Because of their amphipilic structure, biosurfactants have highly emulsifying properties which are important for the extraction of oil, to form stable water–oil emulsions (Bachmann et al. 2014).
1.1.1.2 Bioremediation
Oil spills during the transport, exploration or refining of petroleum products cause huge environmental hazards. The primary transportation method of oil products is via ship, which increases marine oil contamination due to the routine operations of ship washing, and accidents during exploration and transportation (Souza et al. 2014). Conventionally the spilled oil is removed via physicochemical methods, which doesn’t solve the problem in the long term, since the contaminants are simply removed from one environment to another, which often results in the development of even more toxic byproducts. For this reason, the search for biological alternatives, such as biosurfactants, is important to contribute to environmental health (Silva et al. 2014).
Generally, bioremediation is a process where toxic compounds are fully or partially degraded through living organisms such as bacteria or plants.
The production of biosurfactants generally requires a hydrophobic and hydrophilic carbon source in the culture medium. This process is economically and environmentally friendly when using waste products as substrates (Silva et al. 2014). The biodegradation of the oil-derived compounds is based on different mechanisms. Through the production of biosurfactants the bioavailability of the hydrophobic substrate for the producing bacterium increases, whereby the surface tension of the medium around the bacteria reduces, which results in a lower interfacial tension. Another mechanism is a membrane modification through an interaction of the cell surface and the biosurfactant, which increases the hydrophobicity of the cell wall by reducing the lipopolysaccharide index through an adhesion of the hydrocarbons, without damaging the membrane. Through these mechanisms the formation of hydrogen bonds is blocked and the surface/interfacial tension is reduced, enhancing the dispersion of the hydrocarbon into micelles (which breaks down the biomass into drops) and amplifies the bioavailability and biodegradability (Aparna et al. 2011; Santos et al. 2016).
Regarding the environmental industry trehalose lipids are used as microbial-enhanced oil recovery, biodegradation of polycyclic aromatic hydrocarbons or oil-spill treatments, in the cosmetics industry and most importantly in the biomedical field with biologic properties, like anti-microbial, anti-viral (Azuma et al. 1987) and anti-tumor activities (Franzetti et al. 2010; Gudiña et al. 2013; Kadinov et al. 2020). Moreover, they can act as therapeutic agents due to their functions in cell membrane interactions (Franzetti et al. 2010).
1.1.1.3 Agriculture
Great agricultural damage can be caused by plant pathogens, which results in the use of chemical pesticides that have a number of negative effects on the health of humans and the environment. This is why the requirement and search for natural and environmental friendly pesticides is highly relevant. Biosurfactants, show anti-microbial properties against a large group of plant pathogens, or they are able to stimulate the plant immune system. In particular rhamnolipids have been broadly studied in terms of crop protection (Crouzet et al. 2020).
1.1.1.4 Food Industry
Due to their emulsifying and anti-bacterial activities biosurfactants show great potential in the food industry. Because of their low toxicity and biodegradability their use can be explored as additives in food.
An emulsion describes a dispersion of two or more phases that do not blend with each other, resulting in a liquid–liquid separation. Emulsions are usually unstable and sensitive to agglomeration of the inner phase, through the addition of biosurfactants the system can be stabilized by the reduction of the surface and interfacial tension, thereby preventing the coalescence through steric and electrosteric barriers (Santos et al. 2016). Examples of foods that are defined as emulsions would be milk, heavy cream or mayonnaise.
Another use in the food industry is caused by the anti-biofilm properties of biosurfactants. Biofilms are a significant issue since they can lead to faster food spoilage, distribution of diseases or contamination. Biosurfactants have been shown to have strong anti-adhesive activities against bacterial and yeast strains, through modifying the physicochemical properties of surfaces or changing bacterial interactions. Surfaces can therefore be pre-conditioned using biosurfactants (Mnif and Ghribia 2016).
There are several further applications in food, such as controlling fat globule agglomeration, improving texture and consistency of fat-based products or stabilizing aerated systems (Santos et al. 2016).
1.1.1.5 Biomedicine
Biosurfactants show great potential in therapeutical applications, due to their biological activities. They have the great advantage of being mostly non-toxic and stable in extreme conditions. In particular the anti-microbial, and anti-biofilm properties of biosurfactants make them attractive agents for pharmaceutical and medical applications. Anti-bacterial properties of biosurfactants (Sana et al. 2018; Solaiman et al. 2016), anti-fungal (Sarwar et al. 2018), and anti-viral activities (Borsanyiova et al. 2016) have been reported.
Biosurfactants are well known by their membrane permeabilization properties as they can induce pore and ion channel formation in lipid bilayer membranes. Moreover, they are able to destabilize membranes disturbing their integrity and permeability. Also, pore formation in membranes may cause transmembrane ion influxes, including Na+ and K+, which result in membrane disruption and cell death (Fracchia et al. 2015; Matsumoto et al. 2016).
The anti-microbial property (Rodrigues et al. 2006) of the biosurfactant might be due to their capacity to disturb the integrity of the membranes, destabilizing them and leading to cell lysis by increasing the permeability of the membrane, leading to leakage of metabolites. This effect is based on the interruption of the protein conformation that signals important membrane functions leading to changes in the physical structure of the membrane (Banat et al. 2010; Cortés-Sánchez et al. 2013; Fracchia et al. 2015).
Biofilms describe groups of bacteria that colonize or accumulate on a given surface. They often contain bacterial strains that can become highly resistant. Biosurfactants can interfere with microbial adhesion and biofilm formation, by modifying the physicochemical properties of surfaces or changing bacterial interactions (Fracchia et al. 2015).
One example for a widely studied biosurfactant in the medical and pharmaceutical field is trehalose lipid. Studies have shown that trehalose lipid contains growth inhibiting properties against several resistant pathogen. Trehalose lipid (TL1) shows anti-fungal activity by which it inhibits the Chlamydospore germination of the fungus Glomerella cingulata at a concentration of 300 mg L-1. It also shows 30% inhibition of Candida albicans growth (Janek et al. 2018). Furthermore succinoyl-trehalose (STL-1 and -2) presents virucidal activity by inhibiting the herpes simplex virus and influenza virus at concentrations from 11–33 mg L-1. Trehalose lipids were investigated to present high anti-adhesive properties against several microorgansims, such as Candida albicans and Escherichia coli, on polystyrene surface and silicone urethral catheters, therefore it could be a promising coating agent (Janek et al. 2018; Kitamoto et al. 2002).
1.2 Biosynthesis of Glycolipids
The exact synthetic pathways of the majority of glycolipids are not yet fully known. Various pathways are involved in the biosynthesis of precursors for biosurfactant production, depending on the main carbon source used in the fermentation medium.
Generally the last step of the glycolipid-synthesis implicates the linking of glycosyl and lipid precursors. The linking is mostly via o-glycosidic or ester bonds formed by glycosyltransferases or acyltransferases. Glycosyltransferases are highly valuable glycosylation catalysts, inducing the transfer of the glycosyl residue from an activated glycosyl donor (mostly sugar nucleotides or phosphates) to a lipid acceptor, by making glycosidic bonds