Published on: Mar 4, 2016
Transcripts - Polysacccharide
Molecules 2009, 14, 2535-2554; doi:10.3390/molecules14072535
Bacterial Extracellular Polysaccharides Involved in Biofilm
Barbara Vu 1,2, Miao Chen 2, Russell J. Crawford 1 and Elena P. Ivanova 1,*
Faculty of Life and Social Sciences Swinburne University of Technology, PO Box 218, Hawthorn,
Victoria 3122, Australia
CSIRO Minerals, Bayview Avenue, Clayton, Victoria 3168, Australia
* Author to whom correspondence should be addressed; E-mail: firstname.lastname@example.org;
Tel.: +61-3-92145137; Fax: +61-3-98190834
Received: 18 May 2009; in revised form: 26 June 2009 / Accepted: 1 July 2009 /
Published: 13 July 2009
Abstract: Extracellular polymeric substances (EPS) produced by microorganisms are a
complex mixture of biopolymers primarily consisting of polysaccharides, as well as
proteins, nucleic acids, lipids and humic substances. EPS make up the intercellular space of
microbial aggregates and form the structure and architecture of the biofilm matrix. The key
functions of EPS comprise the mediation of the initial attachment of cells to different
substrata and protection against environmental stress and dehydration. The aim of this
review is to present a summary of the current status of the research into the role of EPS in
bacterial attachment followed by biofilm formation. The latter has a profound impact on an
array of biomedical, biotechnology and industrial fields including pharmaceutical and
surgical applications, food engineering, bioremediation and biohydrometallurgy. The
diverse structural variations of EPS produced by bacteria of different taxonomic lineages,
together with examples of biotechnological applications, are discussed. Finally, a range of
novel techniques that can be used in studies involving biofilm-specific polysaccharides is
Keywords: extracellular polymeric substances; biofilms; bioremediation; acidithiobacillus
Molecules 2009, 14 2536
Microorganisms are traditionally studied, characterized and identified as planktonic, single cells
[1-3]. However, detailed studies of sessile communities in different environments have lead to the
conclusion that planktonic microbial growth rarely exists in nature. The investigation of microbial
aggregates on tooth surfaces by Antonie van Leeuwenhoek resulted in the identification of microbial
biofilms [1,4,5]. Heukelekian and Heller found that for marine microbes, growth and activity were
enhanced by the presence of a surface onto which they could adhere . During a study of natural
marine bacteria populations, Zobell also discovered that there were many more microbes found
attached to solid surfaces than were found in the surrounding medium . It has become evident that
bacterial function and growth within a population is a fundamental aspect of bacterial survival and a
typical life style of microorganisms .
The term ‘biofilm’ was coined and described in 1978 . Since then, it has been well documented
that biofilm-associated microbes differ from their planktonic relatives in terms of the genes that are
transcribed . Bacteria can develop biofilms on a number of different surfaces, such as natural
aquatic and soil environments, living tissues, medical devices or industrial or potable water piping
systems [1,10]. Clusters of different microbial populations are found in almost all moist environments
where nutrient flow is available and surface attachment is possible . Biofilms have been found to
protect the microbial community from environmental stresses [10-12]. This is why the formation of
biofilms in natural and industrial environments allow bacteria to develop resistance to bacteriophage,
amoebae, chemically diverse biocides, host immune responses and antibiotics . These important
characteristics have resulted in biofilm science and biofilm engineering emerging as intensively
developing areas of research .
