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In biology, symbiosis is defined as "life together", i.e., that two organisms live in an association with one another. Thus, there are at least three types of relationships based on the quality of the relationship for each member of the symbiotic association.

Types of Symbiotic Associations

1. Mutualism. Both members of the association benefit. For humans, one classic mutualistic association is that of the the lactic acid bacteria that live on the vaginal epithelium of a woman. The bacteria are provided habitat with a constant temperature and supply of nutrients (glycogen) in exchange for the production of lactic acid, which protects the vagina from colonization and disease caused by yeast and other potentially harmful microbes.

Lactobacilli in association with a vaginal epithelial cell (CDC).

2. Commensalism. There is no apparent benefit or harm to either member of the association. A problem with commensal relationships is that if you look at one long enough and hard enough, you often discover that at least one member is being helped or harmed during the association. Consider our relationship with Staphylococcus epidermidis, a consistent inhabitant of the skin of humans. Probably, the bacterium produces lactic acid that protects the skin from colonization by harmful microbes that are less acid tolerant. But it has been suggested that other metabolites that are produced by the bacteria are an important cause of body odors (good or bad, depending on your personal point of view) and possibly associated with certain skin cancers. "Commensalism" best works when the relationship between two organisms is unknown and not obvious.

Staphylococcus epidermidis (CDC).

3. Parasitism. In biology, the term parasite refers to an organism that grows, feeds and is sheltered on or in a different organism while contributing nothing to the survival of its host. In microbiology, the mode of existence of a parasite implies that the parasite is capable of causing damage to the host. This type of a symbiotic association draws our attention because a parasite may become pathogenic if the damage to the host results in disease. Some parasitic bacteria live as normal flora of humans while waiting for an opportunity to cause disease. Other nonindigenous parasites generally always cause disease if they associate with a nonimmune host.

Parasitology, actually a branch of microbiology, refers to the scientific study of parasitism but somehow it developed into a discipline that deals with eucaryotic parasites exclusively.

Bacterial Pathogenesis

A pathogen is a microorganism (or virus) that is able to produce disease. Pathogenicity is the ability of a microorganism to cause disease in another organism, namely the host for the pathogen. As implied above, pathogenicity may be a manifestation of a host-parasite interaction.

In humans, some of the normal bacterial flora (e.g. Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae) are potential pathogens that live in a commensal or parasitic relationship without producing disease. They do not cause disease in their host unless they have an opportunity brought on by some compromise or weakness in the host's anatomical barriers, tissue resistance or immunity. Furthermore, the bacteria are in a position to be transmitted from one host to another, giving them additional opportunities to colonize or infect.

There are some pathogens that do not associate with their host except in the case of disease. These bacteria may be thought of as obligate pathogens, even though some may rarely occur as normal flora, in asymptomatic or recovered carriers, or in some form where they cannot be eliminated by the host.

Opportunistic Pathogens

Bacteria which cause a disease in a compromised host which typically would not occur in a healthy (noncompromised) host are acting as opportunistic pathogens. A member of the normal flora can such as Staphylococcus aureus or E. coli can cause an opportunistic infection, but so can an environmental organism such as Pseudomonas aeruginosa. When a member of the normal flora causes an infectious disease, it sometimes referred to as an endogenous bacterial disease, referring to a disease brought on by bacteria 'from within'. Classic opportunistic infections in humans are dental caries and periodontal disease caused by normal flora of the oral cavity.

A photomicrograph of Pseudomonas aeruginosa, one of the most common opportunistic pathogens of humans. The bacterium causes urinary tract infections, respiratory system infections, dermatitis, soft tissue infections, bacteremia and a variety of systemic infections, particularly in cancer and AIDS patients who are immunosuppressed. CDC.

Infection

The normal flora, as well as any "contaminating" bacteria from the environment, are all found on the body surfaces of the animal; the blood and internal tissues are sterile. If a bacterium, whether or not a component of the normal flora, breaches one of these surfaces, an infection is said to have occurred. Infection does not necessarily lead to infectious disease. In fact, infection probably rarely leads to infectious disease. Some bacteria rarely cause disease if they do infect; some bacteria will usually cause disease if they infect. But other factors, such as the route of entry, the number of infectious bacteria, and (most importantly) the status of the host defenses, play a role in determining the outcome of infection.

Determinants of Virulence

Pathogenic bacteria are able to produce disease because they possess certain structural or biochemical or genetic traits that render them pathogenic or virulent. (The term virulence is best interpreted as referring to the degree of pathogenicity.) The sum of the characteristics that allow a given bacterium to produce disease are the pathogen's determinants of virulence.

Some pathogens may rely on a single determinant of virulence, such as toxin production, to cause damage to their host. Thus, bacteria such as Clostridium tetani and Corynebacterium diphtheriae, which have hardly any invasive characteristics, are able to produce disease, the symptoms of which depend on a single genetic trait in the bacteria: the ability to produce a toxin. Other pathogens, such as Staphylococcus aureus, Streptococcus pyogenes and Pseudomonas aeruginosa, maintain a large repertoire of virulence determinants and consequently are able to produce a more complete range of diseases that affect different tissues in their host.

A photomicrograph of Corynebacterium diphtheriae bacteria using a Gram stain technique. Corynebacterium diphtheriae causes diphtheria that affects the upper respiratory tract, where an inflammatory exudate causes severe obstruction to the breathing airways, and sometimes suffocation. CDC.