Adhesion zones

From the previous discussion, it is apparent that the cell surface of E. coli is a complex system, which under various experimental conditions reveals the outer membrane and its components to have localized regions with different properties than the rest of the cell envelope. The existence of differences within the outer membrane is supported by (1) the multiple areas at which cells rupture, (2) a widening of these areas after penicillin treatment, and (3) studies on LPS release after exposure to EDTA. The question may be posed whether the selective action of these treatments is a reflection of special zones in the other membrane only, or whether it reflects a more general mosaic structure of the envelope, including the inner membrane. 

In ultrathin sections of plasmolyzed E. coli, the separation of outer and inner membranes is incomplete, and there are numerous distinct areas at which the inner membrane remains attached to the outer membrane, whereas the major portion of the inner membrane is retracted from the more rigid outer membrane. This is best observed when cells are 

From ultrathin sections, it is possible to calculate that there are 200-400 adhesion zones per growing cell. The width of the adhesion areas usually measure between 25-50 nm, but occasionally, are much broader. The poles of the cells, as well as the prospective sites of cell division, have a tendency to form a concave area, which often stretches the adhesion areas excessively and leads to their rupture. 

In contrast to growing cells, adhesion zones are not seen in stationary cells, which instead show a few large concave retractions of the inner membrane from the wall. After spontaneous deplasmolysis, which in growing cultures occurs within 10-15 minutes, the membranes are seen in their usual relative positions, more or less in contact with each other. 

The space between the inner and outer membrane, which becomes visible during plasmolysis, may be correlated to what is operationally defined as the periplasmic space. This space appears to contain a number of degradative enzymes, as well as transport proteins that can be selectively released by sudden exposure of the cells to media of low osmotic strength prior to the osmotic shock, the cells are exposed to concentrations of sucrose comparable to those found to plasmolyze most of the cells in cultures of E. coli. A precise morphological localization of the periplasmic enzymes has proven to be difficult. For example, in Pseudomonas aeruginosa, Cheng et al found that alkaline phosphatase can be associated with either the inner or outer membrane, depending on the experimental conditions. 

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The completed exopolysaccharide is then transported to the cell surface through membrane adhesion zones. 

Fig. 1.3 Comparison of elastic Hertzian contact (left) and adhesive JKR contact (right), (a) Hertzian contact Dashed line (sphere) shape of contacting spherical lens prior to pressing to the flat surface by force L. Hertzian contact profile shown by solid line, with radius under external load L aH (b) JKR contact Schematic of adhesion force (adhesive zone model, forces schematically indicated by vectors) further deforming a spherical lens from Hertzian contact (solid line) to JKR contact (dotted line) with radius aJKR. Reproduced from [7] with permission copyright Springer Verlag...

Adhesion occurs for high values of km and low values of ro, as indicated by the wide area between the no- and firm-adhesion zones in Figure 62.3. As ro increases, km has to decrease in order for adhesion to take place. In the simulation (Figure 62.3), both association rate fcf and wall shear rate are kept constant. Varying fcf does not change the shape of the state diagram but shifts the location of the rolling envelope... 

The zone of reversible adhesion (Zone II) will depend not only on the thickness of the layer of surfactant adsorbed on the surface, but also on the coverage of the surface with this layer, i.e., which is arbitrarily taken as unity when the area of the adsorbed surfactant layer is near saturation. 

But in reality, failure occurs in most cases within the adhesion zone. This is the main reason that engineers have some doubts to use adhesives in structural applications. 

When bonding metal substrates, it is the deterioration or corrosion of the oxide layer which destroys the adhesion zone. With plastic substrates, effects like plasticizer migration or residues of release agents are often reasons for failure, whereas moisture plays a less important role. 

Figure 8.9 RFID label component adhesion zones...

Role of Adhesion Zones in Bacterial Cell-Surface Function and Biogenesis... 

Fig. 6. Ultrathin section through E. coli HfrH, revealing the F-pilus insertion at the membrane adhesion zone. The pilus is labeled with bacteriophage MS2. (60,000x)...

Fig. 7. Sites of F-pilus insertion at adhesion zones. Left A lysing cell. Right The pilus seems to interrupt the profile of the outer membrane. (200,000x)...

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