Role of gut microbiota in melanosis coli: from anthraquinone biotransformation to mucosal homeostasis dysbiosis

Melanosis coli (MC) is a benign and usually reversible condition characterized by brownish-black pigmentation of the colonic mucosa and is commonly associated with chronic exposure to anthraquinone laxatives (ALs). The best-established histopathological sequence involves AL-related epithelial apoptosis, phagocytosis of apoptotic bodies by macrophages, and subsequent lipofuscin deposition. Emerging evidence suggests that the gut microbiota (GM) may contribute to this process by converting pharmacologically inactive anthraquinone glycosides into active anthrone metabolites, including rhein anthrone. This narrative review summarizes available MC-specific findings and clearly distinguishes them from mechanistic hypotheses extrapolated from constipation, intestinal barrier, and microbiome literature. We discuss microbial β-glucosidases and reductases involved in AL biotransformation, reported changes in microbial diversity and SCFA-producing taxa in MC or constipation-associated cohorts, and plausible links with barrier dysfunction, bile-acid metabolism, tryptophan-derived metabolites, and LPS-TLR4 signaling. We therefore present the "Microbiota-Apoptosis Axis" as a proposed framework rather than a validated causal pathway. Finally, we review GM-targeted strategies, including probiotics, synbiotics, and fecal microbiota transplantation, while emphasizing that direct clinical evidence in MC remains limited and that cessation of anthraquinone laxatives remains the primary management strategy.