Principle:

Bacterial cells are streaked onto a medium to obtain an independent isolate. This is done to reduce the likelihood of working with a culture which has become contaminated and/or has accumulated mutations.

Time Required:

Less than 5 minutes to streak out each strain; the colonies grow overnight.

Procedure:

1. The cells can be streaked from another plate a stab or from a frozen glycerol stock. Pick up a slightly visible amount of cells on the rounded end of a sterile flat toothpick.

2. To transfer the cells to a media plate begin the streak at one edge of the plate. Press the side of the toothpick containing the cells to the agar plate''s surface and quickly streak the toothpick back and forth across part of the plate''s surface (see #1 in the diagram below). The streaks should lie near one another but should not cross over previous streaks.

3. Turn the toothpick over to the side that did not originally touch the cells or use a new toothpick for the next streaks. These streaks should start by crossing over the last streak then proceeding as before into new areas of the agar plate (#2 in the diagram below).

4. Repeat in new territory on the agar plate with a fresh toothpick ( #3 in the diagram). The cells will be distributed on the plate in decreasing concentration through the streaks and no matter how many cells were on the toothpick to begin with there should be an area on the streaked plate that will produce isolated colonies.

Cover the plate invert it and incubate at 37 degrees C overnight. The plate should appear to have solid streaks of cells as well as isolated colonies. Pick a colony well isolated from the others to use in subsequent work.

Plate appearance after overnight growth.