Bacterial adhesion

<p>As mentioned in the introduction of this thesis bacterial adhesion has been studied from a variety of (mostly practice oriented) starting points. This has resulted in a range of widely divergent approaches. In order to elucidate general principles in bacterial adhesion phenomena, we felt it was necessary to start from a fundamental level i.e. using welldefined model systems. In our study colloid chemical principles are applied to microbial systems. Although both colloid chemists and microbiologists have investigated the behaviour of small microscopic particles, there has been only limited cooperation between them in the past. Nevertheless, this study reveals that such a cooperation can be very fruitful.<p>After a general (Chapter 1) and a theoretical (Chapter 2) introduction, we deal in Chapters 3 and 4 with the relation between bacterial surface characteristics and adhesion to sulphated polystyrene (a hydrophobic, charged surface). The cell surface hydrophobicity and electrokinetic potential were determined by the contact angle measurement and electrophoresis, respectively. Adhesion increases with increasing bacterial hydrophobicity or decreasing electrokinetic potential. The effect of the electrokinetic potential increases with decreasing hydrophobicity. An interesting finding is the increase with growth rate in surface hydrophobicity of bacteria.<p>In Chapter 5 we show that initial adhesion to sulphated polystyrene is reversible and can at least qualitatively be described by the DLVO theory for colloidal stability, i.e., in terms of Van der Waals and electrostatic interactions. From adhesion isotherms we found an adhesion Gibbs energy of -2 to - 3 kT per cell. This corresponds to calculations using DLVO theory that predict adhesion in the so-called secondary minimum, a case where no direct intimate contact is made between bacterium and surface. Finally, the implications of our findings for natural and (bio)technical processes are discussed.<p>In Chapter 6 we report on the applicability of the DLVO theory for the interpretation of bacterial adhesion to glass and to more practical surfaces (Rhine river sediment and protein-coated surfaces). In all these cases adhesion could be interpreted in terms of the hydrophobicity and electrical properties of the surfaces.<p>The possible influences of adhesion on bacterial activity are discussed in Chapter 7, in the form of a critical literature review. Despite the opinion regularly heard that there might be a direct influence of adhesion on bacterial physiology we have not been able to find any experimental evidence in support of this hypothesis. Different activities of attached and free cells are often due to changes in substrate transport (e.g. diffusion, desorption, or convective transport) or differences in hydrophobicity of active and resting cells. For the conversion of adsorbed substrates the dissolved concentration determines the conversion rate. With strongly adsorbing compounds the conversion can become desorption-limited, whereas non-desorbing compounds are often not degraded.<p>In this thesis it is shown that application of colloid chemistry to microbial systems can lead to interesting new viewpoints. More specifically, the DLVO theory for colloidal stability was found to give a quantitative description of the initial stage of bacterial adhesion both to model surfaces as in more applied situations (Chapters 5 and 6). Generally, in the studies dealing with interaction between bacteria themselves or between bacteria and surfaces electrostatic interactions are often neglected, despite the fact that this interaction is often desicive whether strong adhesion can occur or not.<p>The insights derived from a colloid chemical approach can be used, as complementary to a more biological approach, in understanding the (auto-) immobilization of bacteria in natural and biotechnological systems, as e.g. in UASB- reactors.<p>The experimental methods developed in this study may also be successfully applicable in other research areas. Due to the sensitivity of the contact angle and electrophoretic mobility measurements they can for instance be applied as a rapid screening<br/>method of new isolates or cell surface mutants. Especially with urface mutants the methods mentioned here are much faster than conventional biochemical or immunological methods.<p>The contact angle and electrophoretic mobility measurements may also be useful for obtaining information on the structure of the outer part of the cell wall. In particular electrophoresis, at different pH and electrolyte strength, combined with chemical modifications of specific groups (e.g. -NH <sub>2</sub> groups) may be very powerful. Preliminary experiments with lipopolysaccharide mutants of Pseudomonads are very promising. For this and other applications it is necessary to improve the electrochemical characterization of bacteria, especially with respect to the influence of bacterial conductivity.<p>Other areas in microbiology that may be successfully treated by colloid chemical theories concern firstly the biological availability of substances, in particular micro-pollutants, to bacteria. This availability is mainly determined by substrate adsorption to inert solid material and substrate transport through the cell wall and membrane. A second interesting field might be the relation between molecular composition and function or stability of membranes in different bacteria, or under different environmental conditions