VOl3 NO 1
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`JANUARY 1986
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`ISSN 0265-1351
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`Published for the International Union of Microbiological Societies by Blackwell Scientific Publications Ltd
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`FRESENIUS-KABI, Exh. 1024
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`Microbiological Sciences Vol. 3, No.5, 1986
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`145
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`Biosurfactants
`
`DG Cooper*
`
`Many different types of biosurfactants are synthesized by microorganisms. As the structures and properties are
`elucidated, yields increased and costs of recovery from the fermentation media reduced, biosurfactants will become
`important industrial chemicals.
`
`Introduction
`
`Conventional surfactants are currently used for a broad
`range of purposes in a large variety of different applica(cid:173)
`tions. 1 Most requirements for a conventional surfactant
`could be met by a biosurfactant. To justify replacement
`of a synthetic surfactant with a biological compound, it is
`necessary to find a more effective agent for a given
`application, and/or one that can be produced more
`cheaply.
`This article discusses the structures and properties of
`biosurfactants, their production and isolation from fer(cid:173)
`mentation broths, and their potential for commercial
`exploitation (particularly the more unusual compounds
`produced by microbes). These have unique properties
`because of their structures and thus have the best poten(cid:173)
`tial for application to a specific niche.
`A surfactant is a molecule which has both water(cid:173)
`soluble and water-insoluble (usually hydrocarbon) por(cid:173)
`tions.:.! This balance of hydrophilic and hydrophobic
`moieties in the same molecule imparts unusual proper(cid:173)
`ties, including an ability to lower the surface tension of
`water. Unfortunately, the term biosurfactant has gener(cid:173)
`ally been used very loosely to refer to any compound
`which has some influence on interfaces. For example, it
`is often applied to biopolymers which have emulsifying
`properties but do not lower the surface tension of water
`appreciably or demonstrate other characteristics of a
`classical surfactant. This article considers both the bio(cid:173)
`logical compounds which fit the classical definition of a
`surfactant as well as the larger, poorly defined polymers
`or cell fragments which have some form of surface ac(cid:173)
`tivity. Several longer review articles have been publish(cid:173)
`ed on various aspects ofbiosurfactants.:l-:>
`
`Structures
`
`Biosurfactants have many different structures. Most are
`lipids, which have the typical amphiphilic structure of a
`surfactant. The lipophilic portion of lipids is almost
`always the hydrocarbon tail of one or more fatty acids
`(Figure 1) which may be saturated or unsaturated and
`may contain cyclic structures or hydroxyl functions. The
`polar, water-soluble part of a biosurfactant may be as
`
`*Department of Chemical Engineering, McGill University, 3480
`University Street, Montreal, Quebec, Canada H3A 2A7.
`Received 13 December, 1985.
`
`Lipophilic
`
`Hydrophilic
`
`Figure 1. Carboxylic acids and other lipids have the amphipathic
`structure of a surfactant.
`
`simple as a carboxylate or hydroxyl function or a com(cid:173)
`plex mixture of phosphate, carbohydrate, amino acids,
`etc.
`Most biosurfactants are either neutral or negatively
`charged. In anionic biosurfactants the charge is due to a
`carboxylate and/or phosphate or, occasionally, to a sul(cid:173)
`phate group. A small number of cationic biosurfactants
`contain amine functions.
`Biosurfactants may be classified on the basis of their
`lipid types.:l The simple neutral lipid surfactants include
`esters, alcohols and mono-., di- and triglycerides. The
`phospholipids contain diglyceride structures, phosphate
`and a wide range of polar groups. Glycolipids vary from
`the ubiquitous glycosyl glycerides to the many complex
`compounds produced by microbes. Finally, there are
`several examples of lipopeptide biosurfactants.
`Carboxylic acids, neutral lipids and phospholipids are
`well known constituents of all cells and the usual types
`will not be considered here. A more unusual group of
`hydroxycarboxylic acids are common in microbial bio(cid:173)
`surfactants. They have useful surfactant properties on
`their own and are common constituents of complex bio(cid:173)
`surfactants.
