Petri dish bacterial cultures. Masked technician uses a sterile loop to pick a single colony from a petri dish containing streaked bacterial cultures. Several petri dishes are seen containing different coloured nutrient agar gels and different bacterial strains. Bacterial samples are smeared across the gels and individual cells in the samples feed on the nutrients in the agar gels and multiply to form pure colonies. Microorganisms from these colonies can then be removed and analysed.