Streaking a plate for single colonies

Bacteria in liquid culture can be streaked on a plate to obtain individual colonies. It is important for a molecular biologist to work with bacteria that are genetically identical. By streaking for single colonies, one isolated colony will form from a single bacterial cell and thus the colony is genetically identical.

One method is to streak bacteria around the petri plate, working in a circle and diluting the bacterial population as you go. The microbiological loop is either flamed, or a new loop is used, at four steps. These are represented by different colors in the figures below.

In our lab, since we do not have bunson burners on the lab bench, we will use a new plastic loop for each step. The used loops are put in a beaker of alcohol for sterilization instead of flaming.
 

#1 = a loopful of bacteria is obtained from a resuspended liquid culture, and a dime to penny sized circle is made on the plate (BLACK). #2 - a second loop is used to make four lines moving out from the circle (RED). #3 - a third loop is used to make four more lines which overlap the first four (BLUE). #4 - a fourth loop is used to make four more lines which overlap the second four (GREEN). NOTE that the same loop is used to make the final four lines (GREEN).

On this plate, many isolated colonies are seen on the third set of streaks.              Thanks to Dr. S. Morrison for the original hand drawing of this scheme.