Download Lab 5 - Lone Star College

Lab Exercise 5:
Cultivation and Isolation of Bacteria
You’re
never too
old to
streak!
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.
Each species of
bacteria produces a
distinctive colony
appearance.
What are five (5)
ways colonies can be
different from each
other?
1
Nice isolation!
2
4
1
2
3
Streak pattern
4
3
Bacterial growth pattern
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.