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

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.

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

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

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

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

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