`Figure 2 illustrates one common type of hydroxy-acid.
`These a-branced, J3-hydroxy acids are highly variable.:l
`The shorter, corynomycolic acids with 20 to 40 carbon
`atoms are particularly common in biosurfactants; un(cid:173)
`branched, hydroxy acids are also found. The hydroxyl
`group can be either adjacent to the carboxylic group or
`at the opposite end of the hydrocarbon chain. A more
`complex carboxylic acid surfactant is 4-hydroxy-4, 5-
`dicarboxypentadecanoic acid, with
`three carboxyl
`groups and a hydroxyl group in the polar portion. 1
`;
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`Microbiological Sciences Vol. 3, No.5, 1986
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`The lipids with the most unusual structures, and hence
`the greatest potential for unique properties, are the
`microbial glycolipids and lipopeptides. In general, these
`lipids are synthesized as mixtures and in most cases there
`is appreciable variation in the types of fatty acids incor(cid:173)
`porated and even in the polar groups or the points of
`attachment between the two portions.
`The first of these glycolipid surfactants to be studied
`were the sophorose lipids produced by Torulopsis
`species (Figure 3).:;-;; The sophorose is attached to an
`w-hydroxy fatty acid through the hydroxyl moiety,
`resulting in a surfactant structure with a single hydro(cid:173)
`phobic portion between two polar structures. Several
`patents have been issued for this biosurfactant. 7
`,H The
`rhamnolipids from Pseudomonas aeruginosa and the
`trehalose lipids isolated from a variety of bacteria also
`contain various hydroxy fatty acids and carbohydrates
`and have surfactant properties.:l-:i,!J
`The most thoroughly studied lipopeptide surfactant is
`produced by Bacillus subtilis and given the trivial name
`surfactin (Figure 4). :;-;; The seven amino acids in this
`compound form a ring and are bonded to both the
`carboxyl and hydroxyl groups of the acid. Other
`lipopeptide surfactants have been reported but only a
`few have been characterized completely.
`There are also examples of one or two amino acids
`
`Figure 2. An a-branched, {3-hydroxy carboxylic acid. For cory(cid:173)
`nomycolic acids n 1 plus n?. varies between 20 and 40.
`
`CH20H
`
`COOH
`I
`(CH2)n
`I
`0 - , -H
`
`HO
`
`HO
`
`...._ ____ 0
`
`OH
`
`CH 3
`
`Figure 3. One type of glycolipid from Torulopsis species
`containing sophorose and an w-hydroxy carboxylic acid.
`
`0
`II
`CH -(CH2) 9 - CHCH2-c- L-Giu- L-Leu
`I
`I
`l
`Djleu
`
`CH3/
`
`L-Leu-D-Leu- L-Asp- L-Val
`
`Figure 4. Structure of surfactin from Bacillus subtilis.
`
`attached to fatty acids. These biosurfactants are zwitter(cid:173)
`ionic (they can carry both positive and negative
`charges). The most interesting example is cerilipin from
`Gluconobacter cerinus, which contains the unusual
`amino acid taurine: this makes it one of the few biosur(cid:173)
`factants with a sulphate group.
`The remaining types of surface-active compounds are
`polymeric. These are often referred to as biosurfactants
`but a more appropriate word would be bioemulsifiers. In
`general, these are poorly defined polysaccharides and
`often contain some protein or carboxylic acids.:;-;;, w, 11
`The most thoroughly studied bioemulsifier is emulsan,
`produced by Acinetobacter calcoaceticus and composed
`mainly of amino sugars and fatty acids. 1?. For the poly(cid:173)
`
`meric emulisfiers which have been reported, the emul(cid:173)
`sifying properties have been characterized much more
`thoroughly than the structures.
