Adhesion of Bacteria

The colonization of microorganisms in various niches is dependent on their ability to adhere to surfaces and substratum. Adhesion or adherence is defined as the measurable union between a bacterium and substratum. A bacterium is considered to have adhered to a substratum when energy is required to separate the bacterium from the substratum (22).

Adhesion of a bacterium to a substratum, its colonization and finally possible invasion of the tissue is a multi-step process. It usually involves two or more kinetic steps. Firstly, the bacterium approaches the substratum via long distance interactions, such as van der Waals forces and electrostatic forces and becomes loosely attached (22). Complementary adhesion-receptor interaction leads to the formation of a bacterium-cell complex:

Bacteria + Intestinal cell % Bacterium — Intestinal cell complex (1)

where kj and k—j are dissociation constants for the above reaction. At equilibrium, the concentration of the adhered bacteria (ex) can be expressed as:

where em is the maximum value of ex at saturated bacterial concentration (23). The value of em is equivalent to the concentration of adhesion sites on the mucosal surface and x is the concentration of bacterial cells present around the adhesion site. The dissociation constant, kx determines the affinity the bacterial cells have for the adhesion sites on the mucosal surfaces. Thus, the adhesion of a bacterium to the substratum is determined by two major properties: the concentration of the bacterium in the vicinity of the cell receptor (x in the above equation) and the affinity of the bacterium for the receptor (kx in the equation).

Bacterial adhesion is crucial for invasive pathogenic microbes and may be important for certain commensals, prior to colonization of the intestinal mucosa. The receptors for bacterial adhesins are found in three groups of membrane consitituents: integral, peripheral and cell surface coat components. These receptors are chemically proteins, glycoproteins or glycolipids. They fulfill the criteria of a biological receptor because they exhibit specific binding followed by physiologically relevant responses. An example would be membrane-associated fibronectin acting as a receptor molecule for streptococci (22).

Bacterial adhesion to substrata receptors could involve the specific adhesin-receptor interaction and non-specific interactions. The specific adhesion is defined as the association between the bacteria and substratum that requires rigid stereochemical constraints (22). Many bacteria have the ability to produce lectins (24), carbohydrate-specific proteins, which are usually expressed on the bacterial surfaces. Lectins are a subset of adhesins that recognize and bind to a defined carbohydrate sequence present on host glycoproteins. Previous studies reported that there were three main types of adhesin-receptor interactions. The first type was based on the carbohydrate-lectin recognition, the second kind involved protein-protein interaction and the third class, which is the least characterized, involved the binding interactions between hydrophobic moieties of proteins and lipids (25). A well-established example is the type 1 fimbriae (carrying adhesins) of E. coli which recognize D-mannose as the receptor site on the host mucosal surface (26). Binding of some Lactobacillus to human colonic cells is a mannose-specific adherence mechanism (27,28). Their similarity in binding specificity may contribute to competitive exclusion of enteropathogens by some strains of probiotic lactic acid bacteria. Lactic acid bacteria have been shown to exclude enteropathogens from the mucosal surface in in vitro studies (29-32).

On the other hand, the non-specific adhesion is also an association between a bacterium and substratum that may involve the same forces involved in the specific adhesion. However, in non-specific adhesion, a precise stereochemical fit is not necessary. Non-specific interaction comprises the physiochemical forces such as van der Waals, electrostatic forces (33), hydrogen bonding (34), and hydrophobic interactions (35).

The synthesis of adhesins can be switched on and off by the bacteria, depending on the environmental conditions, a process called phase variation (36). Phase variation has been demonstrated in Gram-negative bacteria. However, the environmental regulation of adhesin expression is likely to be present in some commensal and lactic acid bacteria also, since bacteria that are unable to regulate their adhesin expression are often inefficient colonizers (37,38). It has been suggested that the mucosal adhesive properties of the lactic acid bacteria is strain and host dependent, and the mucosal binding of human lactic acid bacteria are strain- and host specific (39,40). The adhesion and colonization of bifidobacteria have been suggested to be disease (allergy, cancer) dependent (41,42). The adhesion to the intestinal mucus of the fecal bifidobacteria from healthy infants was significantly higher than for allergic infants, suggesting a correlation between allergic disease and the composition of the bifidobacteria (41). Surprisingly, bifidobacteria, amongst other bacteria, were generally positively associated with increased risk of colon cancer in a study involving native Japanese and African patients (42). The ability of intestinal bacteria to persist on the intestinal mucosal surface may ultimately be determined by their doubling time in the intestine to maintain a high local concentration. Slowly-dividing bacteria would be expected to be out-competed or washed-out with the intestinal contents (43).

Pregnancy And Childbirth

Pregnancy And Childbirth

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