Biosurfactants and their Potential Applications for Microbes and Mankind: An Overview




Aisha Khan
Amna Butt


Fatima Jinnah Women University,
The Mall,
Rawalpindi,
Pakistan

Correspondence:
Amna Butt
Fatima Jinnah Women University,
The Mall,
Rawalpindi,
Pakistan
Email: ambutt91@yahoo.com

Abstract

Biosurfactants are surface-active compounds synthesized by a wide array of microorganisms. They are "amphipathic" molecules having both hydrophobic and hydrophilic domains. These are capable of lowering the surface tension and the interfacial tension of the growth medium. Biosurfactants have different chemical structures; lipopeptides, glycolipids, neutral lipids and fatty acids. They are non-toxic biomolecules that are biodegradable. Biosurfactants also exhibit strong emulsification of hydrophobic compounds and form stable emulsions. The low water solubility of these hydrophobic compounds confines their accessibility to microorganisms, which is a potential problem for bioremediation of contaminated sites. Microbially produced surfactants enhance the bioavailability of these hydrophobic compounds for bioremediation. Therefore, biosurfactant-enhanced solubility of pollutants has potential applications in bioremediation. Not only are the biosurfactants useful in a variety of industrial processes, they are also of vital importance to the microbes in adhesion, emulsification, bioavailability, desorption and defence strategy.

Key words: Biosurfactant, Biodegradable, Bioremediation, Bioavailability, Surface tension, Surfactants, Specificity, Emulsification



Introduction
Surfactants are low molecular weight chemicals that change the properties of water and other fluids. They are amphiphilic compounds that reduce the surface tension and help formation of emulsions between different liquids (an emulsion is a homogeneous mixture of two liquids) [1] [2]. A good surfactant can lower surface tension (ST) of water from 72 to 35 mN/m and the interfacial tension (IT) water/hexadecane from 40 to 1 mN/m [3]. When surfactants wet a surface, the unwanted substance can be removed. Surfactants are a very important chemical compound which is used in a variety of product with very high volume because of its domestic and industrial applications [2] [4] [5] [6]. Despite all the advantages of surfactants its release into the environment can be a potential danger itself to the environment, being non-degradable and toxic in nature [7]. Currently, almost all the synthetic surfactants being produced are chemically derived from petroleum [8].

Properties of Biosurfactants
Microbial surface active agents (biosurfactants) are important biotechnological products, with a wide range of applications in many industries [9]. Their properties of interest are:

(i) changing surface active phenomena, such as lowering of surface and interfacial tensions;
(ii) wetting and penetrating actions;
(iii) spreading;
(iv) hydrophylicity and hydrophobicity actions;
(v) microbial growth enhancement;
(vi) metal sequestration; and
(vii) Anti-microbial action.

Advantages of Biosurfactants
There are many advantages of biosurfactants if compared to their chemically synthesized counterparts [9]. Some of these are:

• Biodegradability;
• Generally low toxicity;
• Biocompatibility and Digestibility: It allows their application in cosmetics, pharmaceuticals and as functional food additives;
• Availability of raw materials: Biosurfactants can be produced from cheap raw materials which are available in large quantities; the carbon source may come from hydrocarbons, carbohydrates and/or lipids, which may be used separately or in combination with each other;
• Acceptable Production Economics : Depending upon application, biosurfactants can also be produced from industrial wastes and by-products and this is of particular interest for bulk production (e.g. for use in petroleum-related technologies);
• Use in environmental control : Biosurfactants can be efficiently used in handling industrial emulsions, control of oil spills, biodegradation and detoxification of industrial effluents and in bioremediation of contaminated soil;
• Specificity: Biosurfactants are complex organic molecules with specific functional groups and are often specific in their action (this would be of particular interest in detoxification of specific pollutants): de-emulsification of industrial emulsions, specific cosmetic, pharmaceutical, and food applications;
• Effectiveness - at extreme temperatures, pH and salinity. Most of the biosurfactants are high molecular-weight lipid complexes, which are normally produced under aerobic conditions. This is achievable in their ex situ production in aerated bioreactors.

