1. You’re never too old to streak!
Cultivation and Isolation of Bacteria
Lab Exercise 5:
You’re never too old to streak!

2. Each species of bacteria produces a distinctive colony appearance.
What are five (5) ways colonies can be different from each other?
Purpose of streaking:  To obtain pure, isolated colonies.
Principle:  By spreading a large amount of bacteria over the large surface area of a plate, the amount of bacteria is diluted until individual cells are spread on the surface of the plate.  Each individual cell grows into a single colony.  All of the cells in this colony are genetically identical.

3. Streak pattern 1 2 3 4
Bacterial growth pattern
Nice isolation!

4. Starting with a mixed culture, isolation of different species may be obtained by streaking

5. Example of a poorly streaked plate: Bacterial growth is too heavy and isolation is poor.

6. Contamination of a streak plate results from leaving the plate open too long or not shielding properly with the lid.
Correct procedure

7. Which streak plate culture started as a pure culture. How can you tell?
Answer: the one on the right, because all colonies look alike.