2. Microbial extracellular polysaccharides as an integral part of bacterial biofilms
A biofilm can be defined as an aggregation of bacteria, algae, fungi and protozoa enclosed in a
matrix consisting of a mixture of polymeric compounds, primarily polysaccharides, generally referred
to as extracellular polymeric substance (EPS). Over 99% of microorganisms on Earth live within these
biopolymers. The formation of biofilms is a prerequisite for the existence of all microbial aggregates
[14,15] as an essential step in the survival of bacterial populations . The proportion of EPS in
biofilms can comprise between approximately 50-90% of the total organic matter [1,10]. In
Gram-negative bacteria, some of the polysaccharides are neutral or polyanionic. The presence of
uronic acids or ketal-linked pyruvates enhances their anionic properties, thus allowing the association
of divalent cations such as calcium and magnesium to increase the binding force in a developed
biofilm. In some gram-positive bacteria, the chemical composition of their EPS could be slightly
different due to their primarily cationic nature [1,15]. Aside from polysaccharides, biofilms also
consist of proteins, nucleic acids, lipids and humic substances. Often the composition and quantity of
the EPS will vary depending on the type of microorganisms, age of the biofilms and the different
environmental conditions under which the biofilms exist . These include different levels of oxygen
and nitrogen, extent of desiccation, temperature, pH, and availability of nutrients. Their existence in
such a range of environments suggests that these microorganisms are able to respond to their
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environments and change their EPS and adhesion abilities, depending on the properties of the surface
onto which they attach . It has been reported that microbial colonization on solid surfaces can be
affected by diverse range of parameters. For example, the degree of colonization of certain surfaces
has been found to increase with surface roughness because the ‘valleys’ present can allow the microbes
to reside in a protected area with reduced shear forces and the surface roughness provides a surface
with increased surface area for bacterial attachment . Also, microorganisms have been found to
attach more rapidly to hydrophobic and non-polar surfaces than hydrophilic surfaces [1,10]. Cell
surface hydrophobicity, the presence of fimbriae and flagella and the degree of EPS production are
other main factors that have been shown to profoundly influence the rate and degree of attachment of
microbial cells to different surfaces . The three types of forces involved in this process are
electrostatic interactions, hydrogen bonds and London dispersion forces [10,17]. These binding forces
are likely to contribute to the overall stability of biofilm matrices . Different components of EPS
have also been found to influence the extent to which microorganisms can adhere to both hydrophilic
and hydrophobic surfaces. It has also been shown that the formation of EPS leads to irreversible
attachment with different environmental surfaces [1,16,18].
3. Role of EPS in the regulation of biofilm formation
The growth of a biofilm is the result of a complex process that involves the transport of organic and
inorganic molecules and microbial cells to the surface, a subsequent adsorption to the surface and
finally an irreversible attachment aided by the production of EPS . Due to its complexity, the
formation of biofilms is regulated at different stages via diverse mechanisms [20,21]. The most studied
regulatory mechanism that has been found to control the production of EPS, biofilm formation and
differentiation is quorum sensing (QS) regulation [1,20-24]. QS allows bacteria to maintain cell-cell
communication and also regulate the expression of specific genes in response to changes in cell
population density [23,25]. In general, the QS process involves the production, release and detection of
chemical signalling molecules, thus allowing microbial cells to regulate gene expression in a
cell-density-dependent manner . At a given population density, the genes involved in biofilm
differentiation and maturation are activated [1,21].
Two QS processes have been described for bacteria [21,27]. The autoinducer-1 (AI-1) type is
mainly involved in intra-species communication and the AI-2 type is associated with inter-species
interaction . Gram-negative bacteria produce and release AI molecules, which are generally
N-actyl homoserine lactone (AHL) molecules that serve as a function of controlling the cell-population
density. Bacteria detect the accumulation of AHL signals. Above a certain threshold concentration,
these signals are present in sufficient quantity to enable similar transcriptional effectors to activate
silent genes. This alters their cell-density dependent gene expression and therefore their behaviour
[20,22]. In Gram-positive bacteria, communication is carried out with modified oligopeptides
generating the signals and membrane-bound sensor histidine kinases acting as receptors. Signalling is
mediated by many phosphorylation steps, which control the activity of a response regulator. However,
peptide signals are not diffusible across the membrane and therefore the signal release is mediated by
oligopeptide exporters. Normally, signal release occurs concurrently with signal processing and
Molecules 2009, 14 2538
One of the best studied QS processes is the AHL-mediated QS system first described for the
bioluminescent marine bacterium Aliivibrio fischeri (formerly Vibrio fischeri ). This system is
considered to be the ideal model for the QS paradigm in most gram-negative bacteria. There are two
proteins, LuxI and LuxR, which control the expression of the luciferase operon required for light
production in A. fischeri. LuxI is the autoinducer synthase that produces AHL inducers via
S-adenosylmethionine (SAM) and LuxR is the cytoplasmic inducer receptor/DNA-binding
transcriptional activator that requires AHL coinducers to initiate expression of the luciferase-coded
function [21,22,27]. Once produced, AHL molecules diffuse in and out of the cell membrane and
increase in concentration when the cell-population density increases. When the critical threshold
concentration is reached, the AHL is bound by LuxR. The resulting LuxR-AHL complex activates the
transcription of the luciferase operon, and also the expression of LuxI and other genes involved in
different behavioural responses, creating a positive feedback loop, which results in the production of
4. Control of EPS production
QS is known as one of the regulatory pathways for EPS production and biofilm formation in
bacteria [20, 23, 26, 27, 30]. Also, phosphate and polyphosphate metabolism has been associated with
biofilm development and the QS regulatory pathway [28,31]. However, since the QS regulatory system
and biofilm formation and maintenance mechanisms are diverse among different bacterial species, the
role of QS in biofilm formation cannot be described in general terms . For example, in
Pseudomonas aeruginosa, QS is essential for adhesion, proper biofilm formation and virulence factors
[21,32]. They have two QS systems, LasI/LasR and RhlI/RhlR, which work the same way as the
LuxI/LuxR system in A. fischeri . Mutant P. aeruginosa cells that did not produce any QS signals
were found to be more densely populated with a thinner biofilm than the wild type. In addition,
mutation of the LasI gene resulted in an abnormal and undifferentiated biofilm formation process .
