Oxalobacter formigenes produces metabolites and lipids undetectable in oxalotrophic Bifidobacterium animalis

INTRODUCTION In the search for new potential therapies for pathologies of oxalate, such as kidney stone disease and primary hyperoxaluria, the intestinal microbiome has generated significant interest. Resident oxalate-degrading bacteria inhabit the gastrointestinal tract and reduce absorption of dietary oxalate, thereby potentially lowering the potency of oxalate as a risk factor for kidney stone formation. Although several species of bacteria have been shown to degrade oxalate, select strains of Oxalobacter formigenes (O. formigenes) have thus far demonstrated the unique ability among oxalotrophs to initiate a net intestinal oxalate secretion into the lumen from the bloodstream, allowing them to feed on both dietary and endogenous metabolic oxalate. There is significant interest in this function as a potential therapeutic application for circulating oxalate reduction, although its mechanism of action is still poorly understood. Since this species-exclusive, oxalate-regulating function is reported to be dependent on the use of a currently unidentified secreted bioactive compound, there is much interest in whether O. formigenes produces unique biochemicals that are not expressed by other oxalotrophs which lack the ability to transport oxalate. Hence, this study sought to analyze and compare the metabolomes of O. formigenes and another oxalate degrader, Bifidobacterium animalis subsp. lactis (B. animalis), to determine whether O. formigenes could produce features undetectable in another oxalotroph, thus supporting the theory of a species-exclusive secretagogue compound. METHODS A comparative metabolomic analysis of O. formigenes strain HC1 (a human isolate) versus B. animalis, another oxalate-degrading human intestinal microbe, was performed by ultra-high-performance liquid chromatography-high-resolution mass spectrometry (UHPLC-HRMS). Bacteria were cultured independently in anaerobic conditions, harvested, lysed, and extracted by protein precipitation. Metabolite extracts were chromatographically separated and analyzed by UHPLC-HRMS using reverse phase gradient elution (ACE Excel 2 C18-Pentafluorophenyl column) paired with a Q Exactive™ mass spectrometer. OBJECTIVES The purpose of this study was to assess whether O. formigenes potentially produces unique biochemicals from other oxalate degraders to better understand its metabolic profile and provide support for the theoretical production of a species-exclusive secretagogue compound responsible for enhancing intestinal oxalate secretion. RESULTS We report a panel of metabolites and lipids detected in the O. formigenes metabolome which were undetectable in B. animalis, several of which were identified either by mass-to-charge ratio and retention time matching to our method-specific metabolite library or MS/MS fragmentation. Furthermore, re-examination of data from our previous work showed most of these features were also undetected in the metabolomes of Lactobacillus acidophilus and Lactobacillus gasseri, two other intestinal oxalate degraders. CONCLUSIONS Our observation of O. formigenes metabolites and lipids which were undetectable in other oxalotrophs suggests that this bacterium likely holds the ability to produce biochemicals not expressed by at least a selection of other oxalate degraders. These findings provide support for the hypothesized biosynthesis of a species-exclusive secretagogue responsible for the stimulation of net intestinal oxalate secretion.