Microbial conditioning films

Microbial cells transported with the stream of fluid above the surface interact with conditioning films. Immediately after attachment, microorganisms initiate production of slimy adhesive substances, predominantly exopolysaccharides (EPS) that assist the formation of microcolonies and microbial films. EPS create bridges for microbial cells to the substratum and permit negatively charged bacteria to adhere to both negatively and positively charged surfaces. EPS may also control interfacial chemistry at the mineral/biofilm interface. 

Formation of a conditioning film of adsorbed macro-molecular organic components (i.e. proteins and other organic molecules) on the substratum surface prior to microbial deposition. 

The first step in biofilm formation prior to microbial adhesion is the irreversible adsorption of macromolecules, which leads to a conditioning film (humic substances, lipo-polysaccharides, microbial products). This conditioning film alters the effect of the membrane the electrostatic charge and the critical surface tension may change. 

FIGURE 20.1 Subsequent steps in biofllm formation (a) adsorption of organic (proteinaceous) molecules leading to a conditioning film (b) initial cell adhesion (c) anchoring of adhered cells through (polymeric) exudates and (d) microbial colonization of substratum surface. 

Low-molecular-weight surfactants and, especially, proteinaceous molecules usually have formed an adsorbed conditioning film on the substratum surface before the cells or microorganisms arrive. These low-molecular and polymeric surfactants may be excreted by the (microbial) cells themselves. 

The origin of the observed correlation was not established, and the relation was not interpreted as causal. It could be argued that a sustained elevated potential due to as-yet unknown microbial processes altered the passive film characteristics, as is known to occur for metals polarized at anodic potentials. If these conditions thickened the oxide film or decreased the dielectric constant to the point where passive film capacitance was on the order of double-layer capacitance (Cji), the series equivalent oxide would have begun to reflect the contribution from the oxide. In this scenario, decreased C would have appeared as a consequence of sustained elevated potential. 

If the activity of the immobilised catalyst is sufficiently high, the reaction which it mediates occurs essentially at the interface between the catalyst and the substrate solution. In the case of the surface immobilised enzyme or a thin microbial film this will, of course, occur irrespective of the level of activity. Under these conditions the limiting process for transporting substrate from the bulk of the solution to the immobilised enzyme is molecular or convective diffusion through the layer of solution immediate to the carrier. Under steady-state conditions, the rate of reaction at the active sites is equal to the rate at which substrate arrives at the site. This... 

The positive effects of humic substances often occur when the plants are grown in nutrient solution under anexic conditions in thin film isolators. This shows that the beneficial effects are due to the humic substances per se, rather than being mediated by microbial breakdown products. 

Biodegradability of PHB films under aerobic, microaerobic, and anaerobic conditions in the presence and absence of nitrate by microbial populations of soil, sludge from anaerobic and nitrifying/denitrifying reactors. 

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