Origin and effect of Alpha 2.2 Acetobacteraceae in honey bee larvae and description of Parasaccharibacter apium gen. nov., sp. nov.

The honey bee hive environment contains a rich microbial community that differs according to niche. Acetobacteraceae Alpha 2.2 (Alpha 2.2) bacteria are present in the food stores, the forager crop, and larvae but at negligible levels in the nurse and forager midgut and hindgut. We first sought to determine the source of Alpha 2.2 in young larvae by assaying the diversity of microbes in nurse crops, hypopharyngeal glands (HGs), and royal jelly (RJ). Amplicon-based pyrosequencing showed that Alpha 2.2 bacteria occupy each of these environments along with a variety of other bacteria, including Lactobacillus kunkeei. RJ and the crop contained fewer bacteria than the HGs, suggesting that these tissues are rather selective environments. Phylogenetic analyses showed that honey bee-derived Alpha 2.2 bacteria are specific to bees that "nurse" the hive's developing brood with HG secretions and are distinct from the Saccharibacter-type bacteria found in bees that provision their young differently, such as with a pollen ball coated in crop-derived contents. Acetobacteraceae can form symbiotic relationships with insects, so we next tested whether Alpha 2.2 increased larval fitness. We cultured 44 Alpha 2.2 strains from young larvae that grouped into nine distinct clades. Three isolates from these nine clades flourished in royal jelly, and one isolate increased larval survival in vitro. We conclude that Alpha 2.2 bacteria are not gut bacteria but are prolific in the crop-HG-RJ-larva niche, passed to the developing brood through nurse worker feeding behavior. We propose the name Parasaccharibacter apium for this bacterial symbiont of bees in the genus Apis.