MICROBIAL VIRULENCE FACTORS
Infectious agents cause disease in the host by several mechanisms. Most typically, microbial virulence, or the potential to cause disease, involves the ability of the pathogen to gain access to the host by surviving on mucous membranes or by being ingested or inhaled. Pili, submicroscopic hairlike structures on the surface of some bacteria, allow them to attach themselves firmly. Next, the multiplying microbe must have the capability of avoiding or counteracting the normal protective mechanisms of the host. Capsules surround some bacteria, and these structures serve to prevent the microbes from being ingested and killed by defending phagocytic cells. Other bacteria produce toxins that can destroy phagocytes. Certain highly virulent organisms produce disease in a large percentage of people whom they contact, while other microorganisms produce illness in only a small percentage. When a nonimmune person encounters the influenza virus, the organisms are inhaled, multiply in respiratory cells, damage these cells, and cause disease. In contrast, certain organisms produce toxins, making the patient ill without actually being invaded by the microbes.
Clostridium botulinum, a bacterium that grows in improperly preserved foods, produces a potent toxin that when ingested may cause botulism, often leading to paralysis and death. Patients may die by ingesting products of this bacterium without actually being infected by the organism. Other organisms may produce a toxin only after infecting the patient. For example, certain strains of Escherichia coli, a common intestinal bacterium, produce a toxin when colonizing the intestine. This toxin, known as an enterotoxin, causes diarrhea.
In some instances the host's own defenses against infection may be responsible for the damage. For example, infections caused by Mycoplasma pneumoniae may result in inflammation of the airways and lungs. When the microbes grow in the respiratory cells, they do not damage the cells. However, during the host's attempt to kill foreign invaders, the infected cells are attacked and destroyed by immune mechanisms.