LITMUS MILK USE
Litmus is a multi-purposed and sometimes confusing medium because
there are many reactions that you can get with it. The indicator is
litmus, the same one that is used with pink litmus and blue litmus
papers in a chemistry lab to test whether a solution is acid or base. It
starts out as a light lavender color.
The 2 nutrients in milk are casein protein and lactose sugar. A
bacterium can use one of these, both of these, or neither of these,
hence a variety of end results. Sugar users will make acid, causing a
change to pink, whereas casein protein users will produce alkaline
products causing a change towards blue. If the casein is completely hydrolyzed, NH3 is
released and the medium turns a clear tan/brown. In addition, sometimes the litmus indicator
gets reduced and becomes white.
NOTE: It is a good idea to incubate this medium for a week before making a final decision on
the reaction.
OBJECTIVES:
Identify the reactions associated with use of sugar and protein in milk.
MATERIALS NEEDED:
1 litmus milk per unknown
culture of Streptococcus lactis/per table
THE PROCEDURE:
1. Inoculate the tube of litmus milk with a loopful your unknown
bacterium.
2. Your table will run a + control, Streptococcus lactis.
3. Incubate for up to 1 week, at 37 degrees C.
INTERPRETATION:
Your PHOTO ATLAS has some good pictures of the various
reactions.
The major + reactions:
basic reaction
acidic reaction
peptonization (proteolysis)
acid clot/curd
QUESTIONS: 1. You are testing the bacterium's ability to use ________ and ________ in this
medium.
2. What does a basic reaction look like on this medium?
3. The production of acid means that the bacterium is using the _____ in this medium,
producing acid as a by-product.
LAB MANUAL: TABLE OF CONTENTS
8/2009, Jackie Reynolds, Richland College