Question

You were given two test-tube with cultures Escherichia coli and Bacillus subtilis grown on phenol red lactose broth. Unfortunately, labels came off the tubes. How can you determine which test-tube contains what culture without running any additional tests?

Answer 1

Answer-

1. In phenol red lactose broth, lactose is carbohydrate source and phenol red is pH indicator. In this media only Gram negative bacteria like E.coli can grow but Gram positive organism like Bacillus subtilis can not grow.

2. When E.coli grows in this media, acid is produced after fermentation of lactose sugar and production of acid is detected by colour change in pH indicator. Before E.coli growth, colour of broth is red and after acid production by E.coli after growth red colour of broth changes from red to yellow. But Bacillus subtilis can not ferment lactose so that colour o broth does not changes and it will remain red in colour.

3. So that when E.coli and Bacillus subtilis grown on phenol red lactose broth, and unfortunately labels came off the tubes then after 24 hours the tube in which yellow colour appears that indicates E.coli and tube in which red colour does not changes that indicates Bacillus subtilis. In this way without running any additional test we can determine in which test tube E.coli is present and in which test tube Bacillus subtilis is present.