and stability of the bumble bee worker gut microbiome. Bumble bees have simple and host-specific gut microbiomes, and their microbial dynamics may influence health and pollination services. We used 16S rRNA gene sequencing, quantitative PCR and metagenomics to characterize gut microbiomes over the lifespan of Bombus impatiens workers. We also sequenced gut transcriptomes to examine host factors that may control the microbiome. At the community level, microbiome assembly is highly predictable and similar to patterns of primary succession observed in the human gut. However, at the strain level, partitioning of bacterial variants among colonies suggests stochastic colonization events similar to those observed in flies and nematodes. We also find strong differences in temporal dynamics among symbiont species, suggesting ecological differences among microbiome members in colonization and persistence. Finally, we show that both the gut microbiome and host transcriptome-including expression of key immunity genes-stabilize, as opposed to senesce, with age. We suggest that in highly social groups such as bumble bees, maintenance of both microbiomes and immunity contribute to inclusive fitness, and thus remain under selection even in old age. Our findings provide a foundation for exploring the mechanisms and functional outcomes of bee microbiome succession.