Extracellular polymeric substance

Extracellular polymeric substances (EPSs) are natural polymers of high molecular weight secreted by microorganisms into their environment.[1] EPSs establish the functional and structural integrity of biofilms, and are considered the fundamental component that determines the physicochemical properties of a biofilm.[2]

Extracellular polymeric substance matrix formation in a biofilm

EPSs are mostly composed of polysaccharides (exopolysaccharides) and proteins, but include other macromolecules such as DNA, lipids and humic substances. EPSs are the construction material of bacterial settlements and either remain attached to the cell's outer surface, or are secreted into its growth medium. These compounds are important in biofilm formation and cells' attachment to surfaces. EPSs constitute 50% to 90% of a biofilm's total organic matter.[2][3][4]

Exopolysaccharides (also sometimes abbreviated EPSs; EPS sugars thereafter) are the sugar-based parts of EPSs. Microorganisms synthesize a wide spectrum of multifunctional polysaccharides including intracellular polysaccharides, structural polysaccharides and extracellular polysaccharides or exopolysaccharides. Exopolysaccharides generally consist of monosaccharides and some non-carbohydrate substituents (such as acetate, pyruvate, succinate, and phosphate). Owing to the wide diversity in composition, exopolysaccharides have found diverse applications in various food and pharmaceutical industries. Many microbial EPS sugars provide properties that are almost identical to the gums currently in use. With innovative approaches, efforts are underway to supersede the traditionally used plant and algal gums by their microbial counterparts. Moreover, considerable progress has been made in discovering and developing new microbial EPS sugars that possess novel industrial applications.[5] Levan produced by Pantoea agglomerans ZMR7 was reported to decrease the viability of rhabdomyosarcoma (RD) and breast cancer (MDA) cells compared with untreated cancer cells. In addition, it has high antiparasitic activity against the promastigote of Leishmania tropica.[6]

Function

Capsular exopolysaccharides can protect pathogenic bacteria against desiccation and predation, and contribute to their pathogenicity.[7] Sessile bacteria fixed and aggregated in biofilms are less vulnerable compared to drifting planktonic bacteria, as the EPS matrix is able to act as a protective diffusion barrier.[8] The physical and chemical characteristics of bacterial cells can be affected by EPS composition, influencing factors such as cellular recognition, aggregation, and adhesion in their natural environments.[8] Furthermore, the EPS layer acts as a nutrient trap, facilitating bacterial growth.[8]

The exopolysaccharides of some strains of lactic acid bacteria, e.g., Lactococcus lactis subsp. cremoris, contribute a gelatinous texture to fermented milk products (e.g., Viili), and these polysaccharides are also digestible.[9][10] An example of the industrial use of exopolysaccharides is the application of dextran in panettone and other breads in the bakery industry.[11]

Production of EPS followed by adhesion to the surface give rise to the formation of biofilm. compositional support as well as protection of microbial communities from the harsh environments are the major roles of matrix which compromises EPS.[12] One of the challenges of EPS-enriched biofilms is regarding its formation on implant surfaces which would contribute to microbial accumulation, cross-kingdom interaction, antimicrobial resistance, biofilm virulence, and, consequently, peri-implant tissue damage.[13] In 1960s and 1970s, light was shed on the presence of exopolysaccharides in the plaque associated with tooth decay.[14] In the field of paleomicrobiology, dental biofilms and their EPS components on provide the scientists with information about the composition of ancient microbial and host biomolecules as well as the diet.[15] Minerals in EPS were found to contribute to morphogenesis of bacteria (in Bacillus subtilis and Mycobacterium species), structural integrity of the matrix and also associate with medical conditions e.g. calcite generated by Pseudomonas aeruginosa, calcium and magnesium causing catheter encrustation in the biofilms of Proteus mirabilis, Proteus vulgaris, and Providencia rettgeri, presence of CaCO3 in the matrix of B. subtilis and Mycobacterium smegmatis biofilms.[16]

Ecology

Exopolysaccharides can facilitate the attachment of nitrogen-fixing bacteria to plant roots and soil particles, which mediates a symbiotic relationship.[7] This is important for colonization of roots and the rhizosphere, which is a key component of soil food webs and nutrient cycling in ecosystems. It also allows for successful invasion and infection of the host plant.[7]

Bacterial extracellular polymeric substances can aid in bioremediation of heavy metals as they have the capacity to adsorb metal cations, among other dissolved substances.[17] This can be useful in the treatment of wastewater systems, as biofilms are able to bind to and remove metals such as copper, lead, nickel, and cadmium.[17] The binding affinity and metal specificity of EPSs varies, depending on polymer composition as well as factors such as concentration and pH.[17]

In a geomicrobiological context, EPSs have been observed to affect precipitation of minerals, particularly carbonates.[18] EPS may also bind to and trap particles in biofilm suspensions, which can restrict dispersion and element cycling.[18] Sediment stability can be increased by EPS, as it influences cohesion, permeability, and erosion of the sediment.[18] There is evidence that the adhesion and metal-binding ability of EPS affects mineral leaching rates in both environmental and industrial contexts.[18] These interactions between EPS and the abiotic environment allow for EPS to have a large impact on biogeochemical cycling.

