Streptomycetes as Microbial Cell Factories for the Biotechnological Production of Melanin

Abstract

Melanins are complex, polymeric pigments with interesting properties like UV-light absorbance ability, metal ion chelation capacity, antimicrobial action, redox behaviors, and scavenging properties. Based on these characteristics, melanins might be applied in different industrial fields like food packaging, environmental bioremediation, and bioelectronic fields. The actual melanin manufacturing process is not environmentally friendly as it is based on extraction and purification from cuttlefish. Synthetic melanin is available on the market, but it is more expensive than animal-sourced pigment and it requires long chemical procedures. The biotechnological production of microbial melanin, instead, might be a valid alternative. Streptomycetes synthesize melanins as pigments and as extracellular products. In this review, the melanin biotechnological production processes by different Streptomyces strains have been revised according to papers in the literature. The different fermentation strategies to increase melanin production such as the optimization of growth conditions and medium composition or the use of raw sources as growth substrates are here described. Diverse downstream purification processes are also reported as well as all the different analytical methods used to characterize the melanin produced by Streptomyces strains before its application in different fields.

Keywords: Streptomyces; UV-light protection; antimicrobial; antioxidant; heavy metal chelation; melanin; pigment.

Figure 1

Building blocks of the polymeric structures of the different types of melanin.

Figure 2

Metabolic pathways for the biosynthesis of the different types of melanin in Streptomyces strains.

Figure 3

World map showing the countries where the new Streptomyces strains able to produce melanin have been more frequently isolated, according to the literature data.

Figure 4

A representative UV spectrum of melanin produced by Streptomyces strains.

Figure 5

A representative FT-IR spectrum of melanin produced by Streptomyces strains.

Figure 6

Physico-chemical and biological properties of the melanin pigments produced by Streptomyces strains, as tested, identified, and reported in the literature.