Bacterial extracellular polymeric substances: Biosynthesis and interaction with environmental pollutants (2023)

Bacteria exhibit either planktonic (free-living) or sessile (surface-attached) modes of growth. Surface-attached mode of growth develops into structured communities of bacteria encased within a self-produced extracellular polymeric substances (EPS) matrix called biofilm (Rumbaugh and Sauer, 2020). EPS are high molecular weight polymers produced by bacteria, mainly composed of polysaccharides, proteins, lipids, and DNA. These polymers are the most abundant component of the biofilm that provide viscoelasticity, cohesion, protection, and nutrition to the bacteria. EPS is referred to as the “dark matter of biofilm” since only the polysaccharides component of the EPS is well studied. In contrast, other EPS molecules are less explored and cannot be characterized as either carbohydrates, proteins, or lipids (Flemming, 2016; Decho and Gutierrez, 2017). Polysaccharides are present in more amounts and are widely studied components of EPS that vary immensely in structure and function. Molecular weight of EPS ranges from 10 to 1000kDa; however, the chemical nature of EPS varies depending on the species-specific monosaccharides and non-carbohydrate substituents (Jiao et al., 2010; Nwodo et al., 2012). The difference in structure is based on the composition of monosaccharides, chain length, and branching. Extracellular polysaccharides produced by most bacteria range from linear homopolymers (e.g., dextran, curdlan, cellulose) to branched heteropolysaccharides containing three or more monosaccharides (e.g., alginate, xanthan, gellan, emulsan) arranged in a group to form repeating units (Lembre et al., 2012; Khan et al., 2022).

Structural and functional diversity of bacterial extracellular polysaccharides mainly depends on the organization of extracellular polysaccharide genes. Extracellular polysaccharides operon comprises an organized set of genes that regulate EPS synthesis, determine chain length, formation of repeat units, and polymerization and transport of the repeating units. Enzymes for synthesizing nucleotide sugar precursor and nucleotide sugar precursor molecules are crucial for the biosynthesis of extracellular polysaccharides. It has been observed that the over-expression of genes involved in extracellular polysaccharides assembly caused an enhanced yield of EPS (Vandana and Das, 2022).

Extracellular polysaccharides biosynthesis is a multistage process where extracellular polysaccharides are intracellularly synthesized and exported outside. The biosynthesis process starts with the forming of precursor sugar nucleotide (UDP-galactose, UDP-glucose, and dTDP-rhamnose) that forms repeating units. Modification of the extracellular polysaccharides is facilitated with the action of hydrolases and transferases by adding pyruvate, glycerate, succinate, or acetate. Biosynthesis and transport into the extracellular environment are facilitated by three pathways, Wzx/Wzy-dependent pathway, ABC transporter-dependent pathway, and the synthase-dependent pathway (Schmid et al., 2015). Extracellular polysaccharide polymers contain charge, apolar groups such as carboxyl, hydroxyl, sulphydryl, sulfate, phosphate, and hydrophobic regions. In addition, the protein components contain carbonyl and amino groups, while eDNA only contains hydrophobic and phosphate regions. These functional groups play a diverse role, such as nutrient acquisition and bioremediation of toxic pollutants (Wiśniowska and Kowalczyk, 2022).

EPS has a distinct sorption mechanism and binding sites (cationic and anionic), which help remove inorganic and organic pollutants. Bacterial EPS is involved in processes such as bioaccumulation and biosorption of inorganic compounds and degradation of organic compounds to reduce them to CO₂, H₂O, and CH₄ with the help of enzyme activity (Mohapatra et al., 2020). Anionic functional groups of EPS form organometal complexes with metal ions by electrostatic interaction Priyadarshanee and Das, 2021a. In addition, hydrophobic functional groups in EPS bind with the organic pollutants by hydrophobic-hydrophobic interaction, which helps accumulate organic pollutants. Thus, bacterial biofilm-EPS-mediated bioremediation is a cost-effective, sustainable, and eco-friendly technique for restoring the contaminated environment.

Although the properties and biosynthesis mechanism of the extracellular polysaccharides component of EPS have been extensively reviewed, the role of EPS components in the interaction with environmental pollutants has not been reviewed yet. This review summarizes the properties and characterization of EPS based on various analytical techniques. Biosynthesis of extracellular polysaccharides through central carbon metabolic intermediates has been discussed elaborately. Further, the interaction mechanism of EPS matrix components with toxic pollutants has been discussed. In addition, the bioremediation of environmental pollutants by extracellular polymers has been reviewed comprehensively.