3.1: Introduction

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As stated in Lab 2, microorganisms exist in nature as mixed populations. However, to study microorganisms in the laboratory we must have them in the form of a pure culture, that is, one in which all organisms are descendants of the same organism. Two major steps are involved in obtaining pure cultures from a mixed population:

First, the mixture must be diluted until the various individual microorganisms become separated far enough apart on an agar surface that after incubation they form visible colonies isolated from the colonies of other microorganisms. This plate is called an isolation plate.
Then, an isolated colony can be aseptically "picked off" the isolation plate (Figure \(\PageIndex{1}\)) and transferred to new sterile medium (see Fig. \(\PageIndex{3}\)). After incubation, all organisms in the new culture will be descendants of the same organism, that is, a pure culture.

pick off colony_final.jpg — Figure \(\PageIndex{1}\):**Picking a Single Colony 0ff of a Petri Plate in order to Obtain a Pure Culture** Before removing bacteria from the petri plate, first cool the loop by sticking it into the agar away from any growth. (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

Illustration showing how to pick an isolated colony off a petri plate to obtain a pure culture from a mixture. — Figure \(\PageIndex{2}\): **Obtaining Pure Cultures from an Isolation Plate** (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

Figure \(\PageIndex{3}\): **Animation for** **Obtaining Pure Cultures from an Isolation Plate** (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)