Predator-prey interactions between biofilms and bacterivores, such as the soil-dwelling nematode Caenorhabditis elegans, had been extensively studied. Via the production of sticky matrix and formation of aggregates, Yersinia pestis biofilms can prevent feeding by obstructing the mouth of C. elegans.[19] Moreover, Pseudomonas aeruginosa biofilms can impede the slithering motility of C. elegans, termed as 'quagmire phenotype', resulting in trapping of C. elegans within the biofilms and preventing the exploration of nematodes to feed on susceptible biofilms.[20] This significantly reduced the ability of predator to feed and reproduce, thereby promoting the survival of biofilms.

Novel industrial use

Due to the growing need to find a more efficient and environmentally friendly alternative to conventional waste removal methods, industries are paying more attention to the function of bacteria and their EPS sugars in bioremediation.[21]

Researchers found that adding EPS sugars from cyanobacteria to wastewaters removes heavy metals such as copper, cadmium and lead.[21] EPS sugars alone can physically interact with these heavy metals and take them in through biosorption.[21] The efficiency of removal can be optimized by treating the EPS sugars with different acids or bases before adding them to wastewater.[21] Some contaminated soils contain high levels of polycyclic aromatic hydrocarbons (PAHs); EPSs from the bacterium Zoogloea sp. and the fungus Aspergillus niger, are efficient at removing these toxic compounds.[22] EPSs contain enzymes such as oxidoreductase and hydrolase, which are capable of degrading PAHs.[22] The amount of PAH degradation depends on the concentration of EPSs added to the soil. This method proves to be low cost and highly efficient.[22]

In recent years, EPS sugars from marine bacteria have been found to speed up the cleanup of oil spills.[23] During the Deepwater Horizon oil spill in 2010, these EPS-producing bacteria were able to grow and multiply rapidly.[23] It was later found that their EPS sugars dissolved the oil and formed oil aggregates on the ocean surface, which sped up the cleaning process.[23] These oil aggregates also provided a valuable source of nutrients for other marine microbial communities. This let scientists modify and optimize the use of EPS sugars to clean up oil spills.[23]

List of Extracellular polymeric substances
Succinoglycan from Sinorhizobium meliloti
  • acetan (Acetobacter xylinum)
  • alginate (Azotobacter vinelandii)
  • cellulose (Acetobacter xylinum)
  • chitosan (Mucorales spp.)
  • curdlan (Alcaligenes faecalis var. myxogenes)
  • cyclosophorans (Agrobacterium spp., Rhizobium spp. and Xanthomonas spp.)
  • dextran (Leuconostoc mesenteroides, Leuconostoc dextranicum and Lactobacillus hilgardii)
  • emulsan (Acinetobacter calcoaceticus)
  • galactoglucopolysaccharides (Achromobacter spp., Agrobacterium radiobacter, Pseudomonas marginalis, Rhizobium spp. and Zooglea' spp.)
  • galactosaminogalactan (Aspergillus spp.)
  • gellan (Aureomonas elodea and Sphingomonas paucimobilis)
  • glucuronan (Sinorhizobium meliloti)
  • N-acetylglucosamine (Staphylococcus epidermidis)
  • N-acetyl-heparosan (Escherichia coli)
  • hyaluronic acid (Streptococcus equi)
  • indican (Beijerinckia indica)
  • kefiran (Lactobacillus hilgardii)
  • lentinan (Lentinus elodes)
  • levan (Alcaligenes viscosus, Zymomonas mobilis, Bacillus subtilis)
  • pullulan (Aureobasidium pullulans)
  • scleroglucan (Sclerotium rolfsii, Sclerotium delfinii and Sclerotium glucanicum)
  • schizophyllan (Schizophylum commune)
  • stewartan (Pantoea stewartii subsp. stewartii)
  • succinoglycan (Alcaligenes faecalis var. myxogenes, Sinorhizobium meliloti)
  • xanthan (Xanthomonas campestris)
  • welan (Alcaligenes spp.)

New Approaches to Target Biofilms

Application of Nanoparticles has shed new light on how to target biofilms due to their high surface-area-to-volume ratio, their ability to penetrate to the deeper layers of biofilms and the capacity to releasing antimicrobial agents in a controlled way. Studying NP-EPS interactions would provide deeper understanding to develop more effective NP.[12] Some factors that would alter the potentials of the NP to transport antimicrobial agents into the biofilm include physicochemical interactions of the NPs with EPS components, the characteristics of the water spaces (pores) within the EPS matrix and the EPS matrix viscosity.[24] Size and surface properties (charge and functional groups) of the NPs are, respectively, the major determinants of the penetration in and the interaction with the EPS.[12]

See also

References
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