Bio-Rad Microbial Culturing Module - Manual (Page 10)

6
Streaking Plates with Bacteria
Streaking is done to make single colonies from concentrated bacteria. Each colony grows from
one bacterium and thus a colony is a “clone” or group of genetically identical individuals. A tiny
drop of the original bacterial suspension contains millions or billions of individual bacteria and
must be diluted multiple times to isolate single bacteria. Under favorable conditions
E. coli can
double every 20 minutes and thus a single bacterium will multiply to become billions of
genetically identical cells in less than 24 hours.
1. Insert a sterile inoculation loop into a bacterial colony or other sample. Insert the loop
straight into the container without tilting. Remove the loop and gently rub it back and forth
over the agar in the top left hand corner as shown below. The first streak dilutes the cells.
Go back and forth with the loop about a dozen times in the first quadrant. Do not break the
surface of the agar.
2. For subsequent streaks, the goal is to use as much of the surface area of the plate as
possible. Rotate the plate about 45 degrees (so that the streaking motion is comfortable for
your hand) and start the second streak. Do not dip the loop into the starting material
(bacterial colony, rehydrated bacteria, or other sample) again. Go into the previous streak
one or two times and then back and forth as shown about a dozen times.
In subsequent quadrants the cells become more and more dilute, increasing the likelihood of
producing single colonies. Remember a single colony arose from one cell and all the cells in
the colony are genetically identical.
3. Rotate the plate again and repeat streaking into the third quadrant.
4. Rotate the plate again and make the final streak. Do not touch the first quadrant.
5. Place the plates upside down inside the incubator for 16–24 hours at 37°C.