Intestinal colonization with Oxalobacter formigenes and its relation to urinary oxalate excretion in pediatric patients with idiopathic calcium urolithiasis

Deficient butyrate metabolism in the intestinal microbiome is a potential risk factor for recurrent kidney stone disease

Intestinal microbiome dysbiosis is a known risk factor for recurrent kidney stone disease (KSD) with prior data suggesting a role for dysfunctional metabolic pathways other than those directly utilizing oxalate. To identify alternative mechanisms, the current study analyzed differences in the metabolic potential of intestinal microbiomes of patients (n = 17) and live-in controls (n = 17) and determined their relevance to increased risk for KSD using shotgun metagenomic sequencing. We found no differences in the abundance of genes associated with known oxalate degradation pathways, supporting the notion that dysfunction in other metabolic pathways plays a role in KSD. Further analysis showed decreased abundance of key enzymes involved in butyrate biosynthesis in patient intestinal microbiomes. Furthermore, de novo construction of microbial genomes showed that the majority of genes significantly enriched in non-stone formers are affiliated with Faecalibacterium prausnitzii, a major butyrate producer. Specifically pertaining to butyrate metabolism, the majority of abundant genes mapped back to F. prausnitzii, Alistipes spp., and Akkermansia muciniphila. No differences were observed in ascorbate or glyoxylate metabolic pathways. Collectively, these data suggest that impaired bacterial-associated butyrate metabolism may be an oxalate-independent mechanism that contributes to an increased risk for recurrent KSD. This indicates that the role of the intestinal microbiome in recurrent KSD is multi-factorial, which is representative of the highly intertwined metabolic nature of this complex environment. Future bacteria-based treatments must not be restricted to targeting only oxalate metabolism.

Short-Chain Fatty Acids Reduced Renal Calcium Oxalate Stones by Regulating the Expression of Intestinal Oxalate Transporter SLC26A6

Renal calcium oxalate (CaOx) stone is a common urologic disease with a high prevalence and recurrence rate. However, short-chain fatty acids (SCFAs) are less often reported in the prevention of urolithiasis. This study aimed to explore the effect of SCFAs on the renal CaOx stone formation and the underlying mechanisms. Ethylene glycol was used to induce renal CaOx crystals in rats. SCFAs (acetate, propionate, or butyrate) were added as supplements to the drinking water with or without antibiotics. Because intestinal oxalate transporters SLC26A6 and SLC26A3 regulate the excretion and absorption of oxalate in the intestine, we injected adeno-associated virus 9 (AAV9)-SLC26A6-shRNA (short hairpin RNA) and AAV9-SLC26A3 into the tail vein of rats to suppress SLC26A6 and overexpress SLC26A3 expression in the intestine, respectively, to explore the role of SLC26A3 and SLC26A6 (SLC26A3/6) in the reduction of renal CaOx crystals induced by SCFAs. Results showed that SCFAs reduced renal CaOx crystals and urinary oxalate levels but, however, increased the abundance of SCFA-producing bacteria and cecum SCFA levels. SCFA supplements still reduced renal crystals and urinary oxalate after gut microbiota depletion. Propionate and butyrate downregulated intestinal oxalate transporter SLC26A3 expression, while acetate and propionate upregulated SLC26A6 expression, both in vivo and in vitro. AAV9-SLC26A3 exerted a protective effect against renal crystals, while AAV9-SLC26A6-shRNA contributed to the renal crystal formation even though the SCFAs were supplemented. In conclusion, SCFAs could reduce urinary oxalate and renal CaOx stones through the oxalate transporter SLC26A6 in the intestine. SCFAs may be new supplements for preventing the formation of renal CaOx stones. IMPORTANCE Some studies found that the relative abundances of short-chain-fatty-acid (SCFA)-producing bacteria were lower in the gut microbiota of renal stone patients than healthy controls. Our previous study demonstrated that SCFAs could reduce the formation of renal calcium oxalate (CaOx) stones, but the mechanism is still unknown. In this study, we found that SCFAs (acetate, propionate, and butyrate) reduced the formation of renal calcium oxalate (CaOx) crystals and the level of urinary oxalate. Depleting gut microbiota increased the amount of renal crystals in model rats, and SCFA supplements reduced renal crystals and urinary oxalate after gut microbiota depletion. Intestinal oxalate transporter SLC26A6 was a direct target of SCFAs. Our findings suggested that SCFAs could reduce urinary oxalate and renal CaOx stones through the oxalate transporter SLC26A6 in the intestine. SCFAs may be new supplements for preventing the formation of renal CaOx stones.

Defining Dysbiosis in Patients with Urolithiasis

The prevalence of urinary stone disease (USD) is rapidly rising. However, the factors driving this increase are unknown. Recent microbiome studies suggest that dysbiosis may in part contribute to the increasing prevalence. The objective of the current study was to determine the nature and location of dysbiosis associated with USD. We conducted microbiome analysis from the gastrointestinal and urinary tracts, along with a metabolomic analysis of the urinary metabolome, from subjects with an active episode of USD or no history of the disease. Higher rates of antibiotic use among USD patients along with integrated microbiome and metabolomic results support the hypothesis that USD is associated with an antibiotic-driven shift in the microbiome from one that protects against USD to one that promotes the disease. Specifically, our study implicates urinary tract Lactobacillus and Enterobacteriaceae in protective and pathogenic roles for USD, respectively, which conventional, culture-based methods of bacterial analysis from urine and kidney stones would not necessarily detect. Results suggest that antibiotics produce a long-term shift in the microbiome that may increase the risk for USD, with the urinary tract microbiome holding more relevance for USD than the gut microbiome.

