Elective Culture
An elective culture - one that has arisen naturally -
usually has many different types of organisms. The exception occurs when the
environment is extreme as in a hot volcanic spring. Another instance of
relatively few types of organisms occurs when the environment changes as with
the addition of a toxic material. There will be fewer species in a stressed
environment. Microbiological treatment of wastes is particularly interesting
because very complicated mixed elective cultures are constantly changing and
can shift dramatically in response to nutritional or physiological stress.
Pure cultures are the norm for industry but with some exceptions.