In E. coli, cellular functions are controlled by the QS LsrR/LsrK system. Biofilm formation and
architecture were found to be significantly altered in lsrR and lsrK mutants. There were differences
observed in the cell fimbriae and matrix structure and in the thickness of the mutants compared to the
wild type . Lastly, a QS system, AfeI/AfeR, has recently been identified in Acidithiobacillus
ferrooxidans, which is similar to the LuxI/LuxR proteins [20,23]. It was found that the amount of
lipopolysaccharides present was increased in phosphate starved A. ferrooxidans. There was also an
increase in the transcription of the afeI gene, and therefore AHL levels, when cells were cultured in a
low-phosphate medium compared to that of a high-phosphate medium. Since the AHL communication
system is present in A. ferrooxidans, this suggests that QS could regulate the formation of EPS and
biofilms for attachment to solid substrates [20,28].
5. EPS implications in biotechnology
There is a great deal of interest in the EPS of microorganisms used in the food, pharmaceutical,
biomedical, bioremediation and bioleaching fields due to their wide structural diversity and their
physical, rheological and other unique properties [34,35]. An expanding area of biotechnology is the
application of microorganisms in the remediation of environmental effluents produced by the mining
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and metallurgy industries [25,36,37]. Various strategies have been developed to eliminate
contaminants from the environment and to reduce the amount of toxic waste entering the environment
from these industries [11,38]. Over the past few decades, the use of microbes in remediation
techniques has increased in popularity due to their efficiency and their associated economic advantages
compared to traditional chemical and physical treatment methods [11,38,39].
Bioremediation is defined as ‘the elimination, attenuation or transformation of polluting or
contaminating substances by the use of biological processes’ . The applications of bioremediation
technologies are commonly targeted at pollutants such as heavy metals, BTEX hydrocarbons,
petroleum, polycyclic aromatic hydrocarbons, microaromatics, polychlorinated biphenyls, chlorinated
phenols and aliphatics [11,40].
Biofilm-mediated bioremediation has been found to be a more effective and safer alternative to
bioremediation with planktonic bacteria since cells growing within a biofilm have higher chances of
adaptation to different environments and their subsequent survival [1,3,11]. Biofilms maintain optimal
pH conditions, localized solute concentrations and redox potential, allowing cells to improve
mineralization processes . Biofilm reactors are generally used to treat hydrocarbons, heavy metals
and large volumes of dilute aqueous solutions such as industrial and municipal waste water [11,41].
The important role of EPS in the removal of heavy metals from the environment is due to their
involvement in flocculation and ability to bind metal ions from solutions . A major group of
bacteria commonly found in metal contaminated waters is sulfate reducing bacteria (SRB). They have
been shown to be highly efficient in anaerobic degradation of many organic pollutants and in the
precipitation of heavy metals from waste water . Other bacteria applied in the biosorption of toxic
heavy metals in bioremediation processes include Enterobacter and Pseudomonas species .