`
`Properties
`
`The simplest test for surface activity is the measurement
`of the surface tension of an aqueous system. In most
`cases, the preliminary testing is done with the whole
`microbial growth medium. An organism can be con(cid:173)
`sidered promising if it produces compounds which
`
`reduce the surface tension to below 40 mN m- 1• A good
`biosurfactant will produce values below 35 mN m- 1
`, but
`the most effective biosurfactant reported is surfactin
`from Bacillus subtilis (producing a surface tension of 27
`mNm- 1).
`Relatively little work has been done with purified
`biosurfactants. H The presence of other materials in the
`whole broth samples causes some deviation from the
`true value for the active compounds but this is insig(cid:173)
`nificant when compared to the large change from the
`surface tension of pure water (72 mN m - 1). In fact, most
`
`applications of biosurfactants will be with the whole
`broth or only partially purified mixtures because of eco(cid:173)
`nomic considerations. Thus, the surface tensions and
`other properties of these crude systems are the relevant
`data for these uses. These properties have been used to
`screen for suitable biosurfactants.
`Many studies have been carried out to select micro(cid:173)
`organisms for use in the petroleum industry and especi(cid:173)
`ally those suitable for enhanced oil recovery. -t, 1:1- 1;;
`It is possible to select for biosurfactants which can wet
`solid surfaces. It has been known for some time that
`Thiobacillus thiooxidans produces mixtures of phospho(cid:173)
`lipids which wet sulphur particles.:1 A recent study to
`select biosurfactants to enhance peat de-watering, de(cid:173)
`monstrated a correlation between the ability to wet wax,
`extracted from the peat, and the ability to improve water
`loss. 1
`H
`The ability of surfactin to lyse red blood cells is of
`limited use, but this discovery has led to a quick method
`for screening microbes growing on solid media for their
`ability to produce biosurfactants. -t, 17
`Many studies have been published on the testing of
`microbes for their ability to produce emulsifiers. :l,-t, 1x, w
`
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`Microbiological Sciences Vol. 3, No.5, 1986
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`147
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`By altering the aqueous phase it was possible to test for
`stability of the emulsions to pH changes, salt additions,
`heat, etc. A wide range of different oil phases were used
`in the tests including pure hydrocarbons, crude oils and
`vegetable oils. Many of the emulsifiers that were charac(cid:173)
`terized were found to be polymeric, with minimal ability
`to lower surface tension.
`
`Production
`
`Biosurfactants are produced by a wide range of mic(cid:173)
`robes. While all microbes produce lipids which have
`surface activity, most interest lies with those capable of
`producing good yields of extracellular products. The
`highest yield reported is for the sophorose lipids from
`Torulopsis bombicola (35 g g- 1 of substrate). :w Unfor(cid:173)
`tunately, most yields are much less than this and are not
`high enough to be economic. Work has begun to develop
`strain-selection techniques and this will allow the devel(cid:173)
`opment of overproducing mutants. 17
`It has been suggested that microbes release biosurfac(cid:173)
`tants as a mechanism for obtaining water-insoluble sub(cid:173)
`strates.:;,;;, I:; This cannot be the only biological function
`as there are many examples of microbes producing sur(cid:173)
`face-active agents when growing on soluble subs(cid:173)
`trates.:;, IO,:!I Some microbes can produce surfactants
`when grown on many different substrates ranging from
`carbohydrates to hydrocarbons. Changing the substrate
`often alters the structure of the product which, in turn,
`alters its surfactant properties; this can be useful when
`designing a product with the appropriate properties for a
`given application.
`There are many examples in the literature of changes
`in the carboxylic acids incorporated into lipids achieved
`by manipulating the substrate.:;,:!;! It has been shown that
`small variations in carboxylic acids can have dramatic
`effects on surfactant properties. There are also examples
`of more substantial changes, such as modification of the
`polar group in a biosurfactant by changing the substrate
`or growth conditions.
