Honeybee symbionts protect their host from American Foulbrood disease

Recent mass declines, associated to both abiotic and biotic factors, are severely affecting pollinator insects. Despite intense research on the microbial communities associated to the pollinators, little is known about the contribution of gut symbionts to honeybee health. Here we show that selected gut symbionts have a beneficial effect on the larvae by efficiently protecting from Paenibacillus larvae, the causative bacterium of the American Foulbrood Disease, one of the most devastating diseases of the beehive. We have characterized the bacterial microbiome associated to larvae of symptomatic and asymptomatic beehives using multiple approaches targeting 16S rRNA gene including PCR-DGGE, phylochip and 454-based high throughput sequencing. A gut dysbiosis was observed in individuals from symptomatic beehives that were dominated by P. larvae signatures, while asymptomatic animals presented the typical assembly already associated to healthy larvae. Following an intensive screening of bacterial isolates from asymptomatic healthy larvae, we selected two spore-forming bacteria capable of inhibiting P. larvae growth in vitro. Both the strains and their combination reduced the larvae mortality in vivo following exposure to the pathogen. Such a protection was confirmed in the beehive during field experiments. In the beehive, the larvae treated with the strains were protected from P. larvae infection with the combined treatment of the two strains being the most effective. The protection was contributed by multiple controlling factors including direct antagonism against P. larvae, elicitation of the expression of antimicrobial peptides and competitive exclusion of the pathogen due to the colonization of the gut epithelium as shown by fluorescent in situ hybridization.