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

<i>Gilliamella</i> is a genus of bacteria commonly found as symbionts of corbiculate bees. Research into energy metabolism by this genus has predominantly been done through <i>in vivo</i> and <i>in vitro</i> experiments focused on the type species <i>Gilliamella apicola</i>. This study examined 95 publicly available genomes representing at least 18 <i>Gilliamella</i> species isolated predominantly from the hindgut of corbiculate bees. Energy metabolism pathways were found to be highly conserved across not only the <i>Gilliamella</i> but also other members of the family <i>Orbaceae</i>. Evidence suggests <i>Gilliamella</i> 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 <i>Gilliamella</i> and acetoin for some <i>G. apicola</i> strains. According to genomic evidence examined, all <i>Gilliamella</i> 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 <i>Gilliamella</i> species. Uptake of energy sources, i.e. sugars and derivatives, likely relies predominantly on the phospho<i>enol</i>-pyruvate-dependent phosphotransferase system. Differences in the utilized energy sources may confer fitness advantages associated with specific host species.