Bioprospecting of Microorganism-Based Industrial Molecules. Группа авторов

Figure 2.2 Chemical structures for two species of sophorolipids as a result of the condensation of sophorose with oleic acid. On the left side, it is shown the open form of carboxylic acid and on the right side, the cyclic form of the lactone.

      Cyclic esters are called lactones. Hence, there are two types of SL: the acidic forms and the lactone forms. Other less notable molecular variables are (i) the presence or absence of acetyl groups attached to the hydroxyl moieties on the carbohydrate periphery, (ii) the length of the alkyl chain, (iii) the degree of unsaturation (unsaturation = double or triple bonds), (iv) the position of the hydroxyl group in the alkyl chain, (v) the position of the hydroxyl group of the sophorose that serves to build the ether bond with the fatty alcohol, and (vi) the position of the hydroxyl group of sophorose serves to construct the ester bond with the fatty acid in the lactone forms, inter alia.

      Figure 2.2 shows the chemical structures of two SL: one in the acid form (left) and one in the lactone form (right). Both species do not contain any acetyl groups but have a fatty acid moiety of 18 carbon atoms with only one unsaturation in position C‐9 with Z geometry. This fatty acid (oleic acid) must have been previously hydroxylated inside the cell by some biochemical β‐oxidation at position C‐17. This latter allows its association with the free hydroxyl moiety of the anomeric carbon of sophorose, which produces the ether bridge of sophorose‐lipid equally found in both acid and lactone arrangements. Thus, the chemical name for the acid form should be (S,Z)‐17‐{[(2S,3R,4S,5S,6R)‐4,5‐dihydroxy‐6‐hydroxymethyl‐3‐{[(2S,3R,4S,5S,6R)‐3,4,5‐trihydroxy‐6‐hydroxymethyltetrahydro‐2H‐pyran‐2‐yl]oxy}tetrahydro‐2H‐pyran‐2‐yl]oxy}octadec‐9‐enoic acid.

      In the case of lactone, the carboxylic acid was condensed with the hydroxyl group in C‐5 of the other monosaccharide fragment of sophorose. After imagining the number of possible combinations of structural variations, one would not expect microorganisms to produce unique and pure compounds, but rather a wide variety of many different species. Therefore, the selection of microorganism strain, culture conditions, culture media, and substrates are the first fundamental factors playing a decisive role in the complexity (or simplicity) of obtained mixtures.

The other type of interesting glycolipids is rhamnolipids. Like SL, there are abounding varied possible structures. In the case of rhamnolipids, there are even two more features: (i) the number of rhamnose monomers and (ii) the number of condensed fatty acids. So, rhamnolipids can be constituted by one or two units of rhamnose linked to one or two molecules of fatty acids (Figure 2.3). To our knowledge, lactone rhamnolipid is not yet reported.

      The manufacture of surfactants has been an exclusive task for industrial organic chemistry. However, just as microorganisms have been used in industrial processes to afford enzymes, vaccines, antibiotics, wine, and beer, the production of surfactants can also be carried out in this way. Furthermore, the rapid and recent advance on bioprocesses envisions the feasibility of producing BS on a large scale. Research and technological developments have tried to look for cost competitiveness proposing cheaper raw materials and more affordable downstream processes. For example, it has been demonstrated that some agro‐industrial wastes such as molasses from sugar industry [10] or whey from dairy industry [11] can be useful. Other aspect on BS is the possibility to improve physicochemical properties or the productivity of microorganisms via biochemical or genetic engineering techniques.

BS have very attractive properties such as low toxicity and high biodegradability (degraded by other microorganisms); which is very well aligned to current needs imposed by international regulations. It now is considered that biocompatibility of a product is a parameter as important as its cost and performance, so the chemical industry is compelled to implement new strategies for manufacturing more sustainable materials without scarifying efficiency. The numerous advantages of versatile BS compared to synthetic surfactants explain the increasing attention on the topic from the early twenty‐first century (Figure 2.4). Although publishable activity on BS dates back 57 years, the boom has emerged for the last 20 years. Only in the last eight years, more than half of the known references on BS have been reported. BS are recognized for resisting a wide range of pH, salt concentration, and temperature [12] and have the ability to reduce surface tension in exactly the same way as chemical and oleochemical surfactants, so these can find the same application niches, i.e. as the key components of countless formulations for almost any sector of the contemporary industry [13]. Hence, BS are excellent candidates for different industrial applications like oil recovery, detergents, cleaning products, degreasers, fertilizers, agrochemicals, textile products, paints, mining, inter alia. Moreover, BS can attend applications where strong eco‐friendly features are demanded, e.g. bioremediation of soil and water due to hydrocarbon spills [14, 15], water treatment, food processing [16], health, sanitizers, cosmetics, and pharmaceuticals [17].

Figure 2.3 General chemical structures for four types of rhamnolipids: monorhamnose‐monolipid, monorhamnose‐dilipid, dirhamnose‐monolipid, and dirhamnose‐dilipid.

      As it has been observed, classification for a surfactant molecule comprises all the chemical species that share a binary amphiphilic feature. The production of microbial surfactants involves a strong character of sustainability and circular economy. Its production seems to be the right choice that will revolutionize the way the chemical industry, applications, and markets work. Glycolipids such as SL and rhamnolipids appear to be the species with the greatest potential to be developed at larger scales in the coming years. In addition, there are some synergies with other chemical compounds that can enhance surface activities and performances, which makes ipso facto possible the introduction of BS in the market via innovative formulations.

Figure 2.4 Number of publications on biosurfactants from 1963 to April 2020.

Among all primary and secondary metabolites, BS play interesting roles for microbial life. Some authors suggest that their emulsifying properties help microorganisms to adapt to environments, enhancing nutrient availability in soil or water and allowing cell adherence for water‐insoluble substrate transport [18]. Since BS can inhibit microbial growth, those microorganisms producing BS can become predominant in their environment [19].

      As mentioned above, BS are biodegradable and suitable for different industrial applications; for this reason, there is abundant research about their production and natural sources. Biosynthesis of BS are distributed among archaea, bacteria, yeasts, and molds, but depending on the group, genus, and species of microorganisms, BS structures become significantly different.

2.2 Biosynthesis of BS by Archaea and Bacteria

There are few papers related to Archaea as BS producers, compared to bacteria. Those who have been studied are alkaliphiles from the marine environment [20]. In contrast, bacteria produce mainly glycolipids, such as rhamnolipids, trehalolipids, and glucolipids; biosynthesis is restricted to actinobacteria, proteobacteria, cyanobacteria, and firmicutes (87%), other phyla represent between 1.0 and 2.5% of glycolipid‐producing bacteria [21]. On the other hand, lactic acid bacteria (LAB), produce BS showing important roles in LAB colonization