# 3.2: Streak Plate Method of Isolation

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The most common way of separating bacterial cells on the agar surface to obtain isolated colonies is the streak plate method we used in Lab 2 to inoculate a petri plate. It provides a simple and rapid method of diluting the sample by mechanical means. As the loop is streaked across the agar surface, more and more bacteria are rubbed off until individual separated organisms are deposited on the agar. After incubation, the area at the beginning of the streak pattern will show confluent growth while the area near the end of the pattern should show discrete colonies. (See Fig. $$\PageIndex{1A}$$and Fig. $$\PageIndex{1B}$$.).

Fig. $$\PageIndex{1A}$$: Isolation Plate: Mixture of Escherichia coli and Micrococcus luteus Grown on TSA

Fig. $$\PageIndex{1B}$$: Isolation Plate: Mixture of Escherichia coli and Staphylococcus epidermidis Grown on TSA

Streak the inoculum over area "1" as shown above. Then flame the loop and cool it by sticking it into the edge of the agar. Rotate the plate 90 degrees counterclockwise. Rotate the plate so area "1" is on your left. Drag your stetile inoculating loop through area "1" two times and spread out over area "2" as shown above. Then flame the loop and cool it by sticking it into the edge of the agar. Rotate the plate 90 degrees counterclockwise.
(Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)
##### Learning Objectives

Videos reviewing techniques used in this lab: