Litmus Milk Test: Principle, Procedure, and Results

One tube of litmus milk can show several things happening at once: the color shifting, a clot forming, the lower half turning white, the whole thing torn apart by gas. That is what makes the medium powerful and also what makes students freeze when they read it. Each change reports on a different bacterial ability, and learning to read them separately is how one tube tells you what an organism can do to milk.

Litmus milk is an undefined milk-based medium consisting of skim milk and the pH indicator azolitmin (litmus). Azolitmin is pink at pH 4.5 and blue at pH 8.3. Between these extremes, it is purple. In addition to being a pH indicator, litmus is an Eh (oxidation-reduction) indicator. Reduced litmus is white and oxidized litmus is purple.

Litmus milk medium differentiates microorganisms based on their ability to metabolize litmus milk. Four essential reactions in litmus milk are; lactose fermentation, reduction of litmus, casein coagulation, and casein hydrolysis. The combination of these reactions yields different results, each of which can be used to differentiate bacteria.

Figure: Litmus milk test A, Acid reaction. B, Alkaline reaction. C, No change. D, Litmus reduction. E, Clot (note separation of clear fluid from clot at the arrow.) F, Peptonization. (Image source: Ref-1)

Litmus milk is mainly used to differentiate members within the genus Clostridium, and it also differentiates Enterobacteriaceae from other Gram-negative bacilli. Litmus milk also cultivates and maintains cultures of lactic acid bacteria.

Principle

The litmus milk test determines an organism’s ability to metabolize litmus milk. Four basic reactions occur in Litmus Milk: lactose fermentation, reduction of litmus, casein coagulation, and casein hydrolysis.

Skim milk provides nutrients for growth, lactose for fermentation, and protein in the form of casein.

1. Lactose fermentation: Fermentation of lactose is demonstrated when the litmus turns pink as a result of acid production. If sufficient acid is produced, casein in the milk is precipitated and forms an acid clot. Acid clots solidify the medium and can appear pink or white with a pink band at the top depending on the oxidation-reduction status of litmus. Acid clots can be dissolved in alkaline conditions. Fissures or cracks in the clot are evidence of gas production. When gas production is vigorous enough to tear the clot apart, the reaction is called stormy fermentation, described in its own section below.
2. Litmus reduction: Some microorganisms reduce litmus. If litmus becomes reduced during lactose fermentation, it will turn the medium white in the lower tube portion where, the greater reduction rate is.
3. Casein coagulation: Some bacteria produce proteolytic enzymes (caseases) such as rennin, pepsin, or chymotrypsin that coagulate casein and make a curd. A curd differs from an acid clot because it will not dissolve in alkaline conditions.  With some organisms, the curd shrinks, forming a straw-colored fluid called whey at the surface.
4. Casein hydrolysis (peptonization): Some bacteria produce caseases that break casein down into peptides and amino acids. The milk becomes straw-colored and clear, resembling turbid serum, as the solid casein is digested. Breakdown of the protein releases ammonia, which raises the pH and turns the litmus blue. A blue or purple ring at the top of the clear fluid, or bluing of the whole medium, indicates this alkaline, proteolytic reaction.

Stormy fermentation (the stormy clot)

Stormy fermentation is one of the most recognizable reactions in litmus milk, and it is a classic feature of Clostridium perfringens.

What it looks like. The medium first forms an acid clot, then the clot is violently broken apart by trapped gas. Fragments of coagulated casein are pushed up the tube, often forcing the clot or overlying fluid upward. The result is a torn, gas-riddled clot rather than a smooth solid one.

Why it happens. Clostridium perfringens ferments the lactose in milk rapidly and vigorously. Two things happen together. The acid produced lowers the pH enough to coagulate the casein into a clot, and at the same time large volumes of gas (hydrogen and carbon dioxide) are produced. The gas has nowhere to escape from within the solidified clot, so it tears the clot apart. The combination of acid coagulation plus heavy gas production is what gives the "stormy" appearance.

Why it is associated with Clostridium perfringens. Almost all strains of Clostridium perfringens produce this reaction, and it is rapid, often visible within 18 to 24 hours of anaerobic incubation. Because the organism is such an active lactose fermenter and gas producer, the stormy clot is a useful presumptive clue when it is combined with other findings.

An important caution. Stormy fermentation is characteristic of Clostridium perfringens but it is not unique to it. Other vigorous gas-producing lactose fermenters can give a similar picture, so a stormy clot supports the identification rather than confirming it on its own. As with all litmus milk reactions, it should be followed by confirmatory tests.

A practical note on setup. Because Clostridium perfringens is anaerobic, the medium is made oxygen-free for this test, commonly by adding a piece of iron (such as a sterile nail or iron filings) or by layering sterile mineral oil over the surface. This is why some litmus milk tubes are shown with an oil layer on top.

