Previous studies have indicated that exopolymer alters the rheological properties of the local environment. Recently, scientists have used rheology to evaluate exopolymer gels produced by various microbes.
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When the growing conditions are favorable, many bacteria and planktonic algae produce exopolymer. In bacteria, exopolymer forms a capsule on cells’ outer surface, whose main function is to increase the effective diameter of a cell and also aid in better adherence of the bacterial cell onto a surface. In algae, it forms a local network, which either thickens or gels, the water.
Overview of Exopolymer Produced by Microbes
Some of the commonly found bacteria that produce exopolymer as a part of their growing regime are Pseudomonas, Arthrobacter, Bacillus, Anabaena, Nostoc, etc. When exopolymer is released from the cell, it may modify solid surfaces for better attachment of the cell. When the bacterial cells are not under favorable conditions, e.g., under nutrient-deficient conditions, it decreases the size, rounds up, and sheds the exopolymer capsule.
Previously, many researchers have studied rheological thickening on ocean turbulence. However, while conducting these studies, the contribution of exopolymers to the rheological properties was not well included. In the context of bacteria producing exopolymer, this compound promotes superior adherence of the cell on the surface and limits bacterial mobility, which clogs pores.
This leads to poor septic tank performance, reduction in groundwater infiltration in recharge basins, and clogging of systems related to in situ bioremediation process.
The chemical profile of exopolymer is diverse and it depends upon the nature of the bacteria or algae. These extracellular polymeric substances (EPS) are typically composed of proteins, polysaccharides, nucleic acids, lipids, and complex macromolecules.
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These molecules can develop a cross-linked, three-dimensional molecular network that can bind to a high amount of water. Therefore, biofilms or exopolymers are considered to be complex hydrogels that form a protective shield for the bacteria against chemical, biological, and mechanical hazards. It also prevents desiccation. Additionally, the EPS matrix decreases the effectiveness of biocides.
Rheological Properties of Exopolymer Gel
Previous research has shown that the EPS matrix provides the mechanical strength to the biofilm even after the bacteria responsible for its production are killed. Biofilms are found to have the ability to resist physical forces, such as shear stress from flowing water.
This mechanical strength of the EPS matrix is due to two main characters of exopolymers, i.e., the ability to adhere to the surface and the internal cohesion. Both these properties are closely linked to biofilms. Scientists have revealed that the cohesion of the biofilm is reflected via viscoelasticity and this occurs owing to crosslinked polymer networks.
Some of the physical interactions present in the crosslinks of the biofilms are hydrogen bonds, Vander Waals forces, electrostatic interactions, ion bridges, and entanglements.
Generally, alginate-like exopolymers (ALEs) provide structural integrity to sludge and biofilms. Therefore, recently scientists have used this as a model for EPS matrix to evaluate the mechanical properties of biofilm. ALEs form gels in the presence of calcium ions, which impart adhesion and strength to the biofilms. ALE model based on algal alginate helps elucidate the mechanical properties of biofilm EPS.
Previous studies have shown that the accumulation of calcium ions in the thin ALE layers contributes to their density. Recently, scientists have measured the exopolymer gels’ time-dependent viscoelastic properties with a rheometer. The swelling state of hydrogels was reported to affect the adhesion, permeability mechanical strength, and degradation of exopolymers.
Factors on which synaeresis (extraction of the liquid from the gel) depends are the viscoelastic properties, the pore size of the gel, and permeability. An increase of the salt concentration in the upper layer promotes synaeresis and this has been associated with the survival of biofilm during desiccation.
An Experimental Model to Evaluate Exopolymer Gels Using Rheology
Researchers have prepared an ALE gel model, which comprised equal amounts of ALE with varied amounts of calcium carbonate (CaCO3) and glucono-δ-lactone (GdL). GdL, on getting hydrolyzed in water, formed gluconic acid, which contributed to the final volatile suspended solid (VSS).
In this study, the gelling process occurred in a 100% humid environment. Scientists observed that the increase in the Ca2+: ALE ratio caused an increase in the VSS density. Additionally, the ALE network became compact with an increased concentration of CaCO3 owing to a higher number of crosslinks.
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This model showed that during gelation, water is expelled from the samples (synaeresis), which occurs due to contraction in the hydrogel network. Additionally, researchers found that synaeresis was less pronounced at low calcium ion concentrations.
This study found that if the ALE gels are kept undisturbed, their elastic characteristic remains the same. However, when biofilms are exposed to continual forces, prominent viscoelasticity was found with no ruptures. The flow property of the exopolymer gel was found to be slower when the calcium ion concentration was high.
This is because the presence of Ca2+ increased the density of the matrix with more crosslinks needing to be broken with increasing strain. Put simply, an increase of calcium content in the exopolymer gel caused an increase in water removal, which was correlated with the formation of stiffer and brittle gels.
Scientists believed that the ALE gel model is a great platform to study the mechanical properties of exopolymer gels. It could be used to develop methods for biofouling cleaning. In the future, this model could be extended by adding more components that are typically present in biofilms such as DNA.
References and Further Reading
Pfaff, N.M. et al. (2021). Rheological characterisation of alginate-like exopolymer gels crosslinked with calcium. Water Research. 207. 117835. https://www.sciencedirect.com/science/article/pii/S0043135421010290?via%3Dihub
Decho, W.A. and Gutierrez. (2017) Microbial Extracellular Polymeric Substances (EPSs) in Ocean Systems. Frontiers in Microbiology. 8:922. https://www.frontiersin.org/articles/10.3389/fmicb.2017.00922/full
Bhatnagar, M., et al. (2012) Rheology and composition of a multi-utility exopolymer from a desert borne cyanobacterium Anabaena variabilis . Journal of Applied Phycology. 24. pp.1387–1394. https://doi.org/10.1007/s10811-012-9791-7
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