Litmus Milk Reactions Microbiological Applications

Litmus milk reactions: (A) Alkaline. (B) Acid. (C) Upper transparent portion is peptonization; solid white portion in bottom is coagulation and litmus reduction; overall redness is interpreted as acid. (D) Coagulation and litmus reduction in lower half; some peptonization (transparency) and acid in top portion. (E) Litmus indicator is masked by production of soluble pigment (Pseudomonas); some peptonization is present but difficult to see in photo.

Litmus milk contains 10% powdered skim milk and a small amount of litmus as a pH indicator. When the medium is made up, its pH is adjusted to 6.8. It is an excellent growth medium for many organisms and can be very helpful in unknown characterization. In addition to revealing the presence or absence of fermentation, it can detect certain proteolytic characteristics in bacteria. A number of facultative bacteria with strong reducing powers are able to utilize litmus as an alternative electron acceptor to render it colorless.

Previous figure, reveals the color changes that cover the spectrum of litmus milk changes. Since some of the reactions take 4 to 5 days to occur, the cultures should be incubated for at least this period of time; they should be examined every 24 hours, however.

Look for the following reactions:

Acid Reaction
Litmus becomes pink. Typical of fermentative bacteria.

Alkaline Reaction
Litmus turns blue or purple. Many proteolytic bacteria cause this reaction in the first 24 hours.

Litmus Reduction
Culture becomes white; actively reproducing bacteria reduce the O/R potential of medium.

Coagulation Curd formation
Solidification is due to protein coagulation. Tilting tube at 45° will indicate whether or not this has occurred.

Peptonization Medium becomes translucent
It often turns brown at this stage. Caused by proteolytic bacteria.

Ropiness Thick, slimy residue in bottom of tube. Ropiness can be demonstrated with sterile loop.

Fuente: Microbiological Applications - Laboratory Manual in General Microbiology by Benson