Re-culture of a bacteria in a mixed culture of at least four different bacterial species - Click on the image to enlarge it.
A clinical sample seldom contains bacteria in pure culture, they are usually found in a mixed flora. Thus, the pathogenic bacterial species is often isolated in combination with the normal bacterial flora during a bacterial infectious disease. To identify key bacteria in clinical samples, you have to have them in pure culture (see figure). A pure culture thus contains only one species of bacteria, and this can be accomplished by streaking (see below) onto agar plates.