Action of Bacteria in Milk

Bacteria grow well in skim milk. Milk contains
a number of nutrients that bacterial enzymes may
attack and dissimilate to extract energy.

If the cells use lactose sugar there is an acid reaction
with coagulation of the milk protein. This can be seen in litmus milk when the litmus in the milk turns pink. If the cells use casein protein the pH goes up and the litmus in the milk appears violet.

If the milk is reduced, the litmus loses color and appears white, especially at the bottom of the test tube. These reactions are helpful in identifying bacteria because each species has a typical reaction in milk.

The graphic below shows three typical reactions:

• Proteolysis
• Acid Reaction
• Milk Reduction

Note below that proteolysis (left) shows as dark violet due to the rise in pH. Also, the milk solids on the bottom have a soft consistency. In the middle 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. On the right we see complete reduction of milk. Originally all tubes were a light blue color.

E. Bollenbach 11/11/98 Northwestern Connecticut Community-Technical College