The purpose of a streak plate is to produce colonies of a certain organism. These colonies are formed by encouraging the growth of only one organism, which allows the separation of that organism from a sample containing multiple organisms.
A streak plate is prepared with nutrients that are specifically chosen to support the growth of the desired organism. It may also contain chemicals that are toxic to other organisms. These chemicals will kill any organisms that might contaminate the organism you want to study. This combination should result in a plate of isolated colonies of a desired organism.
As well as isolating chosen organisms, streak plates may also be used to support more investigative study of a particular organism. The nutrients in a streak plate can be manipulated to study their effects on the organism, and by setting up different combinations, the effects of changes to the organism's environment can be tested.
A streak plate can also be used to study an organism's normal morphology and lifecycle. In this case, a streak plate can provide an organism with the ideal ratio and amount of nutrients for optimum reproduction and keep any toxic chemicals from reaching the colony. This permits the study of the organism without contamination or interference from any outside factors.