Genomic investigations of diverse corbiculate bee gut-associated Gilliamella reveal conserved pathways for energy metabolism, with diverse and variable energy sources

Abstract

Gilliamella is a genus of bacteria commonly found as symbionts of corbiculate bees. Research into energy metabolism by this genus has predominantly been done through in vivo and in vitro experiments focused on the type species Gilliamella apicola. This study examined 95 publicly available genomes representing at least 18 Gilliamella species isolated predominantly from the hindgut of corbiculate bees. Energy metabolism pathways were found to be highly conserved across not only the Gilliamella but also other members of the family Orbaceae. Evidence suggests Gilliamella are capable of fermentation of both fumarate and pyruvate. Fermentation of the former produces succinate. Fermentation of the latter can produce acetate, ethanol, formate, and both isoforms of lactate for all Gilliamella and acetoin for some G. apicola strains. According to genomic evidence examined, all Gilliamella are only capable of respiration under microoxic conditions, while higher oxygen conditions likely inhibits respiration. Evidence suggests that the glycolysis and pentose phosphate pathways are essential mechanisms for the metabolism of energy sources, with the TCA cycle playing little to no role in energy metabolism for all Gilliamella species. Uptake of energy sources, i.e. sugars and derivatives, likely relies predominantly on the phosphoenol-pyruvate-dependent phosphotransferase system. Differences in the utilized energy sources may confer fitness advantages associated with specific host species.