Quality Control

• Fermentation: Clostridium perfringens (ATCC13124): gas production
• Acid: Lactobacillus acidophilus (ATCC11506): clot formation
• Peptonization: Pseudomonas aeruginosa (ATCC27853): Clearing

Procedure

1. Take a litmus milk medium and inoculate with four drops of 24-hour broth culture.

2. Incubate at 35°C to 37°C in ambient air.

3. Observe daily for seven days and record all changes.

   Look for alkaline reaction (litmus turns blue), acid reaction (litmus turns pink), litmus reduction, acid clot, rennet clot, and peptonization. Multiple changes can occur over the observation period.

Reading litmus milk, the abbreviation key

Litmus milk results are recorded using a set of standard abbreviations. Because several reactions can appear in the same tube, results are often written as a combination.

A single organism may show more than one of these in sequence over the seven-day observation period, so record changes each day rather than only at the end.

Litmus Milk Results and Interpretations

The appearance of Litmus Indicator

Figure: Litmus milk medium(From left to right: Alkaline reaction, uninoculated control, alkaline reaction (K), Digestion (peptonization) (D), acid and gas production (G), acid (A), and acid clots (AC). The clear fluid on the surface of the D and G tubes is a mineral oil used to make the medium anaerobic. (Image source: Ref-2)

Litmus milk reactions of common organisms

Different organisms produce different combinations of reactions, which is what makes litmus milk useful for differentiation. Some representative patterns:

These patterns are presumptive. The medium narrows the possibilities but does not identify an organism by itself, so litmus milk reactions are always confirmed with additional tests.

Appearance of Milk

Peptonization: The conversion of a protein into peptones under the influence of the enzyme pepsin.

Limitation

Litmus media reactions are not specific and should be followed up with additional tests for the definitive identification of microorganisms.

How to Remember

Pink is acid, blue is base. Litmus turns pink when acid is made from lactose, and blue when protein breakdown makes the medium alkaline. Pink from the sugar, blue from the protein.

Two clots, one test. An acid clot is made by acid and dissolves in alkali. A curd (rennet clot) is made by an enzyme and does not dissolve. Acid clot melts away in base, curd stays.

Stormy means a storm in the tube. Clostridium perfringens ferments lactose so hard that gas tears the clot apart. Picture a storm ripping through the clot.

White at the bottom is reduction. Reduced litmus is white, and it shows first in the lower, most oxygen-poor part of the tube.

Key exam facts

Where Students Get Confused

"An acid clot and a curd are the same thing." No. An acid clot forms because acid coagulates the casein, and it dissolves again in alkaline conditions. A curd (rennet clot) forms because an enzyme coagulates the casein, and it does not dissolve in alkali. The way to tell them apart is what happens in base.

"Stormy fermentation confirms Clostridium perfringens." Not on its own. It is highly characteristic and almost all strains produce it, but other vigorous gas-producing lactose fermenters can look similar. It is a strong presumptive clue, confirmed by further testing.

"Blue always means the organism failed to grow." No. Blue is an alkaline reaction, which is an active result. It usually means the organism is breaking down protein and releasing ammonia, not that nothing happened. No change (staying purple) is the true no-reaction result.

"White means contamination." No. White in the lower tube is litmus reduction, a genuine reaction. Reduced litmus is colorless, and it appears first where oxygen is lowest, at the bottom of the tube.

"Peptonization and acid clot are one reaction." No. They are opposite ends. An acid clot is casein coagulated by acid (pink). Peptonization is casein digested by enzymes (clear, blue, alkaline). One builds a clot, the other dissolves the protein away.

Resumen en español

La prueba de leche tornasolada usa leche descremada con el indicador tornasol (azolitmina) para diferenciar bacterias según cómo actúan sobre la leche. El tornasol es rosado en medio ácido, azul en medio alcalino y morado en medio neutro, y se vuelve blanco cuando se reduce. Las cuatro reacciones principales son: fermentación de la lactosa (ácido, color rosado), reducción del tornasol (blanco), coagulación de la caseína (formación de coágulo) e hidrólisis de la caseína o peptonización (líquido claro y color azul alcalino).

La fermentación tormentosa ocurre cuando la fermentación rápida de la lactosa produce ácido y mucho gas que rompe el coágulo. Es característica de Clostridium perfringens, aunque no es exclusiva de este microorganismo. Las reacciones de la leche tornasolada son presuntivas y deben confirmarse con pruebas adicionales.

References and further readings

1. Tille, P. M. (2022). Bailey and Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Leboffe, M. J., and Pierce, B. E. (2021). A Photographic Atlas for the Microbiology Laboratory (5th ed.). Morton Publishing.