Streak Plate Technique

For organisms that grow well on agar plate, streak plate is the method of choice for obtaining pure culture.

The key principles of this method is that, by streaking, a dilution gradient (number of cells decrease as they move across the agar and away from the point of inoculation) is established across the face of the plate as bacterial cells are deposited on the agar surface. Because of this dilution gradient, confluent growth occurs on part of the plate where the bacterial cells are not sufficiently separated; in other regions of the plate where few bacteria are deposited separate macroscopic colonies develop that can easily be seen with naked eye. Each well isolated colony is assumed to arise from a single bacterium and therefore to represent a clone of a pure culture.

Purpose of Streak Plate Technique:  The purpose of the streak plate is to obtain isolated colonies from an inoculum by creating areas of increasing dilution on a single plate.  Isolated colonies represent a clone of cells, being derived from a single precursor cell.

Many different streaking patterns can be used to separate individual bacterial cells on the agar surface.

Methods of making a streak plate to obtain pure cultures.

1. Loop is sterilized, and then a loopful of inoculums is removed from tube
2. A loopful of bacterial cells is streaked across the surface of a sterile solidified nutrient medium.
3. Following the initial streak, subsequent streaks are made at angles to it, the loop being sterilized between streaks.
4. The plates are than incubated under favorable conditions to permit growth of the bacteria.
5. After incubation colonies appear along the points of the streak. It is from such well isolated colonies that pure cultures can usually be obtained.