Natural Surfactants vs. Artificial Surfactants
Natural surfactants are getting preference over the synthetic chemical surfactants with increasing awareness about the environment and growing significance of a sustainable society in harmony with the universal environment. Among the natural surfactants, biosurfactants are the most promising. Bearing in mind the important properties and a wide range of applications of biosurfactants, during recent years much more attention has been given to understand the biochemical properties and physiological role of different classes of biosurfactant on the producing microorganism as well as commercial application of biosurfactants. There are many useful biosurfactants consisting of anionic, cationic, neutral and amphoteric ranging from small fatty acids to large polymers. This wide range results in a broad spectrum of potential applications.

Classification of Biosurfactants:
Biosurfactants are classified into two categories by Rosenberg and Ron (1999) on the basis of their chemical structure and microbial origin [10]. These categories are:

1. High-mass surfactants including i) polymeric and ii) particulate surfactants;
2. Low-mass surfactants including i) glycolipids ,ii) lipopeptides and iii) phospholipids,

High-Mass Surfactants:

i. Polymeric Biosurfactants
Polymeric biosurfactants are high molecular weight biopolymers, which exhibit properties like high viscosity, tensile strength and resistance to shear. The following are the examples of different classes of polymeric biosurfactants. The best studied polymeric biosurfactants are compiled from several well-known components such as emulsan, liposan, mannoprotein, and other polysaccharide-protein complexes [2].

ii. Particulate Biosurfactants
Extracellular membrane vesicles partition hydrocarbons to form a microemulsion which plays an important role in alkane uptake by microbial cells [11] [12]. Vesicles of Acinetobacter sp. with a diameter of 20 to 50 nm and a buoyant density of 1.158 g/cm3 are composed of protein, phospholipid and lipopolysaccharide [13].

Low Mass Surfactants:

i. Glycolipids
These are the most common carbohydrate in combination with long chain aliphatic acid of hydroxyl aliphatic acid [14]. The glycolipid can be categorised as:
• Rhamnolipids (commonly produced by Pseudomonas aeruginosa);
• Trehalolipids (Trehalose lipids were subsequently isolated from Rhodococcus erythropolis by Ristau and Wagner (1983) [15]. They are commonly associated with Actinomycetes, Mycobacterium, Nocardia And Corynebacterium)[14];
• Sophorolipids (produced by different strains of Yeast and Torulopis bombicola and T. petrophilum). Although sophorolipids can lower surface and interfacial tension, they are not effective emulsifying agents [16].

ii. Lipopeptides and Lipoprotein:
These consist of a large number of cyclic lipopetides linked to a fatty acid including Decapeptide antibiotics (gramicidins) and lipopeptide antibiotics (polymyxins) which possess remarkable surface-active properties [14]. Several bacteria are known to produce these antibiotic-like molecules particularly the cyclic lipopeptide surfactin. It is produced by Bacillus subtilus and is one of the most powerful and active biosurfactants [17]. It also possesses anti-bacterial, antiviral, anti-fungal, antimycoplasma and hemolytic activities.

iii. Fatty Acids, Phospholipids, and Neutral Lipids
Large quantities of fatty acid and phospholipid surfactants are produced by several bacteria and yeast during growth on n-alkanes [2] [18] [19] [20] [21].

Potential applications of Biosurfactants
Biosurfactants are one of the most important biotechnology products for industrial and medical applications due to their precise modes of action, low toxicity, relative ease of preparation and extensive applicability. Biosurfactants also play natural physiological roles in increasing bioavailability of hydrophobic molecules and can complex with heavy metals, promoting enhanced degradation of chemical contaminants. They can be used as emulsifiers, de-emulsifiers, wetting and foaming agents, functional food ingredients and as detergents in petroleum, petrochemicals, environmental management, agrochemicals, foods and beverages, cosmetics and pharmaceuticals, commercial laundry detergents and in the mining and metallurgical industries [1].

