Biofilms regions

In fact, significant substrate concentration gradients may exist for cells immobilised in biofilm. Cells located close to the nutrient supply are likely to maintain higher quality and activity compared with cells located relatively further away, leading to differentiation in the quality or activity of the immobilised cell population. This differentiation is more pronounced if there are starvation regions. In practice, zero substrate concentration may exist inside the biofilm, because in these regions the cell physiology may be markedly different from that of the freely suspended cells. 

The resident microbes within the mouth readily form biofilms on teeth. A biofilm consists of a population of bacteria coexisting in an orderly structure at the interface of a solid and a liquid [14] and, within a biofilm, bacteria living in colonies encapsulated in a matrix of extracellular polymer. Oral biofilms are known to vary extensively in structure throughout the colony, with regions of densely packed microorganisms surrounded by open water channels. Each type of bacteria exists in reasonably defined environments which are influenced by surrounding cells, distance from the outer surface and local structure, all of which influence availability of nutrients and ambient pH. 

For bacteria, there may be some adverse effects of living in a biofilm, especially in regions close to the substratum there, bacteria are to some extent cut off from the flux of DOM from the surrounding water. Additionally, bacterial enzymes can be inactivated if they are sorbed to surfaces. On the other hand, fixed positions of bacteria in biofilms support interactions between bacteria either within bacterial clones or between different bacterial species. They take advantage of growing in clones by saving energy spent for the production of extracellular enzymes, and they mutually interact between species in several ways. 

Matlock in Derbyshire (Pentecost, 1999). Another remarkable form, documented only from the mountainous regions of China are the river shoals (Figure 6.5). These consist of large shallow sheets of turbulent water between travertine-dammed lakes, beneath which the tufa builds evenly around a bryophyte and algal biofilm. 

As the biofilm develops, the nutrient availability to the bulk biofilm may become affected. The biofilm, despite its voids and channels, offers a further resistance to mass transfer. The cells within the biofilm consume nutrients that diffuse through the biofilm in response to the difference in concentration between nutrients at the biofilm surface and the cells attached to the conditioning layer. As a consequence, it is entirely possible that cells in the region of the solid surface are likely to become starved of nutrients. The properties of the biofilm may be different, therefore, in the layers where nutrient is available compared with the regions where there is little or no nutrient. For instance, the lack of oxygen may encourage anaerobic species to develop (some bacteria can exist as aerobes or anaerobes), with attendant changes to the quality of the biofilm. 

The change in biofilm accumulation on a surface with time, under flowing water conditions, would be expected to follow the idealized curve in Fig. 3. Three regions can be seen in the diagram ... 

Microorganisms and Effects on Solution Chemistry within Regions of the Biofilm (Ref72, 73)... 

Biofilm study was carried out in the drinking water supply system DWSS"Yovkovtsy , that supply with water the region of Veliko Tamovo, and in the laboratory model of water distribution system. Some experiments were performed 1) to determine microbiological composition of biofilm samples scraped from mild steel or reinforced concrete s main pipe line and concrete tank of DWSS Yovkovtsy 2) to study biofilm formation process on test pipe from PVC, PE, stainless steel and carbon steel in a laboratory model of drinking water distribution system under flow velocity 0,006 cm/s. 3) to study dynamics of biofilm formation process on polypropylene, the pipe material used during the last years in Bulgaria, in a model water distribution system under flow velocities 0.3 m/s, 0.5 m/s, 0.7 m/s and 1 m/s. 

Cracks in an oxide layer can give rise to localised attack of the metal as described in the previous section. Localised corrosion can also occur due to crevices at the metal surface produced during fabrication, e.g. under bolt or rivet heads or lap joints or may even be initiated by deposits such as biofilms or particulate matter. Under these conditions the crevice solution becomes more aggressive than the bulk solution. Rapid metal wastage in these regions is often referred to as crevice corrosion. Turner [1989] has outlined the problems associated with crevice corrosion. 

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