Question

While working in a research lab, your summer student may have contaminated your E. coli sample with a Gram positive organism. Using the BD Media Manual (BD Difco and BBL culture media manual 2nd Ed),

1) List the media you would use to isolate your E. coli, and

2) state the expected colony characteristics on that media since E. coli can ferment lactose.

Remember that E. coli is a Gram negative, lactose fermenting bacteria.

Answer 1

QUESTION 1 and 2

In this case, the best option that you have is to make another culture (taking a sample from the first one) but instead using a BD Media Manual, we will use a MacConkey Agar. This agar is a selective medium which is used to isolate gram negative bacteria. The main mechanism of this is that the agar is composed by bile salts which are able to inhibit the growth of gram positive bacteria. But, this is not the only thing that this agar can make. At the same time, this agar allow you to differentiate between gram negative lactose fermenters with not lactose fermenters.

The main mechanism of this is that the agar has violet crystal which is a pH indicator. When E.coli (which is a lactose fermenter) ferment the lactose, it will produce several products that will decrease the pH of the medium (making it more acid). When the pH becomes more acid, the crystal violet will turn pink-reddish color. So, we could expert pink colonies of E.coli in this culture.