How can I tell if a colony is really only pink and therefore a coliform, if there is a tinge of purple or blue in it (an indication of the presence of both pink and teal green) and therefore E. coli , or if it is teal green alone without any pink and therefore not a true coliform?
A 5. First of all, you should be aware of the fact that E. coli colonies appear earlier on the plate that the other coliforms. This is because the E. coli produces both enzymes and therefore a double dose of dyes which give quicker, darker color, while the other coliforms only produce the one enzyme and the lighter pink color is produced by its activity. So E. coli is visible first and has a definite blue tone, while the other coliforms appear a bit later and look pale pinkish at first. (Teal green appear later also and look very pale green with a light green halo at first.) Second, true teal green colonies will never have a pinkish halo around them, but only a light green halo. True E. coli will either have no noticeable halo or the halo will tend to have a definite pinkish color. If you are still not sure of which colors are associated with the colonies that you are seeing, you can purchase Micrology’s “Test Media for the Determination of E. coli from Coliforms”. There are 2 differen
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