Melanin

Overview

Melanin is a broad group of biologic pigments formed by oxidative polymerization of phenolic and indolic precursors, producing insoluble, dark polymers with strong light-absorbing and redox-active properties.^[1] In humans, melanin is synthesized mainly in melanocytes and is transferred to keratinocytes, contributing to skin, hair, and eye pigmentation.^[2] Across biology, melanin is not one molecule but a family of related polymers, commonly categorized as eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin, reflecting different precursors and organism sources.^[3]

Melanin as a medical barrier and redox regulator

A core medical function of melanin is photoprotection. Eumelanin absorbs ultraviolet (UV) radiation and limits UV penetration, while pheomelanin provides less UV protection and can contribute to oxidative stress under UV exposure, which is clinically relevant to skin cancer susceptibility patterns.^[4] Melanin also binds metals and can neutralize reactive molecules, which helps explain its protective roles in tissues exposed to oxidative injury.^[5] These properties matter beyond dermatology because they represent a conserved host strategy: absorb damaging radiation, buffer oxidative stress, and modulate local chemistry in ways that influence immune activity.^[6][7]

Microbial melanin and host–microbe interactions

Melanin is produced by many bacteria and fungi, where it can act as a survival factor in hostile environments by protecting microbes from heat, radiation, and oxidative stress.^[8] In the opportunistic fungal pathogen Cryptococcus neoformans, melanin is a well-established virulence factor that protects against host immune defenses and supports survival during infection.^[9][10] Mechanistically, Cryptococcus produces melanin using laccase-mediated oxidation of substrates such as L-DOPA, and melanin production is regulated by conserved signaling pathways that control stress resistance and pathogenic fitness.^[11]

In bacteria, melanin-like pigments are typically derived from tyrosine metabolism and can support environmental persistence by reducing oxidative damage and improving stress tolerance.^[12] These microbial melanins are microbiome-relevant because they can influence colonization capacity and resilience under inflammatory or oxidative conditions in the host.^[13]

Why melanin matters in microbiome medicine

Melanin enters microbiome medicine through its role as a functional molecule that can shape microbial survival and potentially influence mucosal homeostasis. In an in vivo mouse study, melanized probiotic E. coli Nissle and soluble fungal allomelanin were tested for intestinal safety and microbiome effects, showing rapid intestinal colonization of melanized bacteria without inducing intestinal inflammation in the acute setting.^[14] This supports a clinically interesting concept: melanized microbes and melanin-based compounds may be developed as biologic tools to support intestinal resilience under environmental stressors, while requiring careful evaluation in inflammatory disease contexts.^[15] In microbiome interpretation, melanin is best understood as a pigment system that can modify oxidative conditions, immune exposure, and microbial fitness rather than as a simple pigment marker.^[16][17][18]

Update History

References

1. Melanin: insights into structure, analysis, and biological activities for future development. Song W, Yang H, Liu S, Yu H, Li D, Li P, Xing R.. (Journal of Materials Chemistry B. 2023)
2. MC1R, eumelanin and pheomelanin: their role in determining the susceptibility to skin cancer. Nasti TH, Timares L.. (Photochem Photobiol. 2015)
3. Melanin: insights into structure, analysis, and biological activities for future development. Song W, Yang H, Liu S, Yu H, Li D, Li P, Xing R.. (Journal of Materials Chemistry B. 2023)
4. MC1R, eumelanin and pheomelanin: their role in determining the susceptibility to skin cancer. Nasti TH, Timares L.. (Photochem Photobiol. 2015)
5. Melanin: insights into structure, analysis, and biological activities for future development. Song W, Yang H, Liu S, Yu H, Li D, Li P, Xing R.. (Journal of Materials Chemistry B. 2023)
6. Melanin: insights into structure, analysis, and biological activities for future development. Song W, Yang H, Liu S, Yu H, Li D, Li P, Xing R.. (Journal of Materials Chemistry B. 2023)
7. MC1R, eumelanin and pheomelanin: their role in determining the susceptibility to skin cancer. Nasti TH, Timares L.. (Photochem Photobiol. 2015)
8. Melanin biosynthesis in bacteria, regulation and production perspectives. Pavan ME, López NI, Pettinari MJ.. (Applied Microbiology and Biotechnology. 2019)
9. Unraveling melanin biosynthesis and signaling networks in Cryptococcus neoformans. Lee D, Jang E-H, Lee M, Kim S-W, Lee Y, Lee K-T, Bahn Y-S.. (mBio. 2019)
10. Melanin and virulence in Cryptococcus neoformans. Casadevall A, Rosas AL, Nosanchuk JD.. (Current Opinion in Microbiology. 2000)
11. Unraveling melanin biosynthesis and signaling networks in Cryptococcus neoformans. Lee D, Jang E-H, Lee M, Kim S-W, Lee Y, Lee K-T, Bahn Y-S.. (mBio. 2019)
12. Melanin biosynthesis in bacteria, regulation and production perspectives. Pavan ME, López NI, Pettinari MJ.. (Applied Microbiology and Biotechnology. 2019)
13. Melanin biosynthesis in bacteria, regulation and production perspectives. Pavan ME, López NI, Pettinari MJ.. (Applied Microbiology and Biotechnology. 2019)
14. Effects of melanized bacteria and soluble melanin on the intestinal homeostasis and microbiome in vivo. Zhang YG, Malo ME, Tschirhart T, Xia Y, Wang Z, Dadachova E, Sun J.. (Toxics. 2023)
15. Effects of melanized bacteria and soluble melanin on the intestinal homeostasis and microbiome in vivo. Zhang YG, Malo ME, Tschirhart T, Xia Y, Wang Z, Dadachova E, Sun J.. (Toxics. 2023)
16. Melanin: insights into structure, analysis, and biological activities for future development. Song W, Yang H, Liu S, Yu H, Li D, Li P, Xing R.. (Journal of Materials Chemistry B. 2023)
17. Melanin biosynthesis in bacteria, regulation and production perspectives. Pavan ME, López NI, Pettinari MJ.. (Applied Microbiology and Biotechnology. 2019)
18. Effects of melanized bacteria and soluble melanin on the intestinal homeostasis and microbiome in vivo. Zhang YG, Malo ME, Tschirhart T, Xia Y, Wang Z, Dadachova E, Sun J.. (Toxics. 2023)