Thioglycollate Broth: Composition, Principle, Uses, and Reading Results