3.1.2 Extracellular polymers (EPS)
EPS is necessary for bacteria to form biofilms and is often used as the backbone of biofilms, playing an essential role in the stages of biofilm formation such as adhesion, proliferation, and microcolony aggregation. Once the bacteria have attached to the interface, several substances such as polysaccharide proteins secreted extracellularly make it easier for the bacteria to adhere to the interface. Then, bacterial aggregation begins to secrete more EPS, which increases the degree of adhesion and interfacial interaction between bacteria [46], gradually turning reversible into irreversible adhesion.
Ma et al. [46] showed that Psl polysaccharides promoted the adhesion and intercellular aggregation of P. aeruginosa PAO1 bacteria, resulting in a more dense biofilm morphological structure. EPS not only affect bacterial adhesion behavior and biofilm formation but also affects the viscoelasticity of biofilms. Firstly, Stoodley [47] and others demonstrated that biofilms are viscoelastic materials using a mathematical model to predict how they deform and fail to detach after shear stress is applied. Later, Dongen et al. [48] determined the viscoelasticity of Streptococcus mutans biofilms by a novel micro indentation device combined with confocal microscopy. By dragging the biofilm to break it, it was possible to find that the tensile strength of the biofilm was mainly dependent on the tensile strength of the EPS. Tsuneda et al. [49] examined cell surface characteristics such as ζ potential and hydrophobicity by a packed bed method, and when EPS was overexpressed, EPS dominated. The number of bacterial adhesions did not vary with the absolute value of ζ potential. In contrast, when EPS is expressed in small amounts, the EPS dominates, and the number of cell adhesions increases with a decrease in the absolute value of the cellular ζ potential.
Harimawan et al. [50] determined the composition of the EPS components produced during interfacial adhesion and found that the EPS layer on the surface of Bacillus subtilis contained more protein compounds such as γ-PGA and peptidoglycan. In contrast, the EPS layer on the surface of Pseudomonas aeruginosa had more polysaccharide compounds such as lipopolysaccharide, alginate, Pel, and Psl. AFM then probed the adhesion between these two bacteria and the substrate surface. The test results showed that the bonding betweenPseudomonas aeruginosa and the interface was more significant than that between Bacillus subtilis and the interface, further demonstrating the presence of polysaccharides in the EPS layer enhanced the adhesion strength.
By studying EPS (with EPS as the control group and without EPS as the experimental group) on E.coli JM109 interfacial adhesion, Zhang[51] found that EPS had no significant effect on bacteria-interface adhesion rate, adhesion number and other behaviors. However, after measuring the vibrational spring constant of the bacteria interface, it was found that EPS had a significant effect on the viscoelasticity of the bacteria interface, with the E.coli JM109 spring constant of the control group being about twice the spring constant of the E.coli JM109 of the experimental group, demonstrating that EPS promoted bacterial-interface adhesion and increased the bacterial-interface interaction force.