Question

If you were using the quadrant streak plate method to plate a very dilute broth culture (with many fewer bacteria than the broth used for the plate pictured here), would you expect to see single, isolated colonies in quadrant 4 or quadrant 3? Explain your answer.   

Answer 1

Quadrant streak plate method is a rapid qualitative isolation method used for isolation of pure culture of organism (mostly bacteria) from a mixed population of organisms. This method allows sequential dilution of the original culture over the entire surface of a fresh agar plate. This method enables us to select and work with individual colonies. Streaking is fast and reliable technique which works especially well when we want to test a large number of bacterial cultures. Since different bacteria often look different simply by inspecting the single colonies by eye we can check for contamination by other bacteria. As the original sample is diluted by streaking it over successive quadrants, the number of organisms decreases. Usually by 3rd or 4th quadrant only a few organisms are transfered on the innoculating loop which produce few isolated colonies. When the plate is streaked with loopful of culture, in the 1st quadrant heavy confluent growth is observed. In the 2nd quadrant heavy growth, in the 3rd quadrant light growth and in the 4th quadrant discrete isolated colonies are obtained.