Biofilms play a critical role in the colonization of microbes on minerals by mediating their
attachment. The dissolution of metal sulfides is thought to occur in the EPS layer and the methond by
which this occurs is based on two assumptions. Firstly, it is assumed that the EPS-complexed iron ions
are reduced by an electron tunneling effect. Secondly, ferrous ion-glucuronic acid complexes are not
stable, and therefore allow the ferrous ions to migrate into the EPS space. If they diffuse towards the
outer membrane, they will be re-oxidised by the enzymatic system of the cells, and can therefore
re-enter the oxidation/reduction cycle . In addition, Sand and Gehrke found that there was a
correlation between the presence of ferric ions within the EPS layer and the extent of bacterial
metabolism . Their results suggested that cells with higher amounts of iron and glucuronic acids
within their exopolymers displayed higher oxidation activity than those with low amounts of these
components. It has also been reported that when there is lack of nutrients in the environment, the
production of EPS is increased to promote the hydrophobic interactions for sorption onto solid
surfaces. Their attachment to the substrate would allow greater chances for adsorption of organic trace
6. EPS roles in bioleaching
In the past few decades, the recovery of metals from low-grade mineral ores using microorganisms
has gained increasing popularity due to these bioprocesses being the economical and environmentally
friendly [38,44]. This process, termed bioleaching, relies on the ability of microbes to oxidize solid
Molecules 2009, 14 2540
compounds, resulting in soluble and extractable elements . Bioleaching is now an established
biotechnological technique for the recovery of heavy metals .
A. ferrooxidans is an acidophilic, obligately chemolithoautotrophic, gram-negative rod that oxidizes
ferrous iron for energy generation . It is one of the most commonly used microorganisms in
bioleaching [47-50]. However, despite many studies of biofilms formed by A. ferrooxidans in
bioleaching processes [42,50-52], the exact chemical structure and physicochemical conditions within
the EPS remain unclear [42,53,54].
Early commercial applications of bioleaching processes using this bacterium to recover a variety of
metals from low-grade ores  included copper  and uranium extraction . However, although
the roles of A. ferrooxidans were known, the mine dumps generated were not conducive to bacterial
activity. In 1980, commercial bioleaching applications were developed that aided the activity of the
microorganisms involved, resulting in a large number of copper heap bioleaching processes and the
biooxidation pretreatment of refractory sulfidic gold ores . Presently, A. ferrooxidans and other
metal solubilising microorganisms are used in industrial leaching processes for the treatment of
low-grade ores, which can contain metal concentrations that are below 0.5% (w/w) . A.
ferrooxidans are able to oxidize different sulfide minerals including arsenopyrite , pyrite,
chalcopyrite, galena, sphalerite and pentlandite  and the main procedures commercially used are
heap leaching, dump leaching, in-situ leaching and reactor leaching .
In bioleaching processes, the extent of interaction between the bacteria and mineral surfaces is very
important. Attachment of the microorganism to the mineral is followed by the oxidation of ferrous to
ferric ions and the reduction of sulfur to sulfate ions . The initial attachment of A. ferrooxidans to
mineral sulfides is non-specific, and driven by chemotactic , electrostatic  and hydrophobic
and hydrophilic interactions . Once cells have attached to the mineral, a monolayered biofilm is
developed over the next few days, covering the cells and mineral surface in an EPS layer. The latter
mediates the contact between bacterial cells and the sulfidic energy source, and is therefore an
essential step in the formation of biofilms and the subsequent bacteria/substrate interactions .
7. Chemical composition of EPS produced by A. ferrooxidans
The EPS produced by A. ferrooxidans consists of neutral sugars, predominantly rhammose, frucose
and glucose, and lipids . The chemical constituents of the EPS vary depending on the type of
substrate upon which the cells are grown. The mode of attachment also differs as a function of
substrate, and hence the expression of different EPS genes will result . The majority of research
investigating the nature of the EPS produced by A. ferrooxidans has taken place in studies involving
pyrite, sulfur and ferrous sulfate substrates [42,50].
7.1. A. ferrooxidans EPS produced on pyrite
The exopolymers produced by A. ferrooxidans grown on pyrite consists of neutral sugars glucose,
rhammose, fucose, xylose and mannose, and C12-C20 saturated fatty acids, as well as some glucuronic
acid residues and complexed ferric ions . The glucuronic acid to ferric ion molar ratio was found to
be approximately 2:1, causing the EPS to have a net positive charge therefore allowing the bacteria to
attach to negatively charged pyrite via electrostatic interaction [42,45,50]. There is no correlation
Molecules 2009, 14 2541
between the amounts of EPS produced with the resulting levels of bacterial metabolic activity.
