Litmus Milk
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Litmus Milk Done by: Alkaser40
Purpose Litmus milk is a milk-based medium used to distinguish between different species of bacteria. The lactose (milk sugar), litmus (pH indicator), and casein (milk protein) contained within the medium can all be metabolized by different types of bacteria. Since milk is usually the first substrate used to maintain bacteria, this test allows for accurate depiction of bacterial types. The addition of litmus, other than explaining the pH type, acts as an oxidation-reduction indicator. The test itself tells whether the bacterium can ferment lactose, reduce litmus, form clots, form gas, or start peptonization.
What to observe? • • • • •
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.
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.
Results Test name
obsevation after a week
Acid reduction
Pink color
Alkaline
Blue color
Control
No change
Diagram
Results Test name
obsevation after a week
Reduction
Pink Top, White Bottom, somewhat solid
clot
Milky color
Peptonization Blue top and white bottom
Diagram
Purpose Litmus milk is a milk-based medium used to distinguish between different species of bacteria. The lactose (milk sugar), litmus (pH indicator), and casein (milk protein) contained within the medium can all be metabolized by different types of bacteria. Since milk is usually the first substrate used to maintain bacteria, this test allows for accurate depiction of bacterial types. The addition of litmus, other than explaining the pH type, acts as an oxidation-reduction indicator. The test itself tells whether the bacterium can ferment lactose, reduce litmus, form clots, form gas, or start peptonization.
What to observe? • • • • •
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.
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.
Results Test name
obsevation after a week
Acid reduction
Pink color
Alkaline
Blue color
Control
No change
Diagram
Results Test name
obsevation after a week
Reduction
Pink Top, White Bottom, somewhat solid
clot
Milky color
Peptonization Blue top and white bottom
Diagram
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