Litmus milk is a milk-based medium used to distinguish between different species of bacteria. The lactose (milk sugar), litmus (pH indicator), and casein (milk protein) contained within the medium can all be metabolized by different types of bacteria.
Since milk is usually the first substrate used to maintain bacteria, this test allows for accurate depiction of bacterial types. The addition of litmus, other than explaining the pH type, acts as an oxidation-reduction indicator. The test itself tells whether the bacterium can ferment lactose, reduce litmus, form clots, form gas, or start peptonization.