Microbial surfactants, called bioemulsifiers, are structural compounds that contain a lipophilic and a hydrophilic moiety inside the same molecule. The hydrophilic group (polar group) is derived from alcohol groups of neural lipids or ester, amino acids or carboxylate group of fatty acids phospholipids(phosphate portions), or carbohydrates of glycolipids.
While, the lipophilic part of the compound is constructed by the hydrocarbon chain of fatty acid. The roles of a biosurfactant include emulsification of water insoluble substrates and exertion of antimicrobial effects on competing microbes.
Microbial Biosurfactants- Examples
Some examples of microorganisms belonging to the glycolipids group that produce important biosurfactants include:
(1) Pseudomonas sp., produces the bioemulsifier rhamnolipids;
(2) Serratia rubidae, produces the Rubiwettins; and
(3) Mycobacterium leprae, produces the Trehalose lipids.
Examples of microorganisms belonging to the neutral and polar lipid group producing important biosurfactants include:
(1) Corynebacterium lepus, which produces fatty acids; and
(2) Rhodotorula sp., which produces polyol lipids.
The examples of microorganisms belonging to the amino acid group producing biosurfactants include:
(1) Pseudomonas fluorescens, produces Viscosin;
(2) Bacillus subtilis, produces surfactin; and
(3) Candida lipolytica, produces liposan.
Examples of microorganisms belonging to the polysaccharide lipid group producing bioemulsifiers include:
(1) Acinetobacter calcoaceticus, which produces emulsan; and
(2) Streptococcus sanguis, which produces lipoteichoic acid.
Microbial Biosurfactants- Today
The global market for detergents in on the rise and the world wide surfactants consumption is expected to reach an approximate 10 billion kilogram by the year 2000.
Biosurfactants are capable of replacing chemically synthesized compounds in different areas of applications. Biosurfactants are less toxic when compared to synthetic tensides; are biodegradable; and are produced on renewable substrates. Further, the physical properties and the chemical structure of biosurfactants can be altered by biological, chemical, or genetic manipulation allowing scientists to tailor the surfactants to specific needs. The key advantages of microbial biosurfactants in industrial applications include: low cost process, high-quality substrate yield, and easy end product recovery.
Pseudomonas aeruginosa Rhamnolipids Composition
The glycolipid biosurfactant containing rhamnose produced by Pseudomonas aeruginosa was described in the year 1949. Rhamnolipid 1 is found in culture supernatants of P. aeruginosa, while rhamnolipid 2 is observed during the cultivation of P. aeruginosa.
Rhamnolipids 3 and 4 are found in the culture supernatants of the resting cells. Rhamnolipid 1 and 2 derivatives of methyl ester were extracted from P. aeruginosa strain 158. The presence of fatty acid homologues in the rhamnolipids were identified by electron impact mass spectrometry and fast atom bombardment. The glycosyl moiety is the building block of rhamnolipids whereas the structural difference depends on the fatty acid residues.
The biosynthesis of P. aeruginosa rhamnolipids initially involved the in vivo method by using important radioactive precursors such as [14C] glycerol and [14C] acetate. The first putative biosynthetic pathway was proposed by Burger. In this pathway, the rhamnolipid synthesis is preceded by the sequential glycosyl transfer reactions, catalyzed by the thymidine-diphospho-rhamnose (TDP-rhamnose) acting as the rhamnosyl donor, while the
L-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate (β-hydroxydecanoic acid) acts as the acceptor. The TDP-rhamnose occurs in majority of gram negative bacteria. The TDP-glucose is formed via several enzymatic steps and is converted to TDP- rhamnose by the reduction, dehydration, and epimerization of the glycosyl moiety.
The β-hydroxydecanoic acid (acceptor substrate) has two formation routes: (1) it rises as a fatty acid degradation intermediate by the β-oxidation cycle. This route is the predominant pathway during the growth on n-alkanes; (2) it occurs as a de novo fatty acid biosynthesis intermediate.
The P. aeruginosa rhamnolipids are produced depending on several environmental and nutritional factors. Rhamnolipids synthesis is found maximum during the stationary growth phase and the late exponential phases of growth. The synthesis takes place under nitrogen limitation conditions thereby demonstrating a direct relationship between improved biosurfactant production and increased glutamine synthetase. Further, rhamnolipid synthesis is also favored non-limiting phosphate concentrations. Among the many mineral salts found in the culture medium, iron has the biggest impact on rhamnolipid synthesis. A three-fold increase in the production of rhamnolipid with no profound changes in the biomass yield was reported after a change to iron-limiting conditions.
Biosurfactants have potential applications in the industrial sector such as emulsification, wetting, emulsion stabilization, phase separation, foaming, corrosion-inhibition, viscosity reduction, de-emulsification, and solubilization. Surfactants area also used in protective coatings, and paints, for processing petrochemical products (textile industry), and for depollution purposes.
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