Pathogen comes from two Greek words: pathos meaning “disease” and genein meaning “to produce.” It refers to an agent or microorganism that is capable of producing disease. Prions are an example of a pathogenic agent. When it is a microorganism spreading disease, the culprit could be a fungus, a protozoan, or a bacterial pathogen. Viruses may be included as microorganisms, but since there is a controversy about whether they are living, they may also be put in the category of agents.
Not all bacteria are pathogenic, but those that are can threaten the lives of animals, people, and plants. Examples of bacterial pathogens include Mycobacterium tuberculosis, Streptococcus, Bacillus anthracis, Rickettsia, Listeria, and Salmonella. A bacteria’s pathogenicity is a measure of its ability or likeliness to cause disease, measured quantitatively as its virulence. The particular factors that account for the virulence of bacterial pathogens are referred to as virulence factors.
The virulence factors include a variety of properties that contribute to its success in establishing itself in or on its host and causing disease. Bacterial pathogens may have relatively few or many virulence factors. Factors that bacterial pathogens may have include producing toxins, having proteins that assist in attachment to a host, and being able to protect its own surface.
Another way of looking at the effectiveness of bacterial pathogens is to consider them in terms of invasiveness and toxigenesis. In this case, the ability to produce toxins that affect tissue both near and far from the point of growth or invasion is separated from all the factors that allow bacteria to successfully invade another organism.
Bacterial pathogens commence their attacks on a host through colonization, establishing themselves in the host's tissues, usually at a point that maintains contact with the external environment. With a human host, this would include the conjunctiva, the digestive tract, the respiratory tract, and the urogenital tract. The bacterial pathogens use adhesins to interact with receptors on the host cells.
The next stage of infection by bacterial pathogens is called invasion. The factors that can help the invading bacteria include substances called invasins that have two purposes: damaging cells of the host and enabling the spread of the bacterial pathogen.
Infectious diseases caused by bacterial pathogens may be treated by one of three different groups of antimicrobial agents. The group of naturally occurring antimicrobials is called antibiotics; the group of chemically synthesized antimicrobials is called chemotherapeutic agents; and the hybrids begin with a naturally occurring substance that is modified. Some members of the first group can now be synthesized.