A. Significance and Role of Biosurfactants to Microbes:
The most important question today about the biosurfactants is why the microbes produce these surfactants and what is the significance and role of biosurfactants to the microorganisms, which produce them. The various roles that a biosurfactant will have could be unique to the physiology and ecology of the producing microorganisms and it is impossible to draw any universal generalizations or to identify one or more functions that are evidently common to all microbial surfactants. Some of the properties of biosurfactants that are of some significance for microbes are as follows:

Advantages of Biosurfactants for Microbes:

I. Adhesion

The most significant role of microbial surfactants is documented for adhesion of the cells to the interfaces. Adhesion is a physiological mechanism for growth and survival of cells in the natural environments. A special case of adhesion is the growth of bacteria on water insoluble hydrocarbons and is one of the primary processes affecting bacterial transport, which determines the bacterial fate in the subsurface. Bacterial adhesion to abiotic surfaces is attributed to attractive interactions between bacteria and the medium. When surfactants are immersed in water, surfactant molecules cause a distortion of the local tetrahedral structure of water and the hydrogen bonds between water molecules are energetically disfavoured, resulting in a decrease in interactions between bacteria and the porous medium. The mass of bacteria eligible for desorption varies directly with the magnitude of the interaction reduction. Since the enzymes necessary for hydrocarbon oxidation are on the cell membrane, the microbe must come into contact with its substrate. The growth of the microbes on certain surfaces is influenced by the biosurfactant, which forms a conditioning film on an interface, thereby stimulating certain microorganisms to attach to the interface, while inhibiting the attachment of others [22]. The microorganisms can use their biosurfactants to regulate their cell surface properties in order to attach or detach from surfaces according to need [10].

II. Emulsification

Many hydrocarbon degrading microorganisms produce extracellular emulsifying agents, the inference being that emulsification plays a role in growth on water immiscible substrates. There is correlation between emulsifier production and growth on hydrocarbons. The majority of
Acinetobacter strains produce high-molecular-mass bioemulsifiers. Emulsifier producing organisms were able to grow on water insoluble substrates while, the mutants, that do not produce emulsifier, grow poorly on hydrocarbons. For the growth of microbe on hydrocarbons, the interfacial surface area between water and oil can be a limiting factor and the evidence that emulsification is a natural process brought about by extracellular agents is indirect and there are certain conceptual difficulties in understanding how emulsification can provide an (evolutionary) advantage for the microorganism producing the emulsifier [23].

III. Bioavailability and Desorption

One of the major reasons for the prolonged persistence of high-molecular-weight hydrophobic compounds is their low water solubility, which increases their sorption to surfaces and limits their availability to biodegrading microorganisms. Biosurfactants can enhance growth on bound substrates by desorbing them from surfaces or by increasing their apparent water solubility. Surfactants that lower interfacial tension are particularly effective in mobilizing bound hydrophobic molecules and making them available for biodegradation. Much less is known about how polymeric biosurfactants increase the apparent solubility's of hydrophobic compounds. Recently, it has been demonstrated that Alasan (a polymeric biosurfactant) increases the apparent solubility's of PAHs 5 to 20-fold and thus significantly increases their rate of biodegradation [24][25].

In addition to adhesion, desorption also plays an important part in the natural growth of the microorganisms. After a certain period of growth, conditions become unfavourable for further development of microorganism e.g., toxin accumulation and impaired transport of necessary nutrients in crowded conditions. Desorption is advantageous at this stage for the cells and need arises for a new habitat. In fact mechanisms for detachment seem to be essential for all attached microorganisms in order to facilitate dispersal and colonization of new surfaces. One of the natural roles of an emulsifier/biosurfactant may be in regulating desorption of the producing strain from hydrophobic surfaces [26].

IV. Defence Strategy

According to Puchkov apart from two main natural roles suggested for surface-active compounds (increasing availability of hydrophobic substrates and regulating attachment and detachment to and from surfaces) the biosurfactants could be an evolutionary defence strategy of microbe as evidenced by high mycocidal activity of the MC secreted by C. humicola. Similar analogy can be made for the lipopeptides biosurfactant producing strains of B. subtilis. The lipopeptide (antibiotic) would have strong influence on the survival of B. subtilis in its natural habitat, the soil and the rhizosphere [27].

