Does bilirubin metabolism by gut bacteria alter microbiome composition and gut immune signaling?

Bilirubin serves as a critical signaling molecule and metabolic substrate within the gut-liver axis. Recent research has moved beyond viewing bilirubin as a mere waste product, identifying it as a modulator of both microbial ecology and mucosal immunity.

Microbial metabolism of bilirubin

The metabolic fate of bilirubin in the digestive tract is governed by a specific subset of the gut microbiota.

• Enzymatic conversion: High-quality biochemical evidence identifies the microbial enzyme bilirubin reductase (BilR) as the primary driver of bilirubin metabolism. BilR reduces unconjugated bilirubin into urobilinogen in the distal intestine.
• Key microbial actors: This function is localized to the phylum Firmicutes, particularly obligate anaerobes within the class Clostridia (e.g., Clostridium perfringens). These bacteria utilize bilirubin as an electron sink to maintain redox balance during fermentation.
• Deconjugation: Before BilR can act, microbial $\beta$-glucuronidases must deconjugate the bilirubin glucuronides secreted in bile. This process regenerates unconjugated bilirubin, which can then be reduced to urobilinogen or reabsorbed through enterohepatic circulation.

Impact on microbiome composition

The delivery of bilirubin to the intestine exerts selective pressure on the microbial community, with effects that vary by life stage.

• Neonatal shifts: In newborns, pathologic hyperbilirubinemia is associated with significant shifts in microbial diversity (beta-diversity). Research shows a decrease in beneficial Bifidobacterium and an increase in genera like Enterococcus and Streptococcus, which possess high $\beta$-glucuronidase activity.
• Adult resilience: While the adult microbiome appears more resilient than the neonatal one, changes in the timing or concentration of bilirubin delivery (such as following cholecystectomy) can still reshape microbial community structures by altering the availability of metabolic substrates.

Mechanistic insights into immune signaling

Bilirubin acts as a potent immunomodulatory ligand that communicates directly with the host immune system in the gut.

• AhR activation: Bilirubin is an endogenous ligand for the Aryl Hydrocarbon Receptor (AhR). When bilirubin reaches the intestinal mucosa, it binds to AhR on epithelial and immune cells (such as ILC3s and macrophages).
• Barrier integrity and IL-22: Activation of the AhR pathway by bilirubin induces the production of IL-22, which promotes epithelial repair and the expression of tight junction proteins (e.g., claudin-1, ZO-1), strengthening the gut barrier.
• Immune tolerance: Bilirubin modulates the adaptive immune response by shifting the balance between Th17 and Treg cells. It suppresses pro-inflammatory cytokines like IL-6 and IL-23 while fostering an IL-10-rich environment that stabilizes regulatory T cells (Tregs), promoting mucosal tolerance.

Bottom line

Bilirubin is actively metabolized by specialized gut bacteria (primarily Clostridia) via the enzyme BilR. Its delivery to the intestine is a homeostatic regulator that shapes the microbiome composition and maintains gut immune health by activating the AhR pathway to enhance barrier function and promote anti-inflammatory Treg responses.