|MadSci Network: Microbiology|
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.
As for streaking media, you should first do the following:
Hope this helps..
-L. Bry, MD/PhD
Dept. Clinical Pathology
Brigham & Womens' Hospital
Harvard Medical School
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