Adherence to Cell Surfaces
Adherence of bacteria to body surface is the most important event in the pathogenesis of disease. Once bacteria enter the body of the host, they must adhere to the cells of a tissue sur-face. If they do not adhere, they will be swept away by mucus and other fluids that bathe the tissue surface.
Most pathogenic microorganisms have more than a single mechanism of host cell attachment. Adherence is important not only during the initial encounter between the pathogen and its host but also throughout the infection cycle. Adherence requires participation of two factors: bacterial adhesins and a receptor on the host cell.
Bacterial adherence to the cell surface is mediated by special-ized molecules. The various molecules that mediate adherence to the cell surface are called adhesins. These adhesins allow the bacteria to adhere to the surface of human cell, thereby pro-moting their ability to cause disease. Microorganisms that lack this mechanism are nonpathogenic. Bacterial adhesins can be divided into two major groups: pili (fimbriae) and nonpilus adhesins (afimbrial adhesins).
Pili: These are the main mechanisms by which bacteria adhereto human cells. They are the fibers that extend from the bacterial surfaces and mediate attachment of bacteria to specific recep-tor on the host cells. The pili of many Gram-negative bacteria bind directly to sugar residues that are part of glycolipids or glycoproteins on the host cells. They also act as a protein scaf-fold to which another more specific adhesive protein is affixed.
The pili of the Gram-negative bacteria have been classified into five different types and are extremely important in the patho-genesis of infections caused by them. Further antigenic varia-tion in the actual structural pilin protein can be an important source of antigenic diversity for the pathogen.
Nonpilus adhesins: These include glycocalyx and otheradhesins present on the bacterial surfaces. Glycocalyx is a polysaccharide “slime layer” secreted by some strains of bac-teria that mediates strong adherence to certain structures, such as catheters, prosthetic implants, and heart valves. For example, the glycocalyx of Staphylococcus epidermidis and that of certain viridans streptococci allows the bacteria to adhere strongly to the endothelium of the heart valve. The matrix formed by these adhesins forming proteins is called a biofilm.
The biofilms are important in pathogenesis because they protect the bacteria by host defense and antibiotics.
The biofilms facilitate colonization of bacteria, especially of surgical appliances, such as artificial valve or indwelling catheters.
· Streptococcus pyogenes makes use of nonpilus adhesins (such aslipoteichoic acid, protein F, and M protein) to bind to epithe-lial cells. The lipoteichoic acid and protein F cause adherence of the streptococci to buccal epithelial cells. M protein acts as an antiphagocytic molecule.
· Recently, it has been shown that certain strains of E. coli and Shigella spp. have surface proteins calledcurli, which help inthe binding of bacteria to the host endothelium as well as to extracellular proteins.
Certain receptors are present on the host cells to which patho-gens adhere and initiate infections. For example, many adhe-sion proteins are present at the tip of the pili of E. coli. These bind specific receptors on the surface of the urinary bladder to initiate urinary tract infections. Similarly, the gonococci adhere to the microvilli of nonciliated cells and start disease process.