Application: Biofilms

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Microogranisms have a profound effect on our supplies of drinking water: some microbes can contaminate drinking water supplies, while other can be used to clean up pollutants. To understand how these processes occur, scientists are investigating the thermodynamics of microbial surfaces.

Under the traditional and extended theory of Dejiaguin, Landau, Verwey, and Overbeek (DLVO), the surface tension is composed of three components:

  • apolar, or Lifshitx-van der Waals (LW) component: This is caused by the asymmetry of the electron cloud surrounding the bacteria.
  • polar, or Lewis acid-base (AB) component: This is due the possibility of covalent bonds (Lewis Acids, electron pair acceptors and Lewis bases, or electron pair donors).
  • electrostatic (EL) component: This is commonly measured by the zeta potential.

Each of these components can be influenced by numerous factors in the microbial environment:

  • nutrient concentrations
  • scaffold-like structures outside of the cell
  • growth rates
  • physiological state (e.g. swimming freely, producing scaffold, or sending out spores).

The surface chemistry has a major effect on whether the bacteria are: (1) hydrophillic or hydrophobic, (2) bound to a porous medium, or (3) embedded in a biofilm.

As a starting point for studying the surface properties, researchers can use contact angle (<math>\theta</math>) measurements. These are related to the thermodynamic properties through the Oss-Chaundhury-Good equation:

<math>\big(1 + \cos(\theta)\big) \gamma_L = 2 \big(\sqrt{\gamma_S^{LW} \gamma_L^{LW}} + \sqrt{\gamma_S^+ \gamma_L^-} + \sqrt{\gamma_S^+ \gamma_L^-}\big)</math>

where S refers to the solid and L to the liquid. The symbols are defined as:

  • <math>\gamma_L</math> = liquid surface tension
  • <math>\gamma^{LW}</math> = Lifshitz-van der Waals component of surace tension
  • <math>\gamma^+</math> = electron-acceptor parameter of Lewis acid-base component of surface tension
  • <math>\gamma^-</math> = electron-donor parameter or Lewis acid-base component of surface tension

To determine all these parameters three contact angle measurements with three different liquids are needed. These can be done by: (1) depositing cells on a flat surface, (2) observing the growth of a colony of cells on a nutrient medium, or (3) using suction to hold a suspension of cells against a flat filter.

For more information, see: Keith A. Strevett, Gang Chen, Microbial surface thermodynamics and applications, Research in Microbiology, Volume 154, Issue 5, June 2003, Pages 329-335