Bacterial-Host-Parasite Relationships in man

Bacteria are consistently associated with the body surfaces of animals. There are many more bacterial cells on the surface of a human (including the gastrointestinal tract) than there are human cells that make up the animal. The bacteria and other microbes that are consistently associated with an animal are called the normal flora, or more properly the "indigenous microbiota", of the animal. These bacteria have a full range of symbiotic interactions with their animal hosts. 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
a) Mutualism
Refers to a symbiotic relationship in which both members in 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.

b) 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. E.g. Staphylococcus epidermidis a common inhabitant of
the human skin

c) Parasitism
Refers to a relationship in which one organism called parasite grows, feeds
and is sheltered on or in a different organism called a host 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 non indigenous parasites generally always cause disease if they associate with a non immune host.