MadSci Network: Microbiology

Re: What is the red medium in Petrie dishes made of?

Date: Tue Jan 19 15:25:49 1999
Posted By: Lynn Bry, MD/PhD, Pathology Resident, Brigham & Women's Hospital, Boston, MA
Area of science: Microbiology
ID: 916669397.Mi

I don't know what kind of "red medium" you are using, but I would guess that it could be "MacConkey's Agar" or "Blood Agar." You should look on the product label to see what kind of media you have. If it is MacConkey's the red coloration comes from the dye crystal violet which is added to the media. MacConkey's contains this dye, a source of carbon (usually the sugar lacotse) and other components such as "peptone" - an alternative energy source dervied from broken-down proteins.

Bacteria capable of fermenting lactose will turn red. They release acids into the media which react with the dye to produce the color change. Organisms that cannot ferment lactose use the peptone for a source of energy. This form of metabolism does not produce abundant acids, so the colonies are white ("non-fermenters").

Alternatively, you could be using "Blood agar" which usually contains 5% sheep's blood. This type of agar can detect "hemolysis" - the "popping" of red blood cells by certain microbial enzymes. Many species of Streptococcus and Staphylococcus produce clear zones of hemolysis around colonies. Escherichia coli, an organism normally found in the intestine, also hemolyzes sheep red blood cells.

MacConkey's agar plates will be translucent. You will not able to see through blood agar plates.

Petrie Plate As for streaking media, you should first do the following:

  1. Create the "K" pattern, shown in the image to the left, on the back of the plate. Use an indelible marker or a grease pencil. You should also label the plates with the date, and from where you took the sample to be streaked.
  2. Next (shown in the image below), rapidly streak your inoculated swab over the region labeled "1". This will be your "primary inoculation." Run the swab lightly over the surface of the media; do not gouge it into the agar (right-most circle..).
  3. Then take a fresh, sterile swab. Go once into the primary area and drag the streak into the region labeled 2 (left-most circle).
  4. Take a second sterile swab, drag once across region 2 and streak into the third area.
  5. Do the same thing with a third sterile swab, streaking from area 3 into area 4 (center circle..).
This process "diltues" the original inoculum from the animal's mouth. The ultimate goal is to obtain single, isolated colonies which you can further characterize.

Streaking a Petrie plate.

Hope this helps..

-L. Bry, MD/PhD
Dept. Clinical Pathology
Brigham & Womens' Hospital
Harvard Medical School
Boston, MA

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