Petri dish culture of the bacterium Escherichia coli, a well used laboratory organism, growing on L medium. The picture shows the pattern of streaking used to isolate single colonies. Parallel lines are drawn across the culture using a platinum wire loop as seen here. These lines are crossed and recrossed at different angles, diluting the culture until colonies originally formed around a single bacterium are isolated. The small blobs in the foreground are single colonies. Streaking is an essential technique for isolating particular strains or mutant bacteria in mixed cultures.