Method: Streaking Bacterial Stocks
Nov. 14
1990
C. Helms
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:
- 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.
- 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.
- 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).
- 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.
References: none
