Bacterial adhesion involves the attachment (or deposition) of bacteria on the surface (solid, gel layer, etc.).
This interaction plays an important role in natural system as well as in environmental engineering.
[1] The low adhesion of bacteria to soil is essential key for the success of in-situ bioremediation in groundwater treatment.
[3] Controlling and preventing the adverse impact of the bacterial deposition on the aquatic environment need a deeply understanding about the mechanisms of this process.
DLVO theory has been used extensively to describe the deposition of bacteria in many current researches.
[1][2][3][4][5][6] DLVO theory describes the interaction potential between charged surfaces.
[2] DLVO theory is applied widely in explaining the aggregation and deposition of colloidal and nano particles such as Fullerene C60 in aquatic system.
[1][2][3][4] The prediction is based on sphere-plate interaction for one cell and the surface.
The electrostatic double layer interactions could be describes by the expression for the constant surface potential [2][3][4][6]
Where ε0is the vacuum permittivity, εr is the relative dielectric permittivity of water, ap is the equivalent spherical radius of the bacteria, κ is the inverse of Debye length, h is the separation distance between the bacterium and the collector surface; ψp and ψc are the surface potentials of the bacterial cell and the collector surface.
The retarded Van der Waals interaction potential was calculated using the expression from Gregory, 1981 .
With A is Hamaker constant for bacteria-water-surface collector (quartz) = 6.5 x 10−21 J and λ is the characteristic wavelength of the dielectric and could be assumed 100 nm, a is the equivalent radius of the bacteria, h is the separation distance from the surface collector to the bacteria.
Thus, the total interaction between bacteria and charged surface can be expressed as follow
Radial stagnant point flow (RSPF) system has currently been used for the experiment of bacterial adhesion with the verification of DLVO theory.
It is a well-characterized experimental system and is useful for visualizing the deposition of individual bacteria on the uniform charge, flat quartz surface.
[1][3] The deposition of bacteria on the surface was observed and estimated through an inverted microscope and recorded at regular intervals (10 s or 20 s) with a digital camera.
They are: All of the bacterial strains have negative zeta potential at experimental pH (5.5 and 5.8) and less become negative at higher ionic strength in both mono and divalent salt solutions.
[2][3][4][6][7] In some experiments, the surface collector was coated with an alginate layer with negative charge for simulating the real conditioning film in natural system.
[1][5] It was concluded that bacterial deposition mainly occurred in a secondary energy minimum by using DLVO theory.
[2] Therefore, the deposit could occur at secondary minimum having the energy from 0.09kT to 8.1kT at 1mM and 31.6 mM ionic strength, respectively.
[2] The conclusion was further proven by the partial release of deposited bacteria when the ionic strength decreased.
[4] This resulted in an observable bacterial deposition despite the very high electrostatic repulsive energy from the DLVO prediction.
The motility of bacteria also has a significant effect on the bacterial adhesion.
Nonmotile and motile bacteria showed different behavior in deposition experiments.
The swimming capacity increase with the ionic strength and 100mM is the optimal concentration for the rotation of flagella.
[7] Despite the electrostatic repulsion energy from DLVO calculation between the bacteria and surface collector, the deposition could occur due to other interactions such as the steric impact of the presence of flagella on the cell environment and the strong hydrophobicity of the cell.