The inoculum is diluted by streaking it across the surface of the agar plate. While streaking in successive areas of the plate, the inoculum is diluted to the point where only one bacterial cell is deposited every few millimeters on the surface of the agar plate. (Ambien) An isolated colony is formed when these lone bacterial cells divide and give rise to thousands and thousands of new bacterial cells. Pure cultures can be obtained by picking well-isolated colonies and re-streaking these on fresh agar plates.
A common assumption is an isolated colony of bacteria is the progeny of a single bacterial cell (i.e. colony is the clone). However, this is not necessarily true. With species in which the cells form a characteristic grouping during cell divisions, the colony-forming unit may develop from a group of cells rather than form a single cell. For example, clusters of staphylococci, chains of streptococci, etc.
A source of bacteria (stock culture, previously streaked agar plate, or any other inoculum)
Inoculation loop
A striker/lighter
Bunsen burner
Lysol (10%v/v)
Agar plate (nutrient agar or any other agar medium)
Paper towels
Tips for the best results
Use only a small amount of inoculum.
Streak lightly so that you do not gouge the agar.
Flame the loop after you streak each quadrant.
Make sure the surface of the plate is free of droplets of condensed moisture.
The purpose of the streak plate is to obtain isolated colonies from an inoculum. Isolated colonies represent a clone of cells derived from a single precursor.
To produce isolated colonies of an organism (primarily bacteria) on an agar plate. This is useful when we separate organisms in a mixed culture (to purify/isolate a particular strain from contaminants) or to study an organism’s colony morphology.
To identify the organism: biochemical tests to identify bacteria are only valid when performed on pure cultures.
Types of Streaking Methods
Many different streaking patterns can be used to separate individual bacterial cells on the agar surface. There are four basic types of streaking methods;
Quadrant streaking
T-streak
Continuous streak
Radiant streak
As the original sample is diluted by streaking it over successive quadrants, the number of organisms decreases. Usually, by the third or fourth quadrant, only a few organisms are transferred, giving discrete colony-forming units (CFUs).