Bacterial pathogenesis is the process through which bacteria cause disease. Pathogenesis depends on a variety of factors in both the bacteria and the individual host, and the progress of pathogenesis can change at any point in response to environmental pressures and other issues. The study of bacterial pathogenesis is important, as it shows the medical and scientific communities how bacteria cause disease, and how this process might be interrupted to avoid or treat illness.
In the host, several things influence the success of bacterial invasions. The first is the health of the host's immune system, which determines resistance and susceptibility. Other factors can include the environment in which the host lives; someone who does not have access to clean water, for example, is susceptible to reinfection with the same bacteria over and over, while someone who is exposed in passing does not face the same issue. Access to medical treatment can also play a role, determining when intervention occurs and how aggressive it will be.
Among bacteria, comparatively few organisms actually cause disease. Many more are neutral, and some are even beneficial. For bacteria to cause disease, they must be capable of either triggering reactions in the body which cause disease, or of releasing their own toxins to cause illness. The bacteria must also be capable of growing quickly enough to bypass the defenses of the immune system, and they must be infectious. Often, the bacteria complete numerous life cycles within the body, sometimes even using the body to perpetuate themselves.
The process of bacterial pathogenesis starts with colonization, in which bacteria settle on or in the body and start to multiply before spreading. As the bacteria spread, they may release toxins which cause the host to feel ill, or they may hijack the host's body to cause cascading reactions. For example, the immune system may go into overdrive and start attacking tissues in the host's body. If the bacteria manage to penetrate and spread, the host will develop an illness.
In some cases, bacterial pathogenesis can be arrested by the host's own body, which fights back the bacteria. The host may feel a bit ill while the body is at work, but will not succumb to infection. In other instances, intervention is needed in the form of antibiotics and other treatments to manage and fight the infection. The interaction between the bacteria and the host can run in any number of directions once bacteria start to colonize; being able to identify and predict the direction of an infection is important. People who study bacterial pathogenesis look at the complex factors which collide in the course of a bacterial infection.