`Relatively few of the microbes known to have some
`surface activity associated with them have been grown
`and monitored in fermenters. There is no standard pat(cid:173)
`tern for all those that have been studied, but many
`synthesize a biosurfactant throughout the exponential
`growth phase. This has been observed for the produc(cid:173)
`tion of both biosurfactants and bioemulsifiers.:;, II,:!:!,:!:;
`Very distinct maxima are observed during the expo(cid:173)
`nential growth of Corynebacterium lepus producing an
`unidentified surfactant.:!:! In the C. lepus fermentation
`the free corynomycolic acid disappears because it is
`incorporated
`into a
`lipopeptide surfactant which
`appears late in the fermentation. Similar behaviour has
`been reported for the production of a bioemulsifier by
`Candida lipolytica. :!-l
`Rhodococcus erythropolis produces glycolipids in two
`stages:;!;; after an early plateau in biosurfactant concen(cid:173)
`tration, there is a second burst of production. Bacillus
`subtilis can be induced to yield a second, large amount of
`
`the lipopeptide surfactin by the addition of iron or man(cid:173)
`ganese salts to the fermenter after active growth is
`over. :!I,:!:!
`Late production of glycolipid by Torulopsis bambi(cid:173)
`cola can be induced by substrate manipulation. :!o This
`yeast requires both carbohydrate and vegetable oil to
`yield biosurfactant; if it is grown on only one substrate
`and the second is added after growth is finished there is
`an immediate burst of product formation.
`The many different modes of biosynthesis of surfac(cid:173)
`tants support the contention that cells are producing
`them for a variety of purposes. In many cases there are
`opportunities to influence the fermentation to increase
`yields and decrease costs.
`All of the above fermentations were aerobic. There
`has been very little investigation of the production of
`these compounds by anaerobic culture. A recent study
`of Bacillus licheniformis characterized a very effective
`biosurfactant isolated after strict anaerobic growth. :!ti
`This compound, which may have an application in in situ
`enhanced oil recovery, was a lipopeptide and appears to
`be very similar to surfactin from Bacillus subtilis.
`A major problem with the economics of biosurfac(cid:173)
`tants is that the fermentations result in dilute aqueous
`solutions of the desired products. Often solvent extrac(cid:173)
`tions are used to recover these compounds.:1,
`, IH Some
`11
`can be recovered by altering the pH and collecting a
`precipitate,:! 1 ,:!fi but crude sophorolipid is unusually easy
`to obtain, separating as a denser phase from the fermen(cid:173)
`tation broth ofT. bombicola. :!o
`
`Applications
`
`Surface-active agents are needed for a very large
`number of diverse applications 1 and there is no industry
`which does not have some use for these compounds.
`Emulsion stabilization is a very common requirement
`for food products, cosmetics, cutting oils, etc. Demulsi(cid:173)
`fiers are required, for example in de-watering of crude
`oil. Surfactants are also useful as soaps and detergents,
`both for simple cleaning applications and for more
`exotic purposes, such as enhanced oil recovery and oil
`spill clean-up.
`Surface-wetting and solid dispersal are important pro(cid:173)
`perties for froth-flotation separation of ores or prepara(cid:173)
`tion of coal slurries for pipelining. Colloid preparations
`are necessary for paints and related products. Penetra(cid:173)
`tion rates of inks, dyes, etc. are important for the pulp
`and paper and textile industries.
`There are many instances where excessive foaming
`must be regulated with surfactants, and the property of
`foam stabilization is also necessary for fire extinguishers
`and in the food industry. Other desirable properties are
`lubrication, corrosion inhibition and static inhibition.
`The above cursory list gives some overall indications
`of the breadth of applications of these compounds in
`industry. The total use of all of these products in the
`United States alone in 1982 was 2.5 million metric tons. 1
`Virtually all of these compounds were synthesized
`
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`MicrobiologicalSciences Vol. 3, No.5, 1986
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`chemically. Large amounts of natural products, especi(cid:173)
`ally lignin and triglycerides, were used as feedstocks but
`a significant portion were prepared from petroleum.
`With so many surfactants already available it is reason(cid:173)
`able to question whether biosurfactants from microbes
`have a future in industry. A strong argument in their
`favour is that any new surfactant is potentially useful.
