Biofilm accumulation

Detachment includes two processes erosion and sloughing. Sloughing is a process in which large pieces of biofilm are rapidly removed, frequently exposing the surface. The causes are not well understood. Biofilm erosion is defined as continuous removal of single cells or small groups of cells from the biofilm surface and is related to shear stress at the biofilm/fluid interface. An increase in shear stress increases the erosion rate and decreases the biofilm accumulation rate. Empirical observations indicate that the erosion rate is related to biofilm thickness and density. 

Microsensors have been used to develop profiles in mixed species biofilms. Figure 10 shows concentration profiles of sulfide, oxygen, and pH in a biofilm accumulated on the surface of a mild steel corrosion coupon. The concentration of sulfide is highest near the metal surface, where iron sulfide forms quickly and covers the steel surface if both ferrous and sulfide ions are available. At low ferrous ion concentrations, adherent and temporarily protective films of iron sulfides are formed on the steel surface, with a consequent reduction in corrosion rate. High rates of SRB-induced corrosion of mild steel are maintained only in high concentrations of ferrous ion. 

It may also be difficult to distinguish between passive and active forms of biocorrosion, as they may occur at the same time biofilm accumulations on metal surfaces will naturally contain live organisms (including slime producers, sulfate reducers, and acid producers), as well as dead bacteria, excretion products, trapped particles, and perhaps oil and grease or other foulants. 

Escher A, Characklis WG (1990) Modeling the initial events in biofilm accumulation. In Characklis WG (ed) Biofilms. Wiley, New York,... 

Depending on the velocity of fluid flow, the thickness varies from 10 to 100 pm, and it may cover from less than 20% to more than 90% of the metal surface. Biofilms or macrofouling in seawater can cause redox reactions that initiate or accelerate corrosion. Biofilms accumulate ions, manganese and iron, in concentrations far above those in the surrounding bulk water. They can also act as a diffusion barrier. Finally, some bacteria are capable of being directly involved in the oxidation or reduction of metal ions, particularly iron and manganese. Such bacteria can shift the chemical equilibrium between Fe, Fe2+, and Fe3+, which often influences the corrosion rate. (Dexter)5... 

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 ... 

Fig. 5 The change in biofilm accumulation on the inside of a tube with three different nutrient concentrations expressed in terms of glucose concentration. (From Ref.. )...

Mott and Bott illustrated the effect of different materials on the accumulation of Pseudomonas fluor-escens biofilms on the inside of tubes under identical operating conditions (see Fig. 9). The differences between the effects of the materials occur for two reasons roughness and surface electrical properties. The quality of the surface, in terms of roughness, on which microorganisms attach, can affect the biofilm accumulation as discussed earlier. The effect of roughness is illustrated in Fig. 9 by the difference of biofilm accumulation between electropolished and as received 316 stainless steel. The rougher stainless steel is seen to be more hospitable to biofilm growth. 

In the light of these results, modification of the surface is a possible way of changing biofilm accumulation, i.e., to reduce roughness where the biofilm represents a nuisance or to use a rough surface where the biofilm performs a useful function. 

Mack D, Fischer W, Krokotsch A et al. (1996) The intercellular adhesin involved in biofilm accumulation of Staphylococcus epidermidis is a linear beta-l,6-hnked glucosaminoglycan purification and structural analysis. J Bacteriol 178 175-183... 

Use of Polysaccharide-Mediated Biofilm Accumulation Mechanisms in Prevention... 

The predominant mechanism of biofilm accumulation in staphylococci involves polysaccharide intercellular adhesin (PIA) [60-66], S. epidermidis strains lacking this adhesin are also regularly isolated from biomaterial-related infections, a fact which prompted a search for an alternative, PIA-independent accumulation mechanism [51, 52, 60-62, 67, 68], The responsible molecule was identified as accumulation associated protein, Aap [69-71], and there may also be a role for additional proteinaceous intercellular adhesins [52], Aap has a similar-acting homolog, SasG, in S. aureus [72], adding to the accumulating evidence that proteinaceous intercellular adhesins are also of importance in S. aureus biofilm formation and device-related infection [52, 72-74],... 

Aap is widespread in clinical isolates [51, 52, 148], and Aap-dependent biofilm formation can be directly demonstrated in clinical isolates [52, 70], so it seems likely that it is, like PIA, an important virulence factor. Aap has significant homology with cell wall protein SasG of S. aureus [149, 150], in which SasG-mediated biofilm accumulation has recently been demonstrated using the same mechanisms [72],... 

In an analysis of the near initial rate of biofilm accumulation Bryers and Characklis [1981] related the development of biofilm to the concept of a continuous stirred tank reactor. They took into account the effects of shear forces and associated removal of biofilm and assumed that the subsequent effects were dependent on the biofilm already on the surface, i.e. 

An important aspect of biofilm formation is the attachment to the surface (see Section 12.2.2) and in some respects, this might be considered the biofilm accumulation rate determining step (excluding reproduction and growth within the film). Fletcher [1977] modelled the adsorption of micro-organisms to a surface with the following assumptions ... 

Bryers, J.D. and Characklis, W.G., 1992, Biofilm accumulation and activity a process analysis approach, in Melo, L.F., Bott, T.R., Fletcher, M. and Capdeville, B. eds. Biofilms - Science and Technology. Kluwer Academic Publishers, Dordrecht, 221 - 250. 

The biofilm profiles suggest that progression of biofilm accumulation exhibits sigmoid behavior. Such progression can practically be divided into three zones (i) induction with biofilm thickness neighboring ca. 1 pm, (ii) exponential or log accumulation, and (iii) a plateau or steady-state zone. 

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