**1. Introduction**

The coloration of textile materials has been performed since ancient times [1,2]. The textile dyeing industry has long been environmentally challenging in several respects. First, there is very high water consumption during all stages of dyeing, which consequently generates a significant amount of contaminated water waste. Second, the use of very harsh chemicals is also a significant threat to the environment, especially given that residue from these chemicals can come into contact with consumers during their use of textile products. Third, the energy consumption during dyeing through the heating and drying stages is also very high. Finally, the dyes that are currently used are mostly of synthetic origin. One of the ways to reduce these issues is to use natural dyes. Natural dyes were in use before synthetic ones [1,3]; however, their extraction and use are technologically time-consuming and very expensive. In the past decade, research has focused on emerging, novel and exciting sources of natural dyes: bacteria [3–5]. Bacterial, or, as they are sometimes called, microbial dyes are byproducts of bacteria that represent a relatively novel and scarcely investigated source of natural dyes, with tremendous potential for dyeing various textile materials in very intense, durable and esthetically beautiful colors, which are nontoxic and safe for human skin, thus producing environmentally sustainable textile products.

Research into the possibility of using bacterial pigments as natural sources of dyes for the textile industry has grown. For this study, the Scopus and ScienceDirect databases were used with several research keywords, including "bacterial pigments dyes microbial textile fibers fabrics dyeing." After refining the search results by excluding all articles related to processes involving the decolorization of textile dyes from wastewater using bacteria or

**Citation:** Kramar, A.; Kostic, M.M. Bacterial Secondary Metabolites as Biopigments for Textile Dyeing. *Textiles* **2022**, *2*, 252–264. https:// doi.org/10.3390/textiles2020013

Academic Editor: Laurent Dufossé

Received: 1 March 2022 Accepted: 15 April 2022 Published: 19 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the dyeing of textiles using plant extracts, there was only a small number of publications in the field relevant to the dyeing of textiles with bacterial pigments, and a slight increase in publication numbers occurring with time (Figure 1).

**Figure 1.** Number of publications per year (for years between 2008 and 2021) regarding textile dyeing using bacterial pigments, according to Scopus database.

In the search for representative articles, the term "microbial pigments" was used, even though microbial pigments cover pigments and dyes derived from both bacteria and fungi [5]. Fungal pigments are also important sources of natural dyes and have potential for the dyeing of textiles [6], but the use of fungal pigments as dye sources is beyond the scope of this study.

One of the first suggestions in the literature, which is not presented in Figure 1, is the possibility of using color pigment, published in 2000 by Shirata et al. [7]. This publication was followed by a study by Alihosseini et al. [8] in 2008; therefore, there was a gap of several years before the interest in bacterial pigments as textile dyes was revived. Together, the authors of these two studies can be considered pioneers in this field, since they produced the first results about the possibility of using a naturally occurring pigment derived from bacteria to dye various fabrics. The following years marked the growth of research groups studying this field, while in 2017, there was even a first mention of the use of biopigment for ink formulation in printing [9].

Their high production yield, non-toxicity and good safety profile make bacterial pigments or dyes novel, sustainable and promising alternatives to synthetic dyes. In this article, important studies regarding the utilization of bacterial pigments on different textile materials and fibers are presented, with a focus on the necessary step of the preparation of pigment solutions, the use of mordants, and other pre- or post-textile treatments to improve dye fixation on specific fibers and increase the exhaustion of the dyebath, as well as the possibility of imparting additional properties to colored materials, in order to obtain high-added-value textile products.

#### **2. Bacterial Dyes**

Bacterial metabolites/secondary metabolites are by-products of bacterial growth [5]. Usually, these byproducts are bacterial responses to external stimuli [10]. Besides color, these metabolites can possess important additional properties, such as potent antimicrobial activity against different pathogens, anticancer activity, antioxidative activity, UV properties, etc. [3,11–13]. These properties suggest that these pigments could be used as functional dyes for different textile materials, since they offer a range of potential applications in addition to their esthetic, qualities (Figure 2). Bacterial pigments, or biopigments, are terms that are used to describe these types of colorant or dyestuff, but from a textile-science point of view, these bacterial metabolites behave more as dyes than as pigments. Even though

they are not soluble in water, they can be considered dyes rather than pigments because it has been shown that they are chemically bound to textile materials, forming bonds with functional groups on fibers and, therefore, behaving as dyes. However, in current research, the term pigment is frequently used, since these bacterial products are usually insoluble in water, they are in powder form after isolation and drying and, in some cases, they are used as suspensions and not solutions.

**Figure 2.** Some of the properties of bacterial pigments that have potential to be transferred to textile materials during dyeing.

If dyes can offer additional properties to textile materials, by binding to textiles in a way that it does not disturb their antimicrobial or anticancer function, for example, then certain functions can be transferred to textile materials after dyeing. The vast range of special properties of bacterial pigments presented in Figure 2 opens up the possibility that dyed textiles possess the same properties. This is one of the most important aspects that should be explored in the future, i.e., whether certain properties of bacterial pigments are retained on textile materials after dyeing.

Several bacterial strains are able to produce pigments or dyes capable of imparting color to textile materials [11]. These bacteria are isolated from various sources, such as soil, water, plants, insects, etc. [11,12]. Among many, the most important are *Serratia, Streptomyces* and *Pseudomonas* [4,13]. The range of colors that these bacteria produce is wide, including pink, red-orange, yellow, blue, green, etc. [14,15]. However, it is important to note that the color of the extracted pigment does not always match the resulting color of the dyed fiber/fabric. As discussed later in this article, the resulting color depends on the nature of the substrate, meaning that various shades can be obtained by the same pigment on different fabrics; the shade also depends on the dyeing conditions (the temperature, pH, and use of mordants).

Pigment production using certain bacteria can be altered by using different conditions during growth. For example, Alihoseini et al. investigated the possibility of mutating *Vibrio gazogenes* to selectively develop the best pigment-producing strain capable of imparting, in addition to color, durable antimicrobial activity against *E. coli* and *S. aureus* to textile materials [16]. On the other hand, Kanelli et al. optimized the culture conditions of *Janthinobacterium lividum* for pigment (violacein) production and the simultaneous dyeing of fabrics, which resulted in dyed fabrics with significant antifungal activity against several *Candida* pathogens, *C. albicans*, *C. parapsilosis* and *C. krusei*, as well as antibacterial properties against *Escherichia coli*, *Staphylococcus aureus* and the methicillin-resistant *S. aureus*, MRSA [17].