`There are so many different applications, each requiring
`a slightly different mix of properties, that it is always
`useful to have new products of this type. Many of the
`structures of the biosurfactants are so different from the
`synthetic compounds that they will have novel combina(cid:173)
`tions of properties. Ideally, one is lboking for a biosur(cid:173)
`factant which has unique characteristics suitable for an
`application with a high enough value to justify the cost of
`the fermentation and product isolation.
`In many of the applications of surfactants there is a
`requirement for a number of compounds with a grada(cid:173)
`tion of some property. For example, in applications
`involving crude oil there is a broad range of types of oils
`and reservoir conditions and each requires slightly dif(cid:173)
`ferent surfactant properties. The ability to make small
`alterations in biosurfactant structure by altering growing
`conditions or the substrate is a simple method of gener(cid:173)
`ating a family of related compounds with a spectrum of
`properties.
`Biosurfactants can be synthesized from renewable
`substrates. Inevitably, petroleum resources are being
`depleted and, at some time in the future, the only source
`of surfactants will be from renewable feedstocks. In
`addition, synthetic surfactants usually require pure
`chemicals, or at least classes of chemicals (i.e. alkanes)
`to produce the desired product. This is not crucial for a
`fermentation: in fact, it is possible to produce biosurfac(cid:173)
`tants by fermenting waste streams containing carbo(cid:173)
`hydrates, fatty acids or similar compounds.
`Finally, biosurfactants are all biodegradable and do
`not present the same pollution problems as some surfac(cid:173)
`tants. There is less likelihood of toxicity problems
`although the intrinsic surfactant properties could be
`deleterious regardless of the source of the compound.
`Industrial application of biosurfactants remains at the
`development stage. The oil industry is a clear target for
`these compounds since it uses large amounts of surface(cid:173)
`active compounds and the application is such that crude
`extracts or even a whole fermentation broth could be
`used. In addition, there is less rigorous testing than there
`is for food and cosmetic formulations, and many of the
`biosurfactant-producing microorganisms can use pet(cid:173)
`roleum products as substrates.
`Applications are also being considered for biosurfac(cid:173)
`tants in industries involving food, cosmetics, pulp and
`paper, coal beneficiation, textiles and ore-processing.
`To be commercially viable, these compounds must be
`produced cheaply and easily; the surfactant properties
`should be uniquely suited to the application.
`The potential use of biosurfactants to de-water fuel(cid:173)
`grade peat is an example of an application where biosur(cid:173)
`factants have unique advantages over conventional
`
`agents. Hi Biosurfactants are added to the process before
`mechanical pressing to improve water loss significantly.
`The wastewater stream contains dissolved organics and
`is a potential pollutant: some of the biosurfactant(cid:173)
`producing microbes, notably Bacillus subtilis, can use
`this waste stream as a substrate. 27 This lowers the bio(cid:173)
`logical oxygen demand of the effluent and produces the
`de-watering additive. As the peat bogs are often in
`remote locations, there is a distinct advantage in pro(cid:173)
`ducing the de-watering agent on site from a readily
`available feedstock.
`
`Conclusions
`
`Biosurfactants are diverse and ubiquitous and there is a
`high probability of finding a compound with the appro(cid:173)
`priate combination of properties for a specific applica(cid:173)
`tion. They can be biosynthesized from inexpensive,
`renewable substrates and they are biodegradable.
`Before most of these compounds can be successfully
`commercialized, it will be necessary to improve yields
`and lower product-separation costs.
`
`References
`
`7
`
`8
`
`Layman PL. Industrial surfactants set for strong growth. Chemi(cid:173)
`cal and Engineering News 1985; 63 (3): 23-48.
`2 Tadros TF, ed. Surfactants. London: Academic Press, 1984.
`3 Cooper DG, Zajic JE. Surface-active compounds from micro(cid:173)
`organisms. Advances in Applied Microbiology 1980; 26: 229-
`53.
`4 Singer ME. Microbial biosurfactants. Microbes and Oil Recovery
`1985; 1: 19-38.
`5 Zajic JE, Seffens W. Biosurfactants. Critical Reviews in Biotech(cid:173)
`nology 1984; 1: 87-107.
