Litmus Milk

Introduction Results Methods Photos

CAUTION: Litmus milk is a complex medium that can produce a diversity 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 acid conditions).

If lactose is fermented, the solution should turn pink. If gas is produced during fermentation, you may be able to see bubble or cracks in the milky medium (but this is often difficult to observe).

If lactose is not fermented and proteins are instead used for energy, the solution will become alkaline and more blue.

Casein protein may be digested. This will coagulate the milk to form a curd (a solid).

The 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.

The litmus may be reduced and become colorless. The culture will then look milk white.

Results

Possible Results Full description Appearance of 7-Day Cultures
No Change   Purple
Alk Alkaline Blue
A Acid Pink
Ac Acid with Curd Acid, somewhat solid
Ac-R Acid with Curd, Litmus reduction Pink Top, White Bottom, somewhat solid
AcG-R Acid and Gas with curd, Litmus reduction Pink Top, White Bottom, broken or cracked solid
AlkP Alkaline with Peptonization Clear brown top, brown bottom
AlkP-R Alkaline with Peptonization, Litmus Reduction Clear brown top, brown bottom

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 control).

3. Incubate both broths at appropriate temperature (which ever 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.