Impact of Nutritional Stress on Honeybee Gut Microbiota, Immunity, and Nosema ceranae Infection.

Honeybees are important pollinators, having an essential role in the ecology of natural and agricultural environments. Honeybee colony losses episodes reported worldwide and have been associated with different pests and pathogens, pesticide exposure, and nutritional stress. This nutritional stress is related to the increase in monoculture areas which leads to a reduction of pollen availability and diversity. In this study, we examined whether nutritional stress affects honeybee gut microbiota, bee immunity, and infection by Nosema ceranae, under laboratory conditions. Consumption of Eucalyptus grandis pollen was used as a nutritionally poor-quality diet to study nutritional stress, in contraposition to the consumption of polyfloral pollen. Honeybees feed with Eucalyptus grandis pollen showed a lower abundance of Lactobacillus mellifer and Lactobacillus apis (Firm-4 and Firm-5, respectively) and Bifidobacterium spp. and a higher abundance of Bartonella apis, than honeybees fed with polyfloral pollen. Besides the impact of nutritional stress on honeybee microbiota, it also decreased the expression levels of vitellogenin and genes associated to immunity (glucose oxidase, hymenoptaecin and lysozyme). Finally, Eucalyptus grandis pollen favored the multiplication of Nosema ceranae. These results show that nutritional stress impacts the honeybee gut microbiota, having consequences on honeybee immunity and pathogen development. Those results may be useful to understand the influence of modern agriculture on honeybee health.

Impact of nutritional stress on the honeybee colony health.

Honeybees Apis mellifera are important pollinators of wild plants and commercial crops. For more than a decade, high percentages of honeybee colony losses have been reported worldwide. Nutritional stress due to habitat depletion, infection by different pests and pathogens and pesticide exposure has been proposed as the major causes. In this study we analyzed how nutritional stress affects colony strength and health. Two groups of colonies were set in a Eucalyptus grandis plantation at the beginning of the flowering period (autumn), replicating a natural scenario with a nutritionally poor food source. While both groups of colonies had access to the pollen available in this plantation, one was supplemented with a polyfloral pollen patty during the entire flowering period. In the short-term, colonies under nutritional stress (which consumed mainly E. grandis pollen) showed higher infection level with Nosema spp. and lower brood and adult bee population, compared to supplemented colonies. On the other hand, these supplemented colonies showed higher infection level with RNA viruses although infection levels were low compared to countries were viral infections have negative impacts. Nutritional stress also had long-term colony effects, because bee population did not recover in spring, as in supplemented colonies did. In conclusion, nutritional stress and Nosema spp. infection had a severe impact on colony strength with consequences in both short and long-term.

Characterization of Nosema ceranae Genetic Variants from Different Geographic Origins.

In recent years, large-scale colony losses of honey bees (Apis mellifera) have been reported and the infection with the microsporidia Nosema ceranae has been involved. However, the effect of N. ceranae at the colony level and its role in colony losses vary in different geographic areas. This difference may be related to the presence of multiple N. ceranae genetic variants resulting in different biological consequences. In this study, we analyzed the genetic diversity of 75 N. ceranae samples obtained from 13 countries and Hawaii through inter-sequence single repetition (ISSR) and evaluated if two of these genetic variants triggered different immune responses when infecting Apis mellifera iberiensis. The genetic diversity analysis showed that 41% of the samples had the same DNA amplification pattern, including samples from most European countries except Spain, while the remaining samples showed high variability. Infection assays were performed to analyze the infection levels and the immune response of bees infected with N. ceranae from Spain and Uruguay. The infected bees presented similar infection levels, and both isolates downregulated the expression of abaecin, confirming the ability of the microsporidia to depress the immune response. Only N. ceranae from Uruguay downregulated the expression level of imd compared to control bees. On the other hand, both genetic variants triggered different expression levels of lysozyme. As imd and lysozyme play important roles in the response to pathogens, these results could reflect differences in the biological consequences of N. ceranae variants in A. mellifera infection.

High prevalence and infection levels of Nosema ceranae in bumblebees Bombus atratus and Bombus bellicosus from Uruguay.

Nosema ceranae is one of the most prevalent pathogens in Apis mellifera and has recently been found in multiple host species including several species of bumblebees. Prevalence and infection intensity of N. ceranae was determined in two species of native bumblebees from Uruguay. Nosema ceranae was the only microsporidia identified and mean prevalence was 72% in Bombus atratus and 63% in Bombus bellicosus, values much higher than those reported elsewhere. The presence of this pathogen in bumblebees may be threatening not only for bumblebee populations, but also to the rest of the native pollinator community and to honeybees.