The skin and mucous membranes provide anatomical barriers against invading microorganisms, but they also supply the foundation for a complex ecosystem, an interacting biological community. The microbial community that resides on humans is important from a medical standpoint because it offers protection from some disease-causing organisms. At the same time, members of the normal flora are a common cause of infection in people who are immunocompromised.
The intimate interactions between the microorganisms and the human body are an example of symbiosis, meaning living together; symbionts are different organisms that live close together on more or less a permanent basis.
Symbiotic Relationships Between Microorganisms and Hosts
Microorganisms that inhabit the human body may have a variety of symbiotic relationships with each other and the human host. These relationships can take on different characteristics depending on the closeness of the association and the relative advantages to each partner. Symbiotic associations can be one of several forms, and these may change, depending on the state of the host and the attributes of the microbes:
■ Mutualism is an association in which both partners benefit. In the large intestine, for example, some bacteria synthesize vitamin K and certain B vitamins. These nutrients are then available for the animal host to absorb, providing an important source of essential vitamins, particularly for hosts lacking a well-balanced diet. The bacteria residing in the intestine benefit as well, as they are supplied with warmth and a variety of different energy sources.
■ Commensalism is an association in which one partner benefits but the other remains unharmed. For example, many bacteria found living on the skin are thought to be neither harmful nor advantageous to the human host, but the bacteria gain by obtaining food and other necessities from the host.
■ Parasitism is an association in which one organism, the parasite, derives benefit at the expense of the other organism, the host. All pathogens are parasites, but medical microbiologists often reserve the word parasite for eukaryotic organisms such as protozoa and helminths that cause disease.
19.2 The Normal Flora 461
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