`6 Zajic JE, Ban T. Spiculisporic acid. Microbes and Oil Recovery
`1985; 1: 310-20.
`Inoue S, Kimura Y, Kinta M. Process for producing a glycolipid
`ester. US Patent 4,2i5,2i3. issued July 29, i980.
`Inoue S, Kimura Y, Kinta M. Dehydrating purification process
`for a fermentation product. US Patent 4,i97,i66. Issued April
`8, i980.
`9 Krestschmer A, Bock H, Wagner F. Chemical and physical
`characterization of interfacial-active lipids from Rhodococcus
`erythropolis grown on n-alkanes. Applied and Environmental
`Microbiology 1982; 44: 864-70.
`10 Cooper DG, Paddock DA. Torulopsis petrophilum and surface
`activity. Applied and Environmental Microbiology 1983; 46:
`1426-9.
`11 Cirigliano MC, Carman GM. Purification and characterization of
`liposan, a bioemulsifier from Candida lipolytica. Applied and
`Environmental Microbiology 1985; 50: 846-50.
`12 Gutnick DL, Rosenberg E, Belsky I, Zosim Z. Emulsans. US
`Patent. issued July 26, i983.
`Janshekar H. Microbial enhanced oil recovery processes. Mic(cid:173)
`robes and Oil Recovery 1985; 1: 54-84.
`Jack TR, Thompson BG. Patents employing microorganisms in
`oil production. In: Zajic JE, Cooper DG, Jack TR, Kosaric N,
`eds. Microbial Enhanced Oil Recovery, Tulsa, Oklahoma:
`Penn Well Books 1983; 14-25.
`15 Zajic JE, Gerson DF. Microbial extraction of bitumen from
`Athabasca oil sand. In: Strausz 0, ed. Oil Sands and Oil Shale,
`New York: Verlag Chemie, 1978, 145-61.
`16 Cooper DG, Pillon DW, Mulligan CN, Sheppard JD. Biological
`additives for improved mechanical de-watering of fuel-grade
`peat. Fue/1986; 65: 255-9.
`
`13
`
`14
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`Microbiological Sciences Vol. 3, No.5, 1986
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`149
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`17 Mulligan CN, Cooper DG, Neufeld RJ. Selection of microbes
`producing biosurfactants in media without hydrocarbons.
`Journal of Fermentation Technology 1984; 62: 311-14.
`18 Akit J, Cooper DG, Manninen Kl, Zajic JE. Investigation of
`potential biosurfactant production among phytopathogenic
`corynebacteria and related soil microbes. Current Micro(cid:173)
`biology 1981; 6: 145-50.
`19 Cooper DG, Liss SN, Longay R, Zajic JE. Surface activity of
`Mycobacterium and Pseudomonas. Journal of Fermentation
`Technology 1981; 59: 97-101.
`20 Cooper DG, Paddock DA. Production of a biosurfactant from
`Torulopsis bombicola. Applied and Environmental Micro(cid:173)
`biology 1984; 47: 173-6.
`21 Cooper DG, Macdonald CR, Duff SJB, Kosaric N. Enhanced
`production of surfactin from Bacillus subtilis by continuous
`product removal and metal cation additions. Applied and En(cid:173)
`vironmental Microbiology 1981; 42:408-12.
`22 Cooper DG. Unusual aspects of biosurfactant production. In:
`Ratledge C, Dawson P, Rattray J, eds. Biotechnology for the
`
`Oils and Fats industry. American Oil Chemists Society, 1984:
`281-7.
`23 Macdonald CR, Cooper DG, Zajic JE. Surface active lipids from
`Nocardia erythropolis grown on hydrocarbons. Applied and
`Environmental Microbiology 1981; 41: 117-23.
`24 Cirigliano MC, Carman GM. Isolation of a bioemulsifier from
`Candida lipolytica. Applied and Environmental Microbiology
`1984; 48:747-50.