However, a correlation was found between the total ferric ion content in the EPS and the level of
bacterial activity. A. ferrooxidans strains that exhibited higher levels of activity were found to possess
greater concentrations of ferric ions complexed within their EPS. Since the presence of ferric ions is a
prerequisite for attachment to pyrite, cells with more iron were found to adhere more rapidly (and in
higher numbers) than cells possessing lower iron concentrations . Cells grown on pyrite also
produce greater levels of EPS than cells grown on sulfur and ferrous sulfate, and cells lacking in EPS
were found not to attach to, or oxidize, pyrite [50,63]. Furthermore, A. ferrooxidans containing higher
concentrations of complexed ferric ions within their exopolymers showed stronger electrostatic
interactions with pyrite than cells with lower concentrations of ferric ions.
7.2. A. ferrooxidans EPS produced on sulfur
The EPS produced by A. ferrooxidans grown on sulfur were found to contain a higher concentration
of lipids, free fatty acids and phosphorous, but a lower concentration of sugars (only glucose and small
traces of glucuronic acid) and virtually no complexed ferric ions or other positively charged groups
compared to that grown on pyrite [45,52]. The cells grown on this substrate displayed hydrophobic
surface properties and did not attach to charged particles, most likely due to the lack of ferric ion
complexes in the EPS .
7.3. A. ferrooxidans EPS produced on ferrous sulfate
The composition of the EPS produced by A. ferrooxidans grown on ferrous sulfate was very similar
to that resulting from growth on pyrite [45,50,52]. However, a smaller amount of EPS was produced
on the ferrous sulfate because of the observation that cells grown using soluble substrates produce only
small amounts of EPS compared to cells grown on solid substrates .
8. Extracellular polysaccharides produced by pathogenic microorganisms
Pathogenic microorganisms associated with biofilms are the focus of intensive research due to their
involvement in a large number of chronic infectious diseases [1,5,13]. Biofilm formation is believed
play an important role in infection immunity and protection toward antimicrobial agents [5,64]. For
example, the gram-positive Staphylococcus epidermidis and the gram-negative P. aeruginosa are the
most prevalent pathogens involved in clinical chronic infections [13,64]. Their growth and
proliferation within a biofilm provides protection from antibiotics and provides them a host defense
mechanism by slowing down or preventing penetration of different agents through the biofilm [5,13].
Other biofilm-associated pathogenic bacteria include the generas Escherichia, Legionella,
Staphylococcus, Streptococcus and Vibrio [5,13,30].
9. Extracellular polysaccharides employed in biotechnology applications
Some of the well studied, commercially used polysaccharides are produced by taxonomically
diverse bacteria [34,35,65-68]. These polysaccharides, together with their biotechnological
applications, are given as follows:
Molecules 2009, 14 2542
9.1. Extracellular polysaccharides produced by gram-positive bacteria, Firmicutes
The phylum Firmicutes has been recognised since 1978, as has its further taxonomic
rearrangements [69-71]. The bacteria from the orders Lactobacillales, Leuconostocaceae and
Streptococcaceae are producers of polysaccharides that are employed in a range of different
commercial applications [35,72,73].
Dextran is a high molecular weight homopolysaccharide of glucose containing numerous
consecutive α-(1,6)-linkages in its backbone . The α-D-glucans have side chains with
α-(1,6)-linkages, which mainly stem from α-(1,3)- and sporadically from α-(1,4)- or α-(1,2)-linkages
(Figure 1). The exact structure of each type of dextran depends on the microbial strain. A majority of
dextrans are produced from sucrose by dextransucrase enzymes, synthesized and secreted largely by
Leuconostoc, Streptococcus and Lactobacillus species [35,72]. Commercially produced dextran is
produced by L. mesenteroides and L. dextranicum . The product is a gel that is widely used as a
molecular sieve for purification and separation of macromolecules such as proteins, nucleic acids and
polysaccharides . Dextran is also used in clinical research and medical applications since it can be
safely consumed .
