Microbial surfactants or biosurfactants are surface active byproducts, synthesized by microorganisms like bacteria, yeasts and fungi. They are also termed as bioemulsifiers and the terms are used synonymously. They have low or high molecular weight, low toxicity, biodegradability, stability at high temperature, pH and salinity and ability to be tailored by methods of genetic engineering. These properties make them superior to chemical surfactants and therefore they have been used for various industrial, environmental and medical applications. They perform different functions like increasing surface area of hydrophobic and water insoluble substrates (like oils), increasing bioavailability of hydrophobic substrates by increasing their solubility or desorption and regulating attachment and detachment of producer microbe to and from surfaces. According to the producing culture and properties bioemulsifiers are classified as Glycolipids, Lipopeptides and Particulate Biosurfactants.
Production, biosynthesis and genetics:
In the laboratory, bioemulsifiers are produced on conventional synthetic defined media containing mineral salts, carbon and nitrogen source suitable for the growth of producing microbe. Nonconventional media like dairy byproducts like whey, leftover from fish market, industrial effluents, molasses, corn steep liquor or sea water have also been used. The medium is inoculated with microbial culture and incubated under controlled temperature, time and shaking/static conditions. How to detect the production of bioemulsifier? For this certain physicochemical assays are carried out; some preliminary detection tests are: measurement of oil emulsification activity (emulsion formation), dye binding, wetting activity, hydrophobicity, hemolytic assay, colorimetric assay and interfacial/surface tension measurement. Estimation of protein, lipid or carbohydrate content will give an idea about chemical composition as well as protocol for further purification can be determined. Structure of purified product can be estimated by advanced physicochemical techniques of chromatography, crystallography or spectrophotometry. The properties like antimicrobial, sporicidal or degradation assays can be performed under in vitro or in vivo conditions.
Biosynthesis pathways of emulsifier production can be elucidated by performing specific assays for regulatory enzymes involved in synthesis of polymers like bioemulsifiers.
Genetic studies are done with respect to the mutants lacking bioemulsifier production. The genes or gene clusters encoding regulator and promoter proteins of biosynthetic pathways and their transcriptional activation can be analysed which would represent genetic basis for bioemulsifier synthesis.
Principle producing microbial genera:
Bacteria, yeasts and fungi are producers of surfactants.
Bacteria are Acinetobacter, Acetobacter, Arthrobacter, Corynebacterium, Brevibacterium, Nocardia, Micrococcus, Bacillus, Paenibacillus, Pseudomonas, Bifidobacter, Serratia, Rhodococcus and Streptococcus etc.
Fungal genera include Penicillium, Aspergillus, Colletotrichum, Verticillum, Fusarium etc.
Biosurfactant producing yeasts are Torulopsis, Candida, Yarrowia, Aureobasidium, Saccharomyces etc.
Important applications of biosurfactants in industry, environment and pharmacy are enlisted:
• Electrocoating process
• Cleaning of oil contaminated vessels
• Large scale on field cleaning of oil spills
• Emulsion stabilizers
• Paint formulation
• Microbially enhanced oil recovery (MEOR)
• Oil spill management
• Bioremediation of xenobiotics and recalcitrants
• Heavy metal recovery
• Cosmetics like moisturizers and other dermatological creams/lotions
• Antibiotics like viscosin, surfactin, gramicidin, erythromycin, polymyxin (anti-viral, bacterial, fungal and yeasts pathogens)
• Other antimicrobial drugs
• Food additives and stabilizers
• Drug delivery system
• Immunization formulations
• Toothpaste/mouthwash/germicidal solutions
• Biofertilizer formulations
• Anti-Mollusks, nematode, insects and fungal formulations
• Biological control of seedling blight, seed rot etc
• Biofilm formation and stabilization
• Quorum sensing
• Pathogenesis related phenomena like tissue invasion
• Establishment of the microbe in its microhabitat
In this article, it was attempted to organize the vast information available on microbial surfactants.
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