hydrophobic sophorosides. Although sophorolipids can lower the surface and interfacial tension, they are not effective emulsifying agents.

Lipopeptides and Lipoprotiens

A large number of cyclic lipopeptides including decapeptide antibiotics and lipopeptide antibiotics, produced by Bacillus brevis and Bacillus polymyxa, respectively, possess remarkable surface-active properties. An aminolipid biosurfactant called serratamolide has been isolated from Serratia marcescens NS.38. Studies on serratamolide-negative mutants showed that the biosurfac-tants increased cell hydrophilicity by blocking the hydrophobic sites on the cell surface.



ch2 ch3

Figure 10. Structure of surfactin obtained from Baxcillus subtilis.

The cyclic lipopeptide surfactin (Fig. 10) produced by B. subtilis ATCC21332, is one of the most powerful biosurfactant. It lowers the surface tension from 72 to 27.9 mN m"1 at concentration as low as 0.005%.

Recently, Yakimov et al34have showed the production ofa new lipopeptide surfactant, lichenysin A, by B. licheniformis BAS-50 containing the long-chain ^-hydroxy fatty acids.

Fatty Acids, Neutral Lipids, Phospholipids

Several bacteria and yeasts produce large quantities of fatty acids and phospholipid surfactants during growth on n-alkanes. The HLB is directly related to the length of the hydrocarbon chain in their structure. In Acinetobactorsp. strain HO1-N phosphatidylethanolamine (Fig. 11) rich vesicles are produced, which form optically clear microemulsion of alkanes in water.

Phosphatidylethanolamine produced by R. erythropolis grown on n-alkane caused a lowering of interfacial tension between water and hexadecane to less than 1 mN m-1 and a cmc of 30 mg l-1.

Polymeric Surfactants

The best-studied polymeric biosurfactants are emulsan, liposan, mannoprotein and other polysaccharide-protein complexes. Acinetobacter calcoaceticus RAG-1 produces a potent poly-anionic amphipathic heteropolysaccharide bioemulsifier called emulsan (Fig. 12). Emulsan is a very effective emulsifying agent for hydrocarbon in water even at a concentration as low as 0.001 to 0.01%. It is one of the most powerful emulsion stabilizers known today and resists inversion even at a water-to-oil ratio of 1:4. Biodispersan is an extracellular, nondialyzable dispersing agent produced by A. calcoaceticus A2. It is an anionic heteropolysaccharide, with an average molecular weight of 51,400 and contains four reducing sugars.

Liposan is an extracellular water soluble emulsifier synthesized by Candida lipolytica and is composed of 83% carbohydrate and 17% protein. The carbohydrate portion is a heteropolysaccharide consisting of glucose, galactose, galactosamine and galacturonic acid.

Particulate Biosurfactants

Extracellular membrane vesicles partition hydrocarbons, to form a microemulsion which plays an important role in alkane uptake by microbial cells. Vesicles of Acinetobacter sp. strain HO1-N with a diameter of 20 to 50 nm and a buoyant density of 1.158 g cm-3 are composed of protein,

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