Slime layer

Slime layers are a mixture of bacterial secretions called extracellular polymers, other metabolic products, bacteria, gases, detritus, and water. Commonly, 99% of the slime layer is water, although much silt and debris may also become entrapped in it. 

Passive corrosion caused by chemically inert substances is the same whether the substance is living or dead. The substance acts as an occluding medium, changes heat conduction, and/or influences flow. Concentration cell corrosion, increased corrosion reaction kinetics, and erosion-corrosion can he caused by biological masses whose metabolic processes do not materially influence corrosion processes. Among these masses are slime layers. 

Slime is a network of secreted strands (extracellular polymers) intermixed with bacteria, water, gases, and extraneous matter. Slime layers occlude surfaces—the biological mat tends to form on and stick to surfaces. Surface shielding is further accelerated by the gathering of dirt, silt, sand, and other materials into the layer. Slime layers produce a stagnant zone next to surfaces that retards convective oxygen transport and increases diffusion distances. These properties naturally promote oxygen concentration cell formation. 

Figure 6.2 Severely pitted aluminum heat exchanger tube. Pits were caused hy sulfate-reducing bacteria beneath a slime layer. The edge of the slime layer is just visible as a ragged border between the light-colored aluminum and the darker, uncoated metal below.

Wastage is caused by biological material accumulating on or near surfaces. Microbial mats such as slime layers and massive accumulations of larger organisms cause most damage. 

Figure 6.1A gelatinous slime layer on a heat exchanger tube sheet. 

Figure 6.13 A dried slime layer peeling off water box surfaces on a small heat exchanger.

Sliming is often more severe on inlet tube sheets than on outlets. Bacterial counts are usually quite high, exceeding tens of millions in most slime layers (Table 6.5). Sulfate-reducer counts are also usually high. In waters taken from such systems, bacteria counts are often several orders of magnitude lower. 

TABLE 6.5 Typical Microbiological Evaluation of Slime Layer in Fig. 6.13 and Typical Counts in Cooling Water Contacting Heavily Slimed Surfaces ... 

Figure 6.14 A generally corroded carbon steel surface after the slime layer was removed.

Any deposit can increase dealloying. Biological accumulations, such as slime layers, are no exception to this rule. In the age of increasing public concern regarding pollution, biological control via chemical treatment can be difficult. However, good biological control is always beneficial. 

Bacillus anthracis, the causative organism of anthrax, possesses a capsule composed of polyglutamic acid the slime layers produced by other organisms are of a carbohydrate nature. 

In the past, copper was believed to be toxic to most microbiological species. Although this may be true in a test tube under laboratory conditions, it is not generally true in the real world. In this real world, microbial communities excrete slime layers which tend to sequester the copper ions and prevent their contact with the actual microbial cells, Aus preventing the copper from killing the microbes. Many cases of MIC in copper and copper alloys have been documented, especially of heat-exchange tubes, potable water, and fire protection system piping. 

Measurements show some variation depending upon the staining solution used and the method of application. In dried and fixed smears, the cell wall and slime layer do not stain with weakly staining dyes such as methylene blue but do stain with the intensely staining pararosaniline, new fuchsin, crystal violet, and methyl violet. The great majority of bacteria have been measured in fixed and stained preparations. In some instances dried, negatively stained smears have been used. Therefore, the method employed should be specified when measurements of bacteria are reported otherwise the results will be of doubtful v alue. 

As has already been stated, Weibull reported the flagella of P. vulgaris to be composed of 98 per cent protein. This contradicts Pijper s statement that flagella are formed from the carbohydrate slime layer that is peeled off into a number of thin, wavy threads. 

Its chemical makeup is considered in Chapter 8. One of the layers is often referred to as the outer membrane. In some bacteria the wall may be as much as 80 nm thick and may be further surrounded by a thick capsule or glycocalyx (slime layer).13 The main function of the wall seems to be to prevent osmotic swelling and bursting of the bacterial cell when the surrounding medium is hypotonic. 

Other bacterial coats. Archaebacteria not only have unusual plasma membranes that contain phytanyl and diphytanyl groups (Section A,3)608 but also have special surface layers (S-Iayers) that may consist of many copies of a single protein that is anchored in the cell membrane.609 The surface protein of the hypothermic Staphylothermus marius consists of a complex structure formed from a tetramer of 92-kDa rods with an equal number of 85-kDa "arms."610 611 S-layers are often formed not only by archaebacteria but also by eubacteria of several types and with quite varied structures.612 14 While many bacteria carry adhesins on pili, in others these adhesive proteins are also components of surface layers.615 Additional sheaths, capsules, or slime layers, often composed of dextrans (Chapter 4) and other carbohydrates, surround some bacteria. 

Slime Layer Microcapsule Capsule Slime 5-500 nm Complex materials that vary in composition mainly polysaccharide but may contain signicant proteins. Responsible for antigenic properties of cells. 

In nature, S. aureus may occur as mucoid strains, in which a slime layer surrounds the cells. Such mucoid strains are less susceptible than nonmucoid strains to chloroxylenol (27), cetrimide (28) or chlorhexidine (13) although there is little difference in response with phenols or chlorinated phenols [111]. Removal of slime by washing renders cells as phenotypically sensitive to biocides as non-mucoid cells [111]. The protective nature of slime could be achieved by its acting as either (i) a physical barrier to biocide penetration, or (ii) a loose layer that interacts with, or absorbs, the biocide. 

Slime layer of organic polymers varies in thickness with eige of the cell and other environmental conditions stores food and binds food and other bacteria into food. 

The importance of biopolymer deposition on the membrane has been highlighted by Chu and Li [34] who noted that such material may accelerate bacterial attachment to the membrane in addition to its contribution to fouling resistance. The formation of a slime layer on the membrane, comprising an EPS matrix with embedded bacteria, may be analogous to a biofilm, the significance of which is the difficulty of its removal by nonchemical methods. The question of whether biofilm formation plays a significant role in MBRs is yet to be addressed. Indeed, a comprehensive model of fouling in MBRs is not yet available. 

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