`25 Rapp P, Bock H, Wray, Wagner F. Formation, isolation and
`characterization of trehalose dimycolates from Rhodococcus
`erythropolis grown on n-alkanes. Journal of General Micro(cid:173)
`biology 1979; 115:491-503.
`Javaheri M, Jenneman GE, Mcinerney MJ, Knapp RJ. Anae(cid:173)
`robic production of a biosurfactant by Bacillus licheniformis
`JF-2. Applied and Environmental Microbiology 1985; 50:
`698-700.
`27 Mulligan CN, Cooper DG. Pressate from peat de-watering as a
`substrate for bacterial growth Applied and Environmental
`Microbiology 1985; 50: 160-2.
`
`26
`
`Biotechnological applications of carboxydotrophic
`bacteria
`
`E Williams* & J Colby
`
`Carbon monoxide (CO) is a widespread pollutant and a hazard to man because of its extremely toxic nature. It is a
`major component of some industrial gas mixtures and may be derived from coal. The carboxydotrophic bacteria
`obtain energy and carbon from the oxidation of CO. These organisms may be used to produce new metabolites, and
`the oxidases from them may be used to produce fuel cells and biosensors for CO.
`
`Introduction
`
`Carbon monoxide is a major atmospheric pollutant
`occurring in rural areas at a concentration of0.1 p.p.m.
`and in urban districts between 50-100 p.p.m. 1 It is a
`colourless, odourless, tasteless and explosive gas with
`flammable limits in air of 12-75% and an ignition point
`in air of 700°C. It is only sparingly soluble in water; 3.3
`ml/100 ml H 20 at 0°C, 2.3 ml/100 ml H 20 at 20°C.
`Carbon monoxide is extremely toxic to aerobic organ(cid:173)
`isms because of its affinity for the metal ions of respir(cid:173)
`atory chain components, and in man it binds easily to
`haemoglobin and can cause rapid and lethal toxaemia.
`Anthropogenic emissions of CO exceed all other pol(cid:173)
`lutants and about 1.4x 10H tare added annually from the
`incomplete combustion of fossil fuels. It is also a major
`component of volcanic gases (1 to 4% ), resulting in a
`further annual addition of 1 x 10H t to the atmosphere and
`is released from the oceans (where supersaturation
`factors > 30 may be found in the surface layers) and
`from the earth's crust. 2 ,:l
`Biogenic contributions to the global production of CO
`are extremely small in comparison to abiogenic sources,
`but a variety of living systems evolve CO in small
`
`*Microbial Technology Group, Department of Microbiology, The
`Medical School, University of Newcastle upon Tyne, Framlington
`Place, Newcastle upon Tyne NE2 4HH, UK.
`Received 8 October, 1985.
`
`amounts. Man produces small amounts of CO from the
`oxidation of haemoglobin by microsomal haem oxy(cid:173)
`genase and in enclosed conditions, such as in sub(cid:173)
`marines, this may accumulate and may reach dangerous
`concentrations. However, man's contact with CO is
`usually from exposure to domestic and industrial
`combustion processes such as faulty domestic heating
`systems, blast furnace gas (25% CO), automobile
`exhaust gases (0.5 to 12% CO) and smoking tobacco
`(cigarette smoke contains 2 to 5% CO, cigar and pipe
`tobacco smoke contains 5 to 14% CO). Carbon mon(cid:173)
`oxide may also accumulate in coal mines and during the
`treatment and transport of sewage.
`
`Industrial gases containing carbon monoxide
`
`Gas mixtures containing CO are widely used as cheap
`feedstocks in many chemical industries and there is
`increasing interest in producing these mixtures from
`coal, lignite and peat, which currently account for about
`47% of the available energy content in recoverable
`reserves of known fossil fuels. -t A coal-based synthesis
`gas industry existed in the United Kingdo~ until the
`1960s converting Town gas into synthesis gas, consisting
`of carbon monoxide and hydrogen, for use in producing
`ammonia by the Haber process or for conversion to
`methanol. Cheaper naphtha and processes based on
`• methane later replaced this coal-based technology in the
`
`
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