Table 4.2 Biosurfactants derived from fungi.
| Fungi | Biosurfactant |
|---|---|
| Torulopsis bombicola | Sophorose lipid |
| Candida bombicola | Sophoro lipids |
| Candida lipolytica | Protein‐lipidpolysaccharide complex |
| Candida lipolytica | Protein‐lipidcarbohydrate complex |
| Candida ishiwadae | glycolipid |
| Candida batistae | sophorolipids |
| Aspergillus ustus | Glycolipoprotein |
| Tichosporon ashii | sophorolipids |
4.5 Classification of Biosurfactants
Origin and composition are the two main factors on the basis of which the classification of biosurfactants has been performed. According to Rosenberg and Ron [46], based on the molecular weight, biosurfactants are categorized into two types. The first one is comprised of those compounds that have low molecular weight compounds with lower surface and interfacial tensions and the second one is comprised of high molecular weight compounds with strong surface binding capacity. The majority of low molecular weight biosurfactants comes under the glycolipids, lipopeptides, and phospholipids category while high molecular weight ones are mainly particulate and polymeric surfactants [47]. Another basis for biosurfactant classification is the presence and type of charge on individual polar moiety. The negatively charged surfactants, i.e. anionic usually have a sulphonate or sulfur group as the chief functional group on their cell surface while positively charged or cationic surfactants mainly possess an ammonium and hydroxyl group. Also, surfactants with a neutral or non‐ionic nature are identified and are the products of a 1, 2‐epoxyethane polymerization reaction. When both positively and negatively charged functional groups are present on the same surfactant molecules, they are identified as amphoteric surfactants [48].
4.6 Types of Biosurfactants
There have been so many forms of biosurfactants and each have a common microbial origin. Some of the broad categories of biosurfactant are now discussed.
4.6.1 Glycolipids
Glycolipids are the most common type of biosurfactants and consist of mono‐, di‐, tri‐, and tetrasaccharides. The saccharides include glucose, mannose, galactose, glucuronic acid, rhamnose, and galactose sulphate. Some of the microorganisms usually have the same fatty acids and phospholipid composition [49, 50]. Carbohydrates in combination with long‐chain aliphatic acids or hydroxyaliphatic acids are the key component of glycolipids [26]. According to Karanth et al. [51], rhamnolipids, trehalolipids, and sophorolipids are the best‐known glycolipids.
4.6.2 Rhamnolipids
Rhamnose and 3‐hydroxy fatty acids containing glycolipid surfactant have been produced by Pseudomonas sp. [52]. As shown in Figure 4.3, one or two molecules of rhamnose are linked to one or two molecules of hydroxyl decanoic acid and represent the basic structure of rhamnolipids. Pseudomonas aeruginosa and Burkholderia sp. play a key role in the production of rhamnolipids and are identified as one of the most effective surfactants in the remediation of hydrodrophobic compounds from contaminated soils [53].
Figure 4.3 Structure of dirhamnolipid.
4.6.3 Sophorolipids
Torulopsis sp., mainly T. bombicola and T. apicola, are the main strains involved in the production of sophorolipids. Asmer et al. [54] proposed the chemical composition of sophorolipids as the dimeric carbohydrate sophorose and long chain hydroxyl fatty acids, linked by a β‐glycosidic bond (Figure 4.4).
Figure 4.4 Structure of sophorolipids.
4.6.4 Trehalolipids
Trehalolipids is another important biosurfactant of glycolipid nature. Many members of the genus Mycobacterium show the presence of the serpentine group because of the presence of trehalose esters on their cell surface. Disaccharide trehalose linked at C‐6 and C‐6 to mycolic acid is associated with most species of Mycobacterium, Norcadia, and Corynebacterium. The trehalolipids have a complex chemical composition and possess the ability to alter the size and structure of their mycolic corrosive ends. They often occur in the form of a complex mixture whose composition varies depending on the strain physiology and growth condition.
4.6.5 Surfactin
Bacillus subtilis produces the most potential biosurfactant, i.e. surfactin is a complex mixture of different amino acids and fatty acid chain that bind with each other through a lactone linkage (Figure 4.5). The reduction in surface tension and interfacial tension of the water molecules occurs in the presence of surfactin. Surfactin has the potential to inactivate herpes and retroviruses.
Figure 4.5 Structure of surfactin.
4.6.6 Lipopeptides and Lipoproteins
According to Rosenberg and Ron [46], these structures consist of lipid molecules in attachment with a polypeptide chain. Antimicrobial action has been shown by many of the biosurfactants against various bacteria, algae, fungi, and viruses. The antifungal and antibacterial property of the lipopeptide, iturin, produced by B. subtilis have been reported by Besson et al. [55] and Singh and Cameotra [13], respectively. A study conducted by Nitschke and Pastore [56] shows lipopeptide activity even after autoclaving between pH 5 and 11 and with a shelf life of six months at −18 °C.
4.6.7 Fatty Acids, Phospholipids, and Neutral Lipids
Huge amounts of unsaturated fats and
