Streaking method

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 share a common principle: spreading a large number of cells over an agar plate in a sequential manner until they are well separated. Each streak is numbered and colored to indicate the order of application. For instance, streak number two begins by transferring bacteria from streak number one using a sterile loop.

After streaking, the plate is incubated, and cells start dividing, forming colonies. These colonies consist of genetically identical clones originating from single cells. Isolated colonies, with less competition for nutrients, tend to grow larger than those in close proximity. They offer an opportunity to establish a pure culture, a genetically identical bacterial population, suitable for further in-depth study in either liquid or solid media. In some cases, the motility or morphology of the colony can also provide useful information for identification.