AimTo examine the role of the Lactobacillus reuteri 100-23C frc gene product in oxalate metabolism, host colonization and the acid stress response. Methods and ResultsGenes encoding putative formyl-CoA transferase (frc) and oxalyl-CoA decarboxylase (oxc) enzymes are present in the genome sequences of Lact.reuteri strains. Two strains isolated from humans harboured an IS200 insertion sequence in the frc ORF and a group 2 intron-associated transposase downstream of the frc gene, both of which were lacking in two strains of animal origin, which contained intact frc and oxc genes. An frc(-) insertional mutant of Lact.reuteri 100-23C was compared with the parent strain with respect to oxalate degradation, colonization of an RLF-mouse host model and growth in the presence of acids. Neither parent nor mutant degraded oxalate in vitro or in vivo. However, the parent outcompeted the frc(-) mutant in the mouse intestine during co-colonization and the frc(-) mutant showed a reduced growth rate in the presence of hydrochloric acid. ConclusionsIntact oxc and frc genes do not ensure oxalate degradation under the conditions tested. The frc gene product is important during host colonization and survival of acid stress by Lact.reuteri 100-23C. Significance and Impact of the StudyOxalate metabolism by oxalate-degrading intestinal bacterial strains may be important in preventing urolithiasis and might lead to the derivation of probiotic products. To produce safe and efficacious probiotics, however, an understanding of the genetic characteristics of potential oxalate degraders must be obtained, together with knowledge of their functional ramifications.