Certain bacteria have specialized structures (e.g., pili) or produce substances (e.g., capsules or glycocalyces) that allow them to adhere to the surface of human cells and enhance their ability to cause disease. These adherence mechanisms are essential for organisms that attach to mucous membranes; mutants that lack these mechanisms are often nonpathogenic. For example, the pili of Neisseria gonorrhoeae and E. coli mediate the attachment of the organisms to the urinary tract epithelium, and the glycocalyx of Staphylococcus epidermidis and certain viridans streptococci allows the organisms to adhere strongly to the endothelium of heart valves. The various molecules that mediate adherence to cell surfaces are called adhesins.
Once bacteria attach, they often form a matrix called a biofilm consisting of various polysaccharides, proteins, and nucleic acids. Biofilms form on foreign bodies such as prosthetic joints, prosthetic heart valves, and intravenous catheters, but can also form on native structures such as heart valves. Biofilms protect bacteria from host immune defenses, such as antibodies and neutrophils, and impede wound healing, resulting in chronic wound infections, especially in people with diabetes. They play an important role in the persistence of Pseudomonas in the lungs of cystic fibrosis patients and in the formation of dental plaque, the precursor of dental caries and periodontal disease.
The production of biofilms by bacteria is controlled by the process of quorum sensing, which allows bacteria to coordinate the synthesis of particular proteins according to the density of the bacterial population. When the concentration of bacteria is low, these proteins are not expressed; but once the population reaches a critical cell density, the individual members sense this and begin to synthesize these proteins, resulting in phenotypic changes that benefit the population as a whole. Examples of behaviors that are controlled by quorum sensing include biofilm formation, expression of virulence factors, and antibiotic resistance, all of which can contribute to pathogenesis.
