Abstract
Bacterial alginate-like exopolymers (ALE) gels have been used in this work as a model for the extracellular polymeric matrix of biofilms. Aim was to relate the mechanical properties and strength of this matrix that make biofilms as persistent to cleaning as they are, to the complex cohesive molecular interactions involved. Mechanical properties of the gels as a function of CaCO3 concentration were investigated using dynamic and static rheology. Gels with relatively low CaCO3 concentrations, between 100 μmol and 300 μmol per g ALE, were found to exhibit similar viscoelastic behaviour as real biofilms, with elastic moduli between 50 Pa and 100 Pa and dissipation factors between 0.2 and 0.3. Increasing CaCO3 concentrations resulted in an increase of the elastic modulus up to 250 Pa, accompanied by an increase in brittleness. At a CaCO3 concentration of 1250 μmol per g ALE this trend stopped, probably due to disturbance of the continuous ALE network by precipitation of salts. Therefore, overdosing of Ca salts can be an adequate approach for the removal of biofouling. All gels exhibited permanent strain hardening under medium strain, and their mechanical properties showed dependency on their strain history. Even after application of an oscillatory strain with 200% amplitude that caused the gel structure to collapse, the gels recovered 65 to 90% of their original shear modulus, for the major part within the first 20 s. Recovery was slightly less for gels with high CaCO3 concentration. In creep tests fitted with a Burgers model with multiple Kelvin elements at least three different interactions in the ALE gels could be distinguished with characteristic retardation times in the range of 10, 100 and 1000 s. Further identification of the mechanisms underlying the gel mechanics will allow the development of targeted strategies to undermine the mechanical strength of biofouling and aid the cleaning process.
Highlights
The complete chemical composition of biofilms is as diverse as the bacterial communities that inhabit them, and in most cases not easy to determine
In previous work (Pfaff et al, 2021), we have found that Ca2+ accumulates in thin alginate-like exopolymers (ALE) layers and influences their density
The ALE gels were prepared with equal amounts of ALE, but different amounts of CaCO3 and glucono-δ-lactone (GdL)
Summary
The complete chemical composition of biofilms is as diverse as the bacterial communities that inhabit them, and in most cases not easy to determine. Polysaccharides, proteins, lipids, nucleic acids and complex macromolecules with mixed functionalities have been identified in EPS (Seviour et al, 2019) These molecules form a physically crosslinked, 3D molecular network with the ability to bind high amounts of water. The EPS matrix, which can provide up to 90% of the biofilms’ organic matter, acts as a protective shield for the bacteria against biological, chemical and mechanical influences. Goal is to relate the mechanical properties and strength of the EPS matrix to the molecular interactions involved in the dynamic crosslinks. Occurrence and peculiarity of synaeresis and its corre lation with other characteristics was a side focus of this work
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