Growing a Pure Culture

A pure culture contains only one species (or strain) of microorganism. Pure cultures are needed to identify a microorganism, to test it with antibiotics, or to use it in industry, because a contaminant would give misleading results.The usual method is the streak plate, which thins out the cells so that some grow as separate colonies:
1. Using aseptic technique, flame an inoculating loop, let it cool and dip it into the mixed culture.
2. Streak the loop in a zigzag across one edge of an agar plate (section 1).
3. Flame and cool the loop. Drag it once through the end of section 1 and streak a new zigzag in section 2.
4. Repeat for sections 3 and 4, flaming the loop between each.Each new section picks up fewer cells than the one before. By the last section, single cells are left well apart. After incubation, each divides repeatedly to form a visible colony. Because a colony comes from one cell, all the cells in it are genetically identical (a clone).To obtain the pure culture, pick one well-separated colony with a sterile loop and transfer it to a fresh agar plate or a tube of sterile nutrient broth. This is called subculturing. Streaking the new plate again should give colonies that all look the same (same colour, shape, edge and size), a check that the culture is pure.Pure cultures can then be kept as stock cultures, often on agar slopes, so they can be used again.