Role of oxalate metabolizing bacteria in calcium oxalate urolithiasis in dogs

Abstract

The incidence of calcium oxalate (CaOx) urolithiasis in dogs, cats and humans has increased steadily over the last two decades. Supersaturation of calcium and oxalate ions in the urine leads to CaOx urinary stone formation in dogs, cats and humans. Therefore, reducing urine concentrations of these stone components are essential to prevent reformation. Oxalate-metabolizing enteric bacteria minimize the freely available oxalate in the gut and reduce the enteric absorption. We hypothesize that decreased colonization of enteric oxalate-degrading bacteria is a risk factor for CaOx urolithiasis in dogs. Fecal samples from dogs with CaOx uroliths, clinically healthy breed matched-dogs, and healthy non-stone forming breed dogs were screened for the presence of Oxalobacter formigenes, Lactobacillus acidophilus, and Bifidobacterium animalis by PCR to detect the species-specific oxalyl CoA decarboxylase (oxc) genes. Further, hind-gut microbiota of the dogs with CaOx stone and healthy dogs was compared by analyzing the sequences of V3-hypervariable region of 16S rDNA libraries. Finally, the E. coli Nissle 1917 (EcN) probiotic strain was engineered to heterologously co-express the oxc, frc and OxlT of O. formigenes or OXDC of B. subtilis.Presence of Oxalobacter formigenes, Lactobacillus acidophilus, and Bifidobacterium animalis was significantly higher in healthy non-stone forming breed dogs than in the dogs with CaOx stones. Based on the 16S rRNA sequences, in total, 1,223 operational taxonomic units (OTUs) were identified at 97% similarity. Principal coordinate analysis, based on the fecal bacterial diversity, revealed that the healthy dogs were clustered together whereas the dogs with CaOx uroliths were dispersed with no apparent pattern. Engineered EcN strains that expressed OXDC of B. subtilis metabolized oxalate in vitro. In the absence of the expression of OxlT of O. formigenes a second version of engineered EcN failed to metabolize oxalate in vitro. Reduced presence of enteric oxalate-metabolizing bacteria is a risk factor for CaOx urolithiasis and fecal microbiota of healthy dogs is distinct from that of dogs affected with CaOx stones. Therefore, enteric colonization by oxalate-metabolizing bacteria may have a preventive effect on CaOx urolithiasis and present a novel therapeutic approach to prevent urinary stones in dogs, cats and humans.