Download Litmus Milk Test

Litmus Milk Test (WINTER 2014 version)
CAUTION: Litmus milk is a complex medium that can produce a variety of results. Because of this,
litmus milk can give quite unreliable results (i.e., the results often give negative results when something
positive should happen). Thus, you would be advised to use litmus milk as a confirmatory test, but not a
definitive test (except as a last resort).
Introduction
Litmus milk is a complex medium that can potentially distinguish among many species of bacteria. Litmus
milk has several components that can be metabolized: lactose (milk sugar); casein (milk protein); and litmus (a
pH indicator that is purple to blue at neutral to alkaline pH and pink under acidic conditions).
If lactose is fermented, the solution should turn pink. If gas is produced during fermentation, you may be able
to see bubbles or cracks in the milky medium (but this is often difficult to observe).
If lactose is not fermented and proteins are metabolized instead, the solution will become alkaline and turn blue
in color.
If casein is digested, the milk will coagulate to form a curd (a solid). Casein may be metabolized all the way
down to individual amino acids. This process, called peptonization, results in a clear (not milky) liquid that is
usually brown in color.
Finally, the litmus may be reduced and become colorless. The culture will then look milk white.
Results
Possible Results
Description
No Change
Alk
Appearance of 7-Day Cultures
Purple
Alkaline
Blue
A
Acid
Pink
Ac
Acid with Curd
Acid, somewhat solid
Acid with Curd, Litmus reduction
Pink Top, White Bottom,
somewhat solid
Acid and Gas with curd, Litmus
reduction
Pink Top, White Bottom, broken
or cracked solid
Alkaline with Peptonization
Clear brown top, brown bottom
Alkaline with Peptonization, Litmus
Reduction
Clear brown top, brown bottom
Ac-R
AcG-R
AlkP
AlkP-R
Methods
1. Obtain two Litmus Milk broths from the back shelf.
2. Inoculate one broth with cells from your stock unknown culture. Leave the other broth uninoculated
(this will act as a negative control).
3. Incubate both broths at an appropriate temperature (whichever is a better growth temperature for your
organism).
4. Observe your cultures over the next several days. Changes should be visible in five to seven days.
Photograph
Figure 1. Several different
results in Litmus milk
Figure 2. A solid curd. Notice
that the culture does not flow
when the test tube is tilted.