Inhibition of urinary stone disease by a multi-species bacterial network ensures healthy oxalate homeostasis

The incidence of urinary stone disease is rapidly increasing, with oxalate being a primary constituent of approximately 80% of all kidney stones. Despite the high dietary exposure to oxalate by many individuals and its potential nephrotoxicity, mammals do not produce enzymes to metabolize this compound, instead relying in part on bacteria within the gut to reduce oxalate absorption and urinary excretion. While considerable research has focused on isolated species of oxalate-degrading bacteria, particularly those with an absolute requirement for oxalate, recent studies have pointed to broader roles for microbiota both in oxalate metabolism and inhibition of urinary stone disease. Here we examined gut microbiota from patients with and live-in individuals without urinary stone disease to determine if healthy individuals harbored a more extensive microbial network associated with oxalate metabolism. We found a gender-specific association between the gut microbiota composition and urinary stone disease. Bacteria enriched in healthy individuals largely overlapped with those that exhibited a significant, positive correlation with Oxalobacter formigenes, a species presumed to be at the center of an oxalate-metabolizing microbial network. Furthermore, differential abundance analyses identified multiple taxa known to also be stimulated by oxalate in rodent models. Interestingly, the presence of these taxa distinguished patients from healthy individuals better than either the relative abundance or colonization of O. formigenes. Thus, our work shows that bacteria stimulated by the presence of oxalate in rodents may, in addition to obligate oxalate users, play a role in the inhibition of urinary stone disease in man.

Calcium Oxalate Urolithiasis: A Case of Missing Microbes?

INTRODUCTION

Urinary stone disease (USD) has known associations with the gut microbiota. Approximately 80% of kidney stones contain oxalate as a primary constituent and diverse oxalate-degrading bacteria exist within the human gut, which may protect against USD. Although bacteriotherapy represents a promising strategy to eliminate oxalate and reduce the risk of USD, oxalate-degrading probiotics have had limited success. To identify limitations of oxalate-degrading probiotics and refine development of bacteriotherapies to prevent USD, we review the literature associated with the gut microbiota and USD.

MATERIALS AND METHODS

A literature search was performed to identify publications that examine the role of oxalate-degrading bacteria or the whole gut microbiota in oxalate metabolism and the pathophysiology of USD. We conducted a meta-analysis of studies that examined the association of the whole gut microbiota with USD. In addition, we evaluated the gut microbiota of healthy individuals and those with comorbidities related to USD using publically available data from the American Gut Project (AGP).

RESULTS

Studies on Oxalobacter formigenes reveal that colonization by this species is not a good predictor of USD risk or urinary oxalate excretion. The species of oxalate-degrading bacteria used in probiotics and duration of administration do not impact efficacy or persistence. Studies focused on the whole gut microbiota reveal broad shifts in the gut microbiota associated with USD and a diverse microbial network is associated with oxalate metabolism. AGP data analysis demonstrated a strong overlap in microbial genera depleted in diseased individuals among USD and comorbidities.

CONCLUSIONS

The associations between the gut microbiota and USD extend beyond individual functional microbial species. Common shifts in the gut microbiota may facilitate the onset of USD and/or comorbidities. The successful development of bacteriotherapies to inhibit USD will need to incorporate strategies that target a broad diversity of bacteria rather than focus on a few specialist species.

The microbiome of the urinary tract--a role beyond infection

Urologists rarely need to consider bacteria beyond their role in infectious disease. However, emerging evidence shows that the microorganisms inhabiting many sites of the body, including the urinary tract--which has long been assumed sterile in healthy individuals--might have a role in maintaining urinary health. Studies of the urinary microbiota have identified remarkable differences between healthy populations and those with urologic diseases. Microorganisms at sites distal to the kidney, bladder and urethra are likely to have a profound effect on urologic health, both positive and negative, owing to their metabolic output and other contributions. Connections between the gut microbiota and renal stone formation have already been discovered. In addition, bacteria are also used in the prevention of bladder cancer recurrence. In the future, urologists will need to consider possible influences of the microbiome in diagnosis and treatment of certain urological conditions. New insights might provide an opportunity to predict the risk of developing certain urological diseases and could enable the development of innovative therapeutic strategies.

Sensitivity of human strains of Oxalobacter formigenes to commonly prescribed antibiotics

OBJECTIVE

To determine the sensitivity of 4 strains of Oxalobacter formigenes (Oxf) found in humans--HC1, Va3, CC13, and OxK--to varying concentrations of commonly prescribed antibiotics. Oxf gut colonization has been associated with a decreased risk of forming recurrent calcium oxalate kidney stones.

METHODS

For each strain and each antibiotic concentration, 100 μL of an overnight culture and 100 μL of the appropriate antibiotic were added to a 7-mL vial of oxalate culture medium containing 20 mM oxalate. On the fourth day, vials were visually examined for growth, and a calcium oxalate precipitation test was performed to determine whether Oxf grew in the presence of the antibiotic.

RESULTS

All 4 Oxf strains were resistant to amoxicillin, amoxicillin/clavulanate, ceftriaxone, cephalexin, and vancomycin, and they were all sensitive to azithromycin, ciprofloxacin, clarithromycin, clindamycin, doxycycline, gentamicin, levofloxacin, metronidazole, and tetracycline. One strain, CC13, was resistant to nitrofurantoin, and the others were sensitive. Differences in minimum inhibitory concentration between strains were demonstrated.

CONCLUSION

Four human strains of Oxf are sensitive to a number of antibiotics commonly used in clinical practice; however, minimum inhibitory concentrations differ between strains.