B. Advantages of Biosurfactants for Mankind:

The characteristics of microbial surfactants, which may be valuable for their commercialization, are as follows:

1. Biodegradability and Controlled Inactivation of Microbial Surfactants

Several chemically synthesized, commercially available surfactants (e.g., perfluorinated anionics) resist biodegradation and accumulate in nature causing ecological problems. Microbial surfactants like all natural products are susceptible to degradation by microorganisms in water and soil [28,29] .

2. Selectivity for Specific Interfaces

Biological molecules have been found to show more specificity as compared to the chemically synthesized materials. Microbial surfactants show a specificity not seen in presently available commercial surfactants for example, specificity of emulsion towards a mixture of aliphatic and aromatic hydrocarbons [30, 31, 32].

3. Surface Modification

An emulsifying or dispersing agent not only causes a reduction in the average particle size but also changes the surface properties of the particle in a fundamental manner. Small quantities of a dispersant can dramatically alter the surface properties of a material such as surface charge, hydrophobicity and most interestingly pattern recognition based on the three dimensional structure of the adherent polymer [1].

4. Diversity of Microbial Surfactants

Microorganisms produce a wider range of surfactant molecules than are available through chemical synthesis. A broad spectrum of surfactants is required to satisfy the industrial demand. Almost every commercial application has a unique set of growth conditions that dictates the optimum type of surfactant formulation, a single isolate often generates chemical variations of the same surfactant, resulting in the production of a surfactant mixture with an associated characteristic surface [33, 34, 35]. Even small differences in the structure of a surfactant can have profound effects on its function and its potential industrial applications [36].

5. Toxicity

Surfactants are one of the major components (10-18%) of detergent and household cleaning products and are used in high volumes. Most of these end up in natural waters and consequently, their impact on the environment has been and continues to be, a worldwide concern. Scientific literature is full of the reports, which describe and discuss the toxic effects of surfactants [37, 38]. The biological surfactants or biosurfactants have an added advantage of being less toxic or nontoxic in comparison to the synthetic surfactants. This property makes them a better candidate for taking care of pollutants in the environment rather than a menace by itself [39].

C. Potential Applications of Biosurfactants for Mankind:
Surfactants are the most important class of industrial chemicals which are used widely in almost every sector of modern industry. Only within the US chemical industry the demand of surfactants has been increased by 300 % during the last decade [40]. At present, the worldwide production is more than three million tonnes per annum (at an estimated value of US $4 billion) and is expected to be greater than over four million tonnes by the end of the century [41, 42].

Because of low toxicity, biodegradable nature and diversity, biosurfactants have gained considerable interest in current years. Biosurfactants are used in many industrial applications like enhanced oil recovery, crude oil drilling, lubricants, surfactant-aided bioremediation of water-insoluble pollutants, health care and food processing [43, 44, 45, 46, 47, 48, 49]. The use of biosurfactants is also common in cosmetic and soap formulations, foods and both dermal and transdermal drug delivery systems [47].

1. Bioremediation by Biosurfactants

Bioremediation aims at providing economical treatments to reduce the concentration of individual or diverse environmental contaminants [50]. 0.08-0.4% of the total worldwide production of oil eventually reaches the sea and contaminates it [51]. In recent years many oil spill accidents have resulted in significant contamination of oceans and seashore environments. More than 105 tonnes of oil were released in the Gulf waters during the Gulf War in 1991 which was a threat to desalination plants and the coastal ecosystem of the Gulf [52].

Due to these incidents attempts were made to develop various chemicals procedures/techniques for combating oil pollution both at sea and along the shoreline. The degradation of hydrocarbons in the environment is due to the ability of biosurfactants to emulsify hydrocarbon-water mixtures. One of the most efficient methods for removing the presence of hydrocarbon-degrading microorganisms in seawater renders biodegradation pollutants [53] [54] [55]. Compared with the chemical surfactants most biosurfactants have lower possible toxicity and shorter persistence in the environment [56] [57]. The ability of a surfactant to enhance the biodegradation of slightly soluble organic compounds depends on the bioavailability of the compound [58].

