Define the Streak Plate Method - Pure Culture Techniques?

The method was developed by Loeffler and Gaffkey in the laboratory of Robert Koch. The streak plate method depends on spatial separation of single cells. The mixed microbial culture is transferred to the edge of an agar plate and then a series of parallel non-overlapping streaks are made in some specific pattern over the surface of the nutrient medium with the help of inoculating loop.

As the microbes are rubbed off the loop on to the medium, there is a continuous reduction in the number of microbes till the last cells to be rubbed off the loop are far enough apart to form isolated/ discrete colonies. The isolated colony can be picked and restreaked on nutrient agar plate to get a pure culture. Streak plate method is the most commonly used method to isolate pure cultures. As discussed above, principle of streak plate method is continuous dilution of the microbes, resulting in separation of individual cells. These cells then form isolated colonies.

Repeated picking and restreaking of isolated colony ultimately results in a pure culture. Different patterns can be used for streaking. Common ones are quadrant (four way streaks) and full plate streak methods. There are other variants of streaking pattern also.The method works well when the organism to be isolated is present in large amounts in a mixture. However, when the amount of desired organism is less, its level has to be increased by using specific enrichment culture before performing streaking.