One drop of bacterial solution contains millions of cells. To identify a species, individual strains have to be grown separately, starting from a single cell. The most commonly used method to isolate strain originating from a single cell is called the streaking method. There are several different ways to streak a plate.

Figure 1:Three streaking patterns.

Streaking methods all follow the same basic principle: a large number of cells are spread over the surface of an agar plate in a sequential fashion until the cells are spread far apart from one another. The numbers and colors refer to the order of the streaks. For example, streak number two is started by placing a sterile loop inside streak number one to pick up bacteria.

Following streaking, the plate is incubated and the cells start to divide and form colonies. Single cells will form colonies of genetically identical clones. Isolated colonies usually grow larger than colonies situated closely together due to less competition for nutrients. These isolated colonies can be used to make a pure culture, a liquid or solid culture in which all bacteria are genetically identical, for further study. In some cases, the motility ormorphology of the colonycan also provide useful information for identification.