Marine Bioremediation by Microbial surfactants

Microorganisms capable of hydrocarbon degradation have often been isolated from marine environments [59]. An emulsifier produced by Pseudomonas aeruginosa SB30 was capable of dispersing oil into fine droplets and inferred that it may be useful in removing oil from contaminated beaches [60]. All of the studies related to marine bioremediation by using biosurfactants are laboratory based and successful bioremediation of exposed marine open sites using biosurfactants remains a challenge.

i. Biosurfactants and Soil Bioremediation

The accumulation and persistence of toxic materials in water and soil represents a major problem today. Various organics are generated as byproducts from various industries (e.g. petroleum and petrochemical, pulp and paper, chemical industries etc.), which may be released into the environment, or are accidentally spilled. Aromatics and their chlorinated derivatives are of primary concern. These are difficult to biodegrade and are toxic. These chemicals are proven carcinogens, so their release to water and soil is prohibited. If, however, they do appear in industrial wastewaters, they must be treated and detoxified by utilizing a combination of methods (chemical, physical, and biological). Biological methods show many advantages, and many organics can be efficiently degraded by aerobic and anaerobic processes. While water treatment is relatively easy to perform, soil bioremediation is much more difficult and complex [61]. The first problem arises due to difficulties in treating soil, especially when pollution is distributed over a large area. Thus, removal of soil from a contaminated site becomes a costly undertaking, even though such ex situ treatment might be well established. This could be accomplished in two ways:
(i) Addition of nutrition to the soil in form of nitrogen, phosphorus and, if necessary, carbon compounds, which would allow the native microbial population to develop and augment, and thus provide more microorganisms for metabolism or co-metabolism of the pollutant in question.
(ii) Produce ex situ a microbial population which is adapted to the pollutant and is capable of metabolizing it efficiently, and then add this population, along with necessary nutrients, to the polluted soil. The added biomass would, under proper conditions, be able to survive in the soil and to further degrade objectionable organics. Both methods are applied whereby method (i) seems to be more popular, but the strategy depends upon the type of the pollutant, the environmental soil conditions, and the availability of the adapted culture.

Bioremediation of soil contaminated with organic chemicals is a viable alternative method for clean-up and remedy of hazardous waste sites. The main objective in this approach is to convert the parent toxicant product into a readily biodegradable one, which is harmless to human health and/or the environment. The biological remediation process can be performed
i) In situ;
ii) In a prepared bed; and
iii) In a slurry reactor system.

In general, biodegradation of the hydrocarbons at any given site will depend upon:
• indigenous soil microbial population,
• hydrocarbon variety and concentration,
• soil structure,
• nutrient availability,
• Oxygen availability.

Soil microorganisms reported to degrade hydrocarbons under favourable conditions include Pseudomonas, Flavobacterium, Achromobacter, Arthrobacter, Micrococcus and Acinetobacter. Hydrocarbons with less than 10 carbon atoms tend to be relatively easy to degrade as long as the concentration is not too high to be toxic to the organisms. Benzene, xylene and toluene are examples of gasoline components that are easily degraded. Complex molecular structures, such as branched paraffins, olefins, or cyclic alkanes, are much more resistant to biodegradation. Soil structure, which is the form of assembly of the soil particles, determines the ability of that soil to transmit air, water, and nutrients to the zone of bioactivity. Another major controlling factor is the variety and balance of nutrients in the soil. Nitrogen and phosphorous are the most common additives. Biodegradation of hydrocarbons in soil can also be efficiently enhanced by addition or in situ production of biosurfactants. It was generally observed that the degradation time, and particularly the adaptation time, for microbes was shortened. Studies with chemical surfactants showed that the degradation of phenanthrene by an unidentified isolate could be increased by a nonionic surfactant based on ethylene glycol [61].

