GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

12. METABOLIC ACTIVITY OF AEROBIC HETEROTROPHS

12.6. INCOMPLETE OXIDATION

12.6.5. Formation of secondary metabolites

12.6.5.5. Microbial surface-active substances

Surface-active substances (Surfactants) are compounds capable of reducing surface tension at a phase boundary. Due to Intermolecular Forces, such substances can concentrate (adsorb) at phase interfaces, lowering surface tension values. Surfactants are subdivided into molecularly soluble ones, which form true solutions, and colloidal ones (micellar-soluble). The Main characteristics of surfactants are as follows:

1) critical micelle concentration (CMC). CMC is defined as the concentration of a surfactant at which micelles are formed in its solution in equilibrium with molecules or ions. In practice, CMC is determined as the maximum concentration of a truly soluble surfactant that can be obtained under given conditions;

2) hydrophilic-lipophilic balance (HLB) value. HLB is determined on a 20-point scale and indicates The ratio of the hydrophilic and hydrophobic parts in a surfactant molecule. The higher the HLB value, the more hydrophilic the compound.

Surfactants of microbial origin are called biosurfactants or bio-surfactants. Their development began in the 1970s; thus, like microbial Lectins, they are a relatively new product of biotechnology. The substrates for surfactant synthesis are predominantly Hydrocarbons (n-alkanes), which are Water-insoluble compounds. Consequently, the synthesis of surfactants by microorganisms can be viewed as a vital function, namely, the emulsification of recalcitrant substrates and their conversion into an assimilable form. From this perspective, biosurfactants can be considered primary metabolites. However, besides hydrocarbons, water-soluble compounds (glucose, ethanol) can also be used as substrates for surfactant synthesis. In such cases, surfactant production is not a vital function, and they can be regarded as secondary metabolites.

Biosurfactants are micellar-soluble compounds, meaning they consist of hydrophobic and hydrophilic parts. Based on their CMC values, surfactants can be divided into three groups:

1) surfactants with a CMC exceeding 7 g/L. Such surfactants do not exhibit detergency properties;

2) typical detergents and emulsifiers, with a CMC ranging from 0.2 to 7.0 g/L;

3) surfactants that are sparingly soluble in water and readily soluble in hydrocarbon media, with a CMC not exceeding 0.2 g/L.

It should be noted that distilled water has a surface tension of 73 mN/m. Effective biosurfactants reduce this value to 29-32 mN/m while maintaining low CMC values.

According to their chemical nature, biosurfactants are divided into five groups (following the Classification of the German scientist F. Wagner, a leading specialist in the field of microbial surfactants): 1) Glycolipids; 2) lipopeptides (Lipoproteins); 3) lipopolysaccharides; 4) Fatty acids and their derivatives; 5) Phospholipids.

Producers of surfactants include Bacteria, Yeasts, Fungi, and microalgae. The best-known microbial surfactants are glycolipids (produced by Agrobacterium species; pseudomonads synthesize a surfactant called rhamnolipid P-1; yeasts of the genera Candida and Torulopsis). Among lipoproteins, Bacillus-derived surfactants such as surfactin and lichenysin are the most prominent, while emulsan is a well-known lipopolysaccharide.

According to the classification by Israeli scientist E. Rosenberg (1999), biosurfactants are divided into low-molecular-weight and high-molecular-weight types. Low-molecular-weight biosurfactants include glycolipids (trehalose Lipids, sophorolipids, rhamnolipids) and lipopeptides (surfactin, polymyxin, gramicidin S). High-molecular-weight biosurfactants comprise Polysaccharides, Proteins, lipopolysaccharides, lipoproteins, or complexes of these compounds. Low-molecular-weight surfactants are capable of lowering interfacial surface tension, whereas high-molecular-weight ones act as effective stabilizers of oil-in-water emulsions, functioning as emulsifiers.