B. Reduction/Curd (arrow denotes gas pocket)
C. Uninoculated Control
D. Acid Formation
E. Proteolysis of casein
F. Alkaline Reaction
By producing acid from the fermentation of lactose present in milk a bacterium may also cause the milk to curdle or clot in the bottom of the tube. Litmus may also act as an electron acceptor thus becoming reduced by bacterial metabolism. This reaction is observed as a white color in the medium.
The photo at the right illustrates clot formation and redution of litmus.A. Represents a tube that produced a curd and reduced litmus.
B. Represents an uninoculated control.