Litmus milk

Litmus milk is a milk-based medium used to distinguish between different species of bacteria.^[1][2] The lactose (milk sugar), litmus (pH indicator), and casein (milk protein) contained within the medium can all be metabolized by different types of bacteria.^[3]

Early in the development of microbiology, milk was used as a convenient, rich growth medium for propagating bacteria. The litmus in the medium acts as both a pH indicator and a redox (oxidation-reduction) indicator. The test itself tells whether the bacterium can ferment lactose, reduce litmus, form clots, form gas, or start peptonization.^[2]