A honey bee symbiont buffers larvae against nutritional stress through lysine supplementation

Honey bees, the worlds most significant agricultural pollinator, have suffered dramatic losses in the last few decades. These losses are largely due to the synergistic effects of multiple stressors, the most pervasive of which is limited nutrition. The effects of poor nutrition are most damaging in the developing larvae of honey bees, who mature into workers unable to meet the needs of their colony. It is therefore essential that we better understand the nutritional landscape experienced by honey bee larvae. In this study, we characterize the metabolic capabilities of a honey bee larvae-associated bacterium, Bombella apis (formerly Parasaccharibacter apium), and its effects on the nutritional resilience of larvae. We found that B. apis is the only bacterium associated with larvae that can withstand the antimicrobial larval diet. Further, we found that B. apis can synthesize all essential amino acids and significantly alters the amino acid content of synthetic larval diet, largely by increasing the essential amino acid lysine. Analyses of gene gain/loss across the phylogeny suggest that two distinct cationic amino acid transporters were gained by B. apis ancestors, and the transporter LysE is conserved across all sequenced strains of B. apis. This result suggests that amino acid export is a key feature conserved within the Bombella clade. Finally, we tested the impact of B. apis on developing honey bee larvae subjected to nutritional stress and found that larvae supplemented with B. apis are bolstered against mass reduction despite limited nutrition. Together, these data suggest an important role of B. apis as a nutritional mutualist of honey bee larvae. ### Competing Interest Statement The authors have declared no competing interest.