© 2005, Lynn Bry. All rights reserved.
Streaking Agar Plates
Goal: Take a source of microorganisms and spread the material over an agar plate to obtain single isolated
colonies. The isolated colonies will allow you to observe specific characterisitcs such as colony size, shape
and color. If special media are you used, you can also determine other traits such as hemolysis (if streaking on
blood agar) or ability to ability to ferment the sugar lactose (if using MacConkey Agar).
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Gloves
Inoculating loop OR sterilized wooden sticks for streaking.
Permanent marker or grease pencil to label your plates beforehand.
o Bunsen burner or sterilizing heater if intending to sterilize the inoculating loop between
streaks.
Swab for collecting the primary inoculum, if intending to collect bacteria from an environmental
source
Agar plate
Incubator, if incubating at a controlled temperature, such as 37°C. However, many common microbial
species will grow on plates left at room temperature, through their growth may be slower than if the
plates were incubated at 37°C.
Antibacterial soap to wash your hand.
5% bleach solution to clean your work area when finished.
Method:
Primary Streak (shown in Yellow).
Hold the plate as though you are going to "paint" the
surface of the agar . If you are right-handed, hold the
plate in your left hand, and the inoculating loop in
your right - as through you would a paint brush. Hold
the plate in your right hand, and inoculating loop in
your left if you are left-handed.
Dip your inoculating loop (sterile swab, or sterile
stick as shown in the above picture) into a broth
culture, or touch it to the material you want to spread
- an isoated colony or swab an area for which you
want to quantitate the microbial species present.
Go back and forth a number of times in a small area of the Petrie plate. The goal is to spread your material
completely over this inital area of the plate.
http://www.madsci.org/~lynn/micro/techniques/streaking/primary.JPG&imgrefurl=http://www.madsci.org/~ly
nn/micro/techniques/streaking/&h=350&w=450&sz=47&hl=en&start=7&tbnid=0w4qpbvhAuVDrM:&tbnh=
99&tbnw=127&prev=/images%3Fq%3DSpread%2BPlate%26svnum%3D10%26hl%3Den%26lr%3D%26sa
%3DG
© 2005, Lynn Bry. All rights reserved.
Secondary Streak (shown in Blue).
Sterilze your inoculating loop, or use a fresh, sterile
inoculating stick or swab. Squelch it in an unused
area of the agar before steaking to cool it. If you were
to use the original, unsterile loop, you will not be
diluting the individual microbes you plates in the first
streak.
Pick up the plate and rotate it 1/4 of a turn to your left
(if right-handed), or to your right (if left handed).
Run the loop through the previous streak 2-3 times,
then draw it along 1/3 of the remaining plate, as
shown by the blue line in the above image.
It's OK if you gouge a little bit of the agar, but with practice you should be able to streak cleanly across the
surface.
Tertiary Streak (Orange)
Rotate the plate another 1/4 turn and
sterilize your inoculating loop or take
a fresh, sterile stick or swab. Squelch
a heated inoculated loop it in an
unused area of the agar before
steaking to cool it.
Run the loop through the previous,
secondary streak 2-3 times, and draw
the streak over a remaining 1/3 of the
plate, as shown.
http://www.madsci.org/~lynn/micro/techniques/streaking/primary.JPG&imgrefurl=http://www.madsci.org/~ly
nn/micro/techniques/streaking/&h=350&w=450&sz=47&hl=en&start=7&tbnid=0w4qpbvhAuVDrM:&tbnh=
99&tbnw=127&prev=/images%3Fq%3DSpread%2BPlate%26svnum%3D10%26hl%3Den%26lr%3D%26sa
%3DG
© 2005, Lynn Bry. All rights reserved.
Quarternary Streak (Black)
Rotate the plate another 1/4 turn and
sterilize the inoculating loop.
Squelch a heated inoculated loop it
in an unused area of the agar before
steaking to cool it.
Run the loop through the previous
tertiary streak 2 times and draw over
the remaining free space in the plate,
being careful not to contact the
primary streak (yellow).
Incubate the plate as needed, and
check 18-24 hours later for growth!
http://www.madsci.org/~lynn/micro/techniques/streaking/primary.JPG&imgrefurl=http://www.madsci.org/~ly
nn/micro/techniques/streaking/&h=350&w=450&sz=47&hl=en&start=7&tbnid=0w4qpbvhAuVDrM:&tbnh=
99&tbnw=127&prev=/images%3Fq%3DSpread%2BPlate%26svnum%3D10%26hl%3Den%26lr%3D%26sa
%3DG