Biosurfactants for a Sustainable Future. Группа авторов

Figure 4.2 Classification of biosurfactants and the respective producing microorganisms.

4.4.1 Biosurfactants of Bacterial Origin

      In the growth medium, the hydrocarbons are emulsified by ionic surfactants excreted by some of the bacteria and yeast. Pseudomonas sp. that produce rhamnolipids (RLs) and Torulopsis sp. that are mainly involved in the production of sophorolipids are some examples of these groups of biosurfactants [35, 36].

Some bacterial species have the ability to alter their cell membrane structure by producing some nonionic or lipopolysaccharide biosurfactants. Examples of some nonionic trehalose corynomycolates producing bacterial strains are: Rhodococcus erythropolis, Arthrobacter sp., and various Mycobacterium sp. [37]. Acinetobacter sp. produce lipopolysaccharides, such as emulsan, and Bacillus subtilis produces extensive quantities of lipoproteins, such as surfactin and subtilisin [38, 39]. Table 4.1 depicts biosurfactants produced by various strains of bacteria.

Table 4.1 Biosurfactants derived from bacteria.

Bacteria	Biosurfactant
Serratia marcescens	Serrawettin
Rhodotorula glutinis, Rhodotorula graminis	Polyol lipids
Rhodococcus erythropolis, Corynebacterium sp. Mycobacterium sp., Arhtrobacter sp., Nocardia erythropolis	Trehalose lipids
Pseudomonas sp., Thiobacillus thiooxidans, Agrobacterium sp.	Ornithine lipids
Pseudomonas fluorescens, Leuconostoc mesenteriods	Viscosin
Pseudomonas aeruginosa, Pseudomonas chlororaphis, Serratia rubidea	Rhamnolipids
Pseudomonas fluorescens, Debaryomyces polmorphus	Carbohydrate‐lipid
Pseudomonas aeruginosa	Protein PA
Lactobacillus fermentum	Diglycosyl diglycerides

4.4.2 Biosurfactanats of Fungal Origin

Only a few species of fungi are identified for the production of biosurfactants in comparison to bacterial species. Some of the typical fungal strains explored for the production of biosurfactants, as investigated by researchers are, Candida bombicola [40], Candida ishiwadae [41], Candida lipolytica [42], Candida batistae [43], Aspergillus ustus [44], and Trichosporon ashii [45]. The best part of these fungal strains is that they have produced biosurfactants using low‐cost raw materials as their growth substrate. Glycolipids and sophorolipids are one of the most important class of biosurfactants produced by these fungal strains. The various biofactants produced by fungi are shown in Table 4.2.

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