ii. Role of Microbial Surfactants in Bioremediation of Oil Pollutants
Oil pollution is an environmental problem of increasing importance. Hydrocarbon-degrading microorganisms, adapted to grow and thrive in oil-containing environments, have an important role in the biological treatment of this pollution. One of the limiting factors in this process is the bioavailability of many fractions of the oil. The hydrocarbon-degrading microorganisms produce biosurfactants of diverse chemical nature and molecular size. These surface-active materials increase the surface area of hydrophobic water-insoluble substrates and increase their bioavailability, thereby enhancing the growth of bacteria and the rate of bioremediation. Oil-contamination of soil is a common problem and its physical treatment methods or remediation techniques can be difficult or economically not feasible. One of the most economically feasible methods includes in situ bioremediation by the use of microorganisms which is the partial simplification or complete destruction of the molecular structure of environmental pollutants. Numerous attempts have been made to successfully remediate the oil contaminated soil by using microbial inoculation and by biosurfactant treatment. The rhamnolipid biosurfactant produced by P. aeruginosa stimulates the uptake of hydrophobic compounds finally leading to its degradation. Study has shown that the bacteria are efficient biosurfactant producers in petroleum oil-contaminated soil which offers the advantage of a continuous supply of natural, nontoxic and biodegradable biosurfactants by bacteria at low cost for solubilizing the hydrophobic oil hydrocarbons prior to biodegradation [62].

2. Application of Biosurfactant in Petroleum Industry
Indigenous or injected biosurfactant-producing microorganisms are exploited in oil recovery in oil-producing wells. Microbial enhanced oil recovery (MEOR) is often implemented by direct injection of nutrients with microbes that are able to produce desired products for mobilization of oil, by injection of a consortium or specific microorganisms or by injection of the purified microbial products (e.g., biosurfactants). These processes are followed by reservoir re-pressurization, interfacial reduction of tension/oil viscosity and selective plugging of the most permeable zones to move the additional oil to the producing wells. Oil recovery was shown to be increased by 30-200 % with injection of biosurfactants, bacteria (e.g., P. aerugi- nosa, X. campestri, B. licheniformis) and nutrients [63]. However, application of MEOR requires thorough research on a case-by-case basis taking into account the physical-chemical conditions and soil and rock formation characteristics.

MEOR is a powerful technique to recover oil, especially from reservoirs with low permeability or crude oil with high viscosity, but the uncertainties on the results and costs are a major barrier to its widespread use. Oilfield emulsions are formed at various stages of petroleum exploration, production and oil recovery and processing, and represent one of the major problems for the petroleum industry, which requires a de-emulsification process in order to recover oil from these emulsions [64]. Biosurfactants have the potential to replace the use of chemical de-emulsifiers in situ, saving on transport of the oil emulsion and providing a more environmentally-friendly solution. Among the bacteria species, Acinetobacter and Pseudomonas species are the main de-emulsifiers in the mixed cultures [65]. The microorganisms exploit the dual hydrophobic/hydrophilic nature of biosurfactants or hydrophobic cell surfaces to disrupt the emulsions. Glycolipids (e.g., rhamnolipids), glycoproteins, phospholipids and polysaccharides are among the microbial tools to displace the emulsifiers from the oil-water interface [12]. Microbes and biosurfactants are in general readily biodegradable, which allows a cheap and easy removal of the de-emulsifier after this process.

3. Biosurfactants and oil storage tank cleaning:
Biosurfactants are also used for oil storage tank cleaning. Surfactants are used for reducing the viscosity of heavy oils which facilitates recovery, transportation and pipelining [66]. A glycolipid surfactant reduces the viscosity of heavy crude oil by 50%. This surfactant is produced by Gram-negative, rod-shaped bacterial isolate H13A [67]. These cleaning processes are economical and less hazardous to persons involved in the process compared to conventional processes [68]. This leads to less disposal of oily sludge in the natural environment and is an environmentally sound technology.

