Adhesion in bacteriological contexts refers to the ability of bacteria to stick to surfaces (e.g. tissues or foreign material i.e. prostheses). This ability can be more or less specific. Colonization of bacteria to a surface and the formation of a biofilm begins with adhesion to the surface. If the surface consists of an inert foreign material, the adhesion is usually non-specific.
For specific adhesion to take place, there must be a receptor on the surface and a ligand on the bacterium, which can bind to the surface. Bacteria use the so-called adhesins as ligands to attach to different receptors on cells in the host animal tissue. Adhesins are proteins or polysaccharides and their receptors can also be made of proteins or polysaccharides. Adhesins are important virulence factors because they contribute to the ability of bacteria to colonize different tissues. Many pathogenic bacteria can express a wide variety of different adhesins, which are expressed during different phases of the infection. Inhibition of one of these adhesins usually results in the bacterium losing its pathogenicity. Attempts to use anti-adhesin antibodies as vaccines have in some cases been successful in model systems.
A well-characterized bacterial adhesin is FimH, which is bound to pili and produced by uropathogenic Escherichia coli (UPEC). FimH binds to mannose and CD48 is a type of human immunoglobulin, which is glycosylated and contains mannose residues. TLR4 is a member of the so-called the toll-like- receptor family, which consists of human membrane proteins that are important for innate immunity. The image is adapted from Wikipedia, Bacterial adhesion. - Click on the image to enlarge it.
Adhesin-receptor systems in bacteria
Fimbriae (= common pili) often function as adhesins and sometimes a particular polypeptide, which is localized at the tip of the fimbria is the actual ligand. Membrane proteins may also act as adhesins.
Many bacteria produce extracellular polysaccharides, which constitute unspecific adhesion factors. The extracellular polysaccharide poly-N-acetylglucosamine (PNAG) works e.g. as an adhesion factor for some staphylococci and allows them to stick to certain abiotic (non-living) surfaces and form a biofilm there.