Bacterial pigments: A colorful palette reservoir for biotechnological applications

Abstract

Synthetic derivatives are currently used instead of pigments in many applicative fields, from food to feed, from pharmaceutical to diagnostic, from agronomy to industry. Progress in organic chemistry allowed to obtain rather cheap compounds covering the whole color spectrum. However, several concerns arise from this chemical approach, as it is mainly based on nonrenewable resources such as fossil oil, and the toxicity or carcinogenic properties of products and/or precursors may be harmful for personnel involved in the productive processes. In this scenario, microorganisms and their pigments represent a colorful world to discover and reconsider. Each living bacterial strain may be a source of secondary metabolites with peculiar functions. The aim of this review is to link the physiological role of bacterial pigments with their potential use in different biotechnological fields. This enormous potential supports the big challenge for the development of strategies useful to identify, produce, and purify the right pigment for the desired application. At the end of this ideal journey through the world of bacterial pigments, the attention will be focused on melanin compounds, whose production relies upon different techniques ranging from natural producers, heterologous hosts, or isolated enzymes. In a green workflow, the microorganisms represent the starting and final point of pigment production.

Keywords: E. coli; bioactive compound; biotechnology; enzyme; expression systems; gene expression; melanin; microbial metabolism; pigments.

FIGURE 1

Schematic representation of the main physiological roles of bacterial pigments. The cited pigments produced by phototrophic and nonphototrophic microorganisms are involved in both light‐dependent and light‐independent mechanisms. Each pigment family involved in capturing sunlight radiation absorbs visible light in the specified range. In the dark, pigments increase the microbial fitness acting as virulence factors, antioxidants, antimicrobials

FIGURE 2

Chemical structures of photosynthetic and nonphotosynthetic pigments cited in the text. Chlorophyll a, bacteriochlorophyll a, β‐carotene, and phycocyanobilin were chosen as representative photosynthetic pigments. Among bacterial pigments not involved in photosynthetic processes, staphyloxanthin, isolalloxazine, melanins (shown as a polymer of homogentisic acid), pyocyanin, prodigiosin, indigo, and violacein were included

FIGURE 3

Applications of bacterial pigments in medical (red), alimentary (yellow), industrial (white), and environmental/agricultural (green) biotechnology. For each field the pigments cited in the text are depicted

FIGURE 4

Application of bacterial pigments in four branches of biotechnology: medical (red), alimentary (yellow), industrial (white), and environmental/agricultural (green) applications. For each field, the pigments cited in the text are depicted

FIGURE 5

Schematic representation of four different biotechnological strategies used to produce pyomelanin. The Pseudomonas balearica strain was grown under different cultivation parameters to optimize the production of pyomelanin (panel A). A pyomelanin hyperproducer strain was obtained in Pseudomonas aeruginosa PAO1 upon knock out of hmgA gene codifying homogentisate‐1,2‐dioxygenase and involved in tyrosine catabolism, as represented in panel B. In panel C, Escherichia coli cells overexpress hpd gene codifying 4‐hydroxyphenylpiruvate dioxygenase from PAO1 strain. This approach represents the use of a heterologous host to produce large amounts of pyomelanin for biotechnological purposes. The chance to produce pyomelanin can be pursued through tyrosinase as isolated enzyme. Tyrosine is transformed into L‐DOPA that, in turn, can lead to the synthesis of melanin‐like polymers