Given a mixture of a grampositive and gram negative bacteriu

Given a mixture of a gram-positive and gram- negative bacterium, MacConkey and Triple Sugar Iron Agar, describe the steps you would take to eventually obtain a pure culture of each organism. State the usefulness of MacConkey agar. What are the precaution you must take during the experiment?

Solution

The mixture is inoculated on Mac conkey medium by streaking so that individual colonies are obtained. Lactose fermenting bacteria form pink colonies on mac conkey media and non fermentors form pale or colorless colonies. By confirming with grams stain, the colonies are again streaked on fresh mac conkey plates for obtaining pure growth.

If both gram positive and gram negative bacteria are lactose fermentors, they can be further differentiated by using triple sugar iron agar medium

Macconkey is a differential media which helps to identify lactose fermentors from non lactose fermentors.

Precautions : stealing must be done such that we obtain individual colonies. Not confluent growth.