Figure 1. The structure of dextran with branching at C3 .
A product that is employed in the dairy industry is kefiran, a capsular polysaccharide produced by
Lactobacillus species including L. rhamnosus, L. kefir and L. kefiranofasciens, which is found in kefir
grains [73,75]. This polysaccharide is considered safe since kefir has been traditionally consumed. Its
Molecules 2009, 14 2543
has also been found to have antibacterial, antifungal and antitumor activity [35,75]. Kefiran is a clear
or yellow polysaccharide gel excreted by kefir grains. It is a water-soluble branched glucogalactan
with similar ratios of D-glucose and D-galactose residues (Figure 2) . Kefiran is primarily used to
produce traditional self-carbonated, slightly alcoholic fermented milk  but can also enhance the
viscosity and viscoelasticity of acid milk gels  and serve to prevent or control come commonly
occurring diseases .
Figure 2. The structure of kefiran, the polysaccharide found in kefir grains .
9.2. Extracellular polysaccharides produced by gram-negative bacteria, Proteobacteria
The phylum Proteobacteria was established on the basis of phylogenetic analysis of 16S rRNA
gene sequences. The phylum contains five classes of gram-negative bacteria including
Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria . The polysaccharides
produced by these gram-negative bacteria possess a high degree of structural diversity, consisting of
either homopolysaccharides, where polymers are generally composed of D-glucose, or
heteropolysaccharides, where the repeating units range from disaccharides to octasaccharides with
unusual side chains .
This class of bacteria are comprised of a heterogeneous group of microorganisms . The main
polysaccharides produced include cellulose and gellan, which are primarily produced by bacteria of the
orders Rhodospirillales and Sphingomonadales, respectively.
Cellulose is primarily produced by bacteria of the genera Acetobacter, particularly A. xylinum,
however, bacteria belonging to the Agrobacterium, Pseudomonas and Rhizobium genera can also
produce cellulose. Cellulose possesses a simple structure consisting of the monosaccharide glucose
in 1→ 4-β-glycosidic links (Figure 3) . Bacterial cellulose can be produced as a highly pure
polymer, therefore it is used in specific applications to a greater extent than plant cellulose .
Cellulose can be utilized in food supplements such as a food matrix, dietary fibre or thickening or
suspending agents. The commercially available products Biofill®, Bioprocess® and Gengiflex® are
cellulose formulations that are used biomedicine for surgery and wound dressings and dental implants
Molecules 2009, 14 2544
Figure 3. The structure of bacterial cellulose .
Gellan is a multifunctional gelling agent that is produced by the non-pathogenic bacterium
Sphingomonas elodea ATCC 31461. It is a linear, anionic polysaccharide consisting of a
tetrasaccharide repeating unit comprising of two molecules of D-glucose, D-glucuronic acid and
L-rhamnose (Figure 4) . In its native form, gellan forms an elastic gel and in solution, it is able to
hold particles in suspension without significantly altering the viscosity of the solution . It also
shows thermal and acid stability, elasticity and rigidity, high transparency and good flavour release.
Gellan is commercially available as Gelrite, a substitute of agar, and Kelcogel® F and Kelcogel®
LT100, which are food-grade gellans. It is also used as an excipient for drug delivery applications .
Figure 4. The primary structure of gellan gum .
Bacteria of the genus Alcaligenes, which belong to the order Burkholderiales, are the producers of
two commercially available products – curdlan and welan . Curdlan is a low molecular weight,
water soluble linear polysaccharide made up of β-1,3-linked glucose residues (Figure 5) [35,81].
Figure 5. Chemical structure of curdlan .
This polysaccharide is produced mainly by Alcaligenes faecalis and also Agrobacterium. However,
commercially produced curdlan comes from a mutant strain of Alcaligenes faecalis var. myxogenes
. Curdlan is unique in its ability to form an elastic gel when its aqueous suspension is heated above
Molecules 2009, 14 2545
55 ºC , making it appealing for use in the food and pharmaceutical industries . It is useful in
improving the texture and stability of foods and can be used as a drug delivery polymer .
Welan is an anionic polysaccharide made up of D-glucose, D-glucuronic acid and L-rhammose units
with singular side chains containing either L-rhammose or L-mannose substituted on C3 of every
1,4-linked glucose repeating unit (Figure 6) . It is produced by the Alcaligenes species ATCC
31555 and has the same backbone repeating units as gellan gum but with different side chains .
