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.