Microbial adhesion to biosurfaces

Adhesion and wetting in an aqueous environment: theoretical assessment of sensitivity to the solid surface energy

The sensitivity of contact angles in an aqueous environment to the surface energy of the solid is discussed. It is demonstrated that this sensitivity is much higher in an aqueous than in a vapor environment. The Girifalco-Good equation, in combination with the Owens-Wendt equation, is used for the approximate demonstrations. It is shown that the transition from complete wetting to complete dewetting by the aqueous phase in a solid-liquid-liquid system occurs over a much narrower range of the surface energy of the solid than in a solid-liquid-vapor system. It is also demonstrated that the contact angle may be extremely sensitive to small variations in the relationship between surface tensions and the corresponding interfacial tension.

An evaluation of the outer membrane charge and softness of Thiobacillus ferrooxidans by the Ohshima's electrophoretic model of a "soft" particle

The surface charge of bacterial cells plays an important role in their interfacial physiology and adhesion to substrata mediated by the electrostatic double-layer interaction. The surface charge or potential of biological cells is generally calculated from the experimentally measurable electrophoretic velocity of these cells migrating in an external electric field, applying the well-known Smoluchowski equation which is valid for "hard" particles with a sharp interface. However, bacterial cells possessing a structured outer membrane of a finite thickness (dependent on the ionic strength and pH of the surrounding liquid medium) are expected to obey Ohshima's electrophoretic mobility equation derived recently for "soft" particles. The electrophoretic mobility of Thiobacillus ferrooxidans was measured here by the fully automated technique of electrophoretic light scattering, based on the proportionality between the mobility and the Doppler shift in the frequency of light scattered by electrophoresing cells. Agreement was obtained between the experimentally determined electrophoretic mobility expressed as a function of low ionic strength (60-6000 mumol/L) at different pH values and the best-fit theoretical predictions of the "soft" particle electrophoresis theory, which is better than in the case of applying the Smoluchowski formula. The best-fit surface-charge and softness parameters predict a rather rigid and low-charge outer membrane of the bacterium examined, as compared to the parameters obtained for other bacteria in media of high ionic strength.