Which method is commonly used to isolate pure bacterial cultures?

The streak plate method is commonly used to isolate pure bacterial cultures because it allows for the separation of individual bacterial cells on the surface of an agar plate. By using an inoculating loop to streak the surface, the cells are diluted over the area of the plate. With careful technique, this results in discrete colonies forming from individual cells, making it easier to identify and pick out pure colonies for further study or experimentation.

This method is particularly advantageous due to its simplicity and effectiveness in obtaining isolated colonies quickly. The agar medium provides a stable environment for bacterial growth and the streaking technique reduces the density of bacteria as they are spread, facilitating the isolation of only specific, pure colonies.

Other methods, while useful in other contexts, typically do not focus on isolating single colonies in the same way. For instance, the broth culture method may grow a large population of bacteria but does not inherently separate them into individual colonies. Serial dilution is effective for quantifying the number of organisms or reducing concentration in a sample, but it does not directly yield isolated colonies on a solid medium like the streak plate method does. Direct plating may yield growth from a mixed culture but doesn't guarantee the isolation of pure colonies as efficiently as streak plating.