Welan retains stability and viscosity at elevated temperatures making it ideal for industrial
applications, particularly in oil-well drilling and cement systems as a stabilizer and viscosifier [35,84].
Figure 6. The primary structure of welan gum .
R = CH3 or CH2OH
Commercially available polysaccharides produced by bacteria of this class include bacteria of the
orders Enterobacteriales (N-acetylheparosan), Pseudomonadales (alginate) and Xanthomonadales
Heparin is a highly sulfated, linear polysaccharide consisting of alternating D-glucosamine residues
1→ 4 linked to either L-iduronic acid or D-glucuronic acid (Figure 7) . This glycosaminoglycan
(GAG) is primarily extracted from porcine intestinal tissues or bovine lung tissue and is widely used as
a clinical anticoagulant. However, these mast cells may also nest bacteria or viruses, particularly in
bovine heparin, thus limiting the uses of animal heparin . The biosynthesis of heparin requires the
formation of non-sulfated polysaccharide chains covalently bound to a protein core, followed by
modifications of the polymers . Although microorganisms do not produce heparin , the
polysaccharide produced by E. coli was found to have the same structure, as the non-sulfated precursor
polysaccharide in heparin biosynthesis [66,87]. The alternative use of bacterial polysaccharides as
sources of heparin synthesis may eliminate concerns associated with mammalian sources of heparin,
and also be useful in medical research .
Alginate is produced by Pseudomonas species and Azotobacter chroococcum and A. vinelandii. It is
a linear copolymer comprised of 1-4-linked β-D-mannuronic acid (M) and C-5-epimer α-L-guluronic
acid (G) (Figure 8) . These residues can be arranged in different blocks such as homopolymeric
blocks of consecutive G-residues (G-blocks) or M-residues (M-blocks), alternating G and M residues
(MG-blocks) or randomly organized blocks. The content of G-blocks stabilizes and improves the
gelling characteristics of alginate . Alginate is commercially used in different industrial
Molecules 2009, 14 2546
applications as viscosifiers, stabilizers, gel-forming, film-forming or water-binding agents. It is also
used in the pharmaceutical industry as a wound dressings, dental materials and for encapsulation of
cells and enzymes for slow release [35,67].
Figure 7. The structure of: (a) the major disaccharide sequence, (b) the minor disaccharide
sequence of heparin and (c) the polysaccharide produced by E. coli .
α-L-IdoAp2S(1-4)-α-D-GlcNpS6S(1-4) X = SO3- or H, Y = SO3-, CH3CO or H
Figure 8. The structure of (a) β-D-mannuronic acid, (b) α-L-guluronic acid and (c) alginate .
Xanthan is a well studied polysaccharide produced by most strains of Xanthomonas campestris. It is
a high molecular weight, branched anionic heteropolysaccharide. The main chain is composed of
glucose units while the side chain is a trisaccharide, made up of α- D-mannose with an acetyl group,
Molecules 2009, 14 2547
β-D-glucuronic acid and a terminal β-D-mannose unit linked to a pyruvate group (Figure 9) . The
trisaccharide side chains align with the backbone via non-covalent interactions to stabilize the structure
. Due to its highly pseudoplastic and suspending properties, xanthan is commonly used as a
suspending agent and an emulsion stabilizer, primarily in food applications but is also used in
cosmetic, pharmaceutical and industrial formulations .
Figure 9. The structure of xanthan .
10. Novel techniques in the study of biofilm-specific polysaccharides
A detailed exploration of biofilm matrices, including studies of their thickness, growth and
detachment, EPS chemical composition and distribution remains limited due to the lack of sufficiently
sensitive analytical techniques . Recent progress in the surface and structure analysis of biofilms
has resulted from the development of advanced microscopy and spectroscopy techniques, atomic force
microscopy (AFM), confocal laser scanning microscopy (CLSM), infrared spectroscopy, nuclear
magnetic resonance imaging (NMRI), Raman spectroscopy (RM) and scanning electron microscopy
(SEM) [89, 90]. However, combinations of standard methods and the development of novel techniques
to study and characterize different biofilms would be useful in many biomedical and industrial
research applications [89,91,92].
