This differential test distinguishes organisms that can ferment lactose. Lactose fermentation causes a lowering of the pH, which in turn causes the litmus to change from purple to pink. Litmus reduction causes the litmus to turn white. Additionally, this test can distinguish organisms that precipitate casein. Sometimes the accumulation of acid following lactose fermentation can lead to the precipitation of casein, forming an acid clot. If the casein is completely hydrolized, NH3 is released and the medium turns brown.
Figure 1. In the front tube, Escherichia coli has fermented acid in the top of the tube and reduced litmus as seen by the white unbroken curd. Standing tubes: to the left is the uninoculated control. The second tube from the left displays Bacillus cereus' reaction in litmus milk, peptonization with an alkaline reaction and a soft curd. The third tube from the left contains Proteus vulgaris, illustrating an alkaline reaction where litmus is reduced. Finally, in the tube on the right, Streptococcus lactis shows a litmus reduction with a red band of acid on the top and no digestion.
Figure 2. In the tube on the left, note that proteolysis shows as dark violet due to the rise in pH. Also, the curd formation on the bottom has a soft consistency. In the middle tube there is an acid fermentation as seen by the red litmus reaction on top. Lower in the tube, the litmus has been reduced and becomes white. In the tube on the right, we see complete reduction of milk. Originally all tubes were a light blue color.
The above was obtained by photographing the results of milk fermentation with a Sony Mavica digital camera during the spring semester of 1998 in the microbiology laboratory of Professor Edward Bollenbach at Northwestern Connecticut Community-Technical College, Park Place, Winsted, CT 06098.