Question

Escherichia coli can ferment glucose and lactose, but not sucrose. Proteus vulgaris can ferment glucose and sucrose, but not lactose. What color would the phenol red tests be for both organisms if you tested all three sugars?

Answer 1

Inorder to test the ability of tha bacteria to ferment lactose or sucrose, we are suppose to do the carbohydrate fermentation test individually for these sugars. We cannot perform TSI here because TSI doesnt gives us specific result. We can only differentiate between glucose and lactose, sucrose in TSI.

So the best way to find is inoculating them in carbohydrate fermentation tubes with respective sugars and look for the colour change. If the sugar is fermented, the red colour of the broth changes to yellow colour. The colour change is due to the acid base indicator- phenol red. In acidic pH PR is yellow, neutral pH it is red and alkaline pH it is pink or fuschia.

Name of the organism	Glucose	Lactose	Sucrose
E coli	yellow colour	yellow colour	red colour or no change
P vulgarius	yellow colour	no change	yellow colour

(colour after 24hrs of incubation)