CLSM is a standard tool used for biofilm analysis. This is a well understood technique, which
provides information on the 3D structure of biofilms and aids in the identification and distribution of
the different components using fluorescent stains . However, along with the prerequisite of
staining the sample, which can have low specificity, CLSM has spatial resolution limitations, thus
analyzing the distribution of nanometer-sized biopolymers within the EPS matrix is not possible .
When used in conjunction with RM, which gives a deeper insight into the composition and structural
information of EPS , the requirement for staining is avoided and information on the EPS
components can be acquired in situ in a non-destructive way. EPS constituents such as polysaccharides
and proteins can be chemically classified, and changes in the chemical composition of the aging
biofilm matrix, not detectable by CLSM, can be revealed .
Molecules 2009, 14 2548
Although CLSM is the typical method used for investigation of biofilms, it it a technique that only
gives information on the distribution and amount of stainable EPS components . On the other
hand, AFM is a powerful technique that is capable of imaging the surface morphology under hydrated
conditions and in aqueous solutions . This technique requires minimal sample preparation and
creates 3D images with nanometer or sub-nanometer resolutions . Since this technique does not
require samples to be coated or stained, imaging can be carried out on surfaces in their native state and
under physiological conditions. The use of AFM allows visualization of the formation of biofilms and
EPS distribution and cells within it . The combination of CLSM and AFM provides an enhanced
insight into biofilms with high-resolution images, whereby CLSM can be used for fluorescence
imaging and AFM can give a more detailed image of selected sections of the sample .
Recently, there has been increasing interest in the application of AFM and RM spectroscopy on the
formation of biofilms . The combination of these two powerful techniques allows high resolution
visualization of the biofilm and analysis of its components . This allows an analysis of the biofilms
and determination of the presence of macromolecules at different stages growth, and hence can provide
valuable information regarding bacterial attachment to different surfaces .
Optical coherence tomography (OCT) is an emerging, high-resolution medical and biological
imaging technology that is able to achieve optical ranging of biological and non-biological structures,
similar to that obtained using ultrasound analysis. Since OCT uses short wavelengths of near-infrared
light rather than sound, the resolution of the image can be up to 100 times higher than that of standard
imaging techniques . OCT involves a depth-resolved analysis of backscattered light using an
interferometer that can generate a 2D or 3D image. When this technology is applied to study a biofilm
structure, a 3D image of the biofilm developed is reconstructed and a more detailed visualization of
structure within the biofilm can be acquired through 2D sections of 3D images at different planes.
Although the data obtained can be similar to data collected using other techniques, additional
information can be provided using OCT showing that this technology can be a useful imaging tool for
characterization of biofilm formation under different conditions [92,96].
Sum-frequency-generation (SFG) spectroscopy is a surface-specific non-linear optical technique
used as a complementary technique to characterize biological molecules at interfaces [91,97,98]. This
technique has attracted much attention due to its high surface specificity and broad applications. SFG
has been successfully applied in areas of surface science including chemistry, biology and materials
science, for example, interfaces of pure liquids and electrochemical processes . Howell et al. used
SFG spectroscopy to study the extracellular matrix under a layer of cells attached to a solid . There
were no differences found in the SFG spectra between samples with cells and samples without cells
and tests carried out with live cells were not visibly affected by irradiation with SFG laser pulses.
These results indicated that the SFG spectroscopy is able to probe the layer between cells and a solid
substrate thus this technique could potentially be applied to in vitro studies of extracellular matrix
under live cells.
Although microorganisms predominantly exist as multi-cellular communities within biofilms in
most environments, scientists are still exploring these complex systems in order to understand the
Molecules 2009, 14 2549
complexity of the interactions within the biofilms, and also their function in bacterial attachment and
proliferation. Intense research of many different polysaccharides produced by a diverse range of
bacteria has been commercially applied in food and biomedical fields. However, little is known about
the role of EPS and biofilms in bioleaching applications. In order to improve the understanding of
biofilm systems, the development of sensitive analytical techniques is required. Significant advances
have been made to reveal new insights into biofilms and their constituents. The expansion of
knowledge in relation to molecular mechanisms involved in bacterial-mineral attachment may be
relevant in the enhancement of bioleaching timing and efficiency.
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