Host-derived nitrate fuels indole production by Escherichia coli to drive chronic kidney disease progression.

Impaired clearance of indoxyl sulfate in CKD mice elevated mucosal expression of the gene encoding inducible nitric oxide synthase (iNOS).

• Adenine-induced CKD mouse model was used to establish the relationship between indoxyl sulfate accumulation and iNOS expression.
• Indoxyl sulfate is a uremic toxin produced from microbiota-derived indole in the liver.
• Elevated iNOS expression was localized to the intestinal mucosa.

The rise in luminal nitrate levels resulting from iNOS activity promoted Escherichia coli growth through nitrate respiration.

• Nitrate serves as an electron acceptor for anaerobic respiration by E. coli.
• This mechanism links the host inflammatory response (iNOS induction) to expansion of Enterobacteriaceae in the gut.
• CKD is associated with elevated fecal abundance of Enterobacteriaceae, and this study identifies host-derived nitrate as an ecological driver of that shift.

Fecal microbiota from CKD patients generated more indole than feces of healthy controls during anaerobic culture, but only in the presence of nitrate.

• Anaerobic culture conditions were used to compare indole production between CKD patient feces and healthy control feces.
• Indole production differences were nitrate-dependent, being observed only when nitrate was present in the culture.
• This finding implicates host-derived nitrate as a key driver of elevated indole production by the CKD microbiota.

Nitrate enhanced indole production by E. coli, thereby worsening renal pathology in CKD mice.

• Indole is the precursor to indoxyl sulfate, a uremic toxin associated with CKD progression.
• Nitrate-driven indole production by E. coli creates a feed-forward loop in which indoxyl sulfate accumulation leads to more nitrate production, which drives more indole synthesis.
• Worsened renal pathology was demonstrated in the adenine-induced CKD mouse model.

Inhibition of iNOS mitigated renal pathology worsening in CKD mice.

• iNOS inhibition was used as an intervention in the adenine-induced CKD mouse model.
• Mitigating iNOS activity reduced the availability of host-derived nitrate, thereby interrupting the feed-forward loop between indoxyl sulfate accumulation, nitrate production, and E. coli-driven indole synthesis.
• This finding identifies iNOS as a potential therapeutic target to slow CKD progression.

CKD is linked to an elevated fecal abundance of Enterobacteriaceae, but the ecological drivers of this shift were previously unclear.

• This paper identifies host-derived nitrate as the ecological driver promoting Enterobacteriaceae (specifically E. coli) expansion in CKD.
• Nitrate respiration provides E. coli a metabolic advantage in the anaerobic gut environment during CKD.
• The study connects the host uremic state to microbiome compositional changes through the iNOS-nitrate axis.