4. Biosurfactants in Medicinal and Therapeutic Industry:

Biosurfactants also have some medicinal and therapeutic applications. Some of these applications are as follows:

• Surfactin is one of the earliest known biosurfactants having various pharmacological applications such as inhibiting fibrin clot formation and haemolysis [69]. It has also been reported to have an antitumor activity and anti-fungal properties [70]. Itokawa et al. (1994) have provided the details about the use of surfactin against human immunodeficiency virus 1 (HIV-1). Vollenbroich et al. proposed the use of surfactin in the virus safety enhancement of biotechnological and pharmaceutical products. The anti-viral action of surfactin is due to a physiochemical interaction between the membrane active surfactant and the virus lipid membrane [72].
• Thimon et al. (1995) described Iturin as an anti-fungal biosurfactant. It is a lipopeptide produced by B. subtilis, which affects the external structure and membrane structure of yeast cells [73].
• Naruse et al. (1990) provided the detail about the inhibitory effect of pumilacidin (surfactin analog) on herpes simplex virus 1 (HSV-1). They also reported the defence against gastric ulcers in vivo.
• Isoda et al. (1997) investigated the biological activities of microbial glycolipids of C. Antarctica T-34 and reported an induction of cell differentiation in the human promyelocytic leukemia cell line HL60 [75].
• The reports on antibiotic effects (Neu et al. 1990) and inhibition of HIV virus growth in white blood corpuscles have opened up new fields for their applications [22].
• Kosaric (1996) describes possible applications as emulsifying aids for drug transport to the infection site, for supplementing pulmonary surfactant and as adjuvants for vaccines. Respiration failure in premature infants is caused by a deficiency in pulmonary surfactant [9].
• With the bacterial cloning of the gene for the protein molecule of the surfactant, the fermentative production of this product for medical application is now possible [76].
• The succinoyl-trehalose lipid of Rhodococcus erythropolis has been reported to inhibit HSV and influenza virus) [77].
• Biosurfactants have been found to inhibit the adhesion of pathogenic organisms to solid surfaces or to infection sites and have been shown to inhibit formation of biofilms on different surfaces, including polyvivyl wells and vinyl urethral catheters [78]. Pre-coating the catheters and medical devices by biosurfactant solution can have potential applications for treating.

5. Biosurfactants for agricultural use

The global concerns about pesticide pollution have encouraged the efforts to find alternative biological control technologies. Biosurfactants have potential for the biological control of zoosporic plant pathogens [79]. For the hydrophilization of heavy soils to obtain good wetting ability and also to achieve equal distribution of fertilizers and pesticides in the soils Surface active agents are needed.

6. Biosurfactants use in mining

For the dispersion of inorganic minerals in mining and manufacturing processes biosurfactants may be used. Also for the stabilization of coal slurries to aid the transportation of coal Kao Chemical Corporation (Japan) used Pseudomonas, Corynebacterium, Nocardia, Arthrobacter, Bacillus and Alcaligenes to produce biosurfactants. Biosurfactants were also tested for the solubilization of coal and partial solubilization was achieved [80].

7. Biosurfactants in the Cosmetic Industry

Multifunctional biosurfactants have several cosmetic applications because of their exceptional surface properties such as detergency, wetting, and emulsifying, solubilizing, dispersing and foaming effects [81] [82]. Biosurfactants are known to have advantages over synthetic surfactants such as low irritancy or anti-irritating effects, better moisturizing properties and compatibility with skin, therefore, are highly on demand [83]. The most widely used biosurfactant glycolipids in cosmetics are sophorolipids, rhamnolipids and mannosylerythritol lipids. Sophorolipids have good skin compatibility and excellent moisturizing properties, rhamnolipids are natural surfactants and emulsifiers that can replace petrochemical based surfactants used in most of the cosmetic products. They also have been used in acne pads, anti-dandruff, anti-wrinkle and anti-ageing products, deodorants, nail care products and toothpastes in several different formulations, because of their high surface and emulsifying activities [84]. Mannosylerythritol lipids are generally used in skin care formulations as the active ingredient to prevent skin roughness [85].

8. Biosurfactants use in the food industry

• Biosurfactants are used as emulsifiers for the processing of raw materials in the food industry;
• Surface active compounds are also used in bakery and meat products, where they influence the rheological characteristics of flour and the emulsification of partially broken fat tissue [70];
• In food industries worldwide, Lecithin and its derivatives are currently in use as emulsifier [86];
• Busscher et al. (1996) established that a biosurfactant produced by thermophilic dairy Streptococcus spp could be used for fouling control of heat exchanger plates in pasteurizers, as they retard the colonization of S. thermophilus responsible for fouling [87];
• Biosurfactants are also utilized as fat stabilizer and anti-spattering agents during cooking of oil and fats [88].
• In food processing the addition of rhamnolipid surfactants improve the texture and shelf-life of starch-containing products, modify rheological properties and stability of wheat dough [89]. L-Rhamnose is the precursor of high-quality flavour. Biosurfactants act as controlling consistency in bakery and ice cream formulation.

9. Biosurfactant as a Substitute of Synthetic Chemical Surfactant in Commercial Laundry Detergents

Almost all surfactants, an important component used in modern day commercial laundry detergents, are chemically synthesized and exert toxicity to fresh water living organisms. Furthermore, these components often produce undesirable effects. Therefore, growing public unrest about the environmental hazards and risks associated with chemical surfactants has stimulated the search for eco-friendly, natural substitutes of chemical surfactants in laundry detergents. Crude CLP biosurfactants showed good emulsion formation capability with vegetable oils and demonstrated excellent compatibility and stability with commercial laundry detergents favouring their inclusion in laundry detergents formulations [90].

10. Other Applications of Biosurfactants

Other important commercial and industrial applications of microbial surfactants includes using biosurfactants in paper industry, textiles and ceramics industries and paint industries due to their enhanced mixing properties. These are also used as dewatering agents in pressing peat [90].

Conclusion

During the recent years there is an increasing environmental awareness and therefore, it might be reasonable to assume that microbial surfactants have a promising role to play in the years to come. Considering the importance of biosurfactants, there is an urgent need to gain a greater understanding of the physiology, genetics and biochemistry of biosurfactant-producing strains and to improve the process technology to reduce production costs for commercial level production of biosurfactants. Therefore, an extensive cooperation among different science disciplines is needed in order to fully characterize the biochemical properties of biosurfactant and exploration of their potential applications in different industrial sectors. Compared with chemical surfactants, the biosurfactants have the advantage of biodegradability and lower toxicity that make them more appropriate for replacing chemicals. There are several types of biosurfactants in market but no single biosurfactant is suitable for all potential applications. Chemically synthesized compounds are cheaper than the biosurfactants available in the market basically because biosurfactants have their high production costs and the lack in comprehensive toxicity testing. The cost of production of biosurfactants may be significantly reduced for selected applications such as using sterilized or pasteurized fermentation broth without any need for extraction, concentration or purification. The crude product may be used by oil industries and environmental bioremediation in many applications. Strategies like medium and downstream process-optimization may also have a positive impact on cost reduction. It is very alarming that large chemical companies seem not to be interested in research in these areas.

The usefulness of biosurfactants in bioremediation is however expected to gain more importance in the coming years. Their success in bioremediation will require precise targeting to the physical conditions and chemical nature of the pollutant affected areas. Encouraging results have been obtained for the use of biosurfactants in hydrocarbon pollution control in marine biotopes in closed systems (oil storage tanks) and, although many laboratory studies indicate potential for use in open environments, a lot remains to be demonstrated in pollution treatment in marine environments or coastal areas.

The usefulness of biosurfactants in other fields is promising. The progress is astonishing particularly in personal and health care products, cosmetics and as therapeutic agents. Lipases had been used for the enzymatic synthesis of man-made surfactants and have given a whole new dimension to biosurfactant production. With increased efforts on developing improved application technologies, strain improvement and production processes, biosurfactants are expected to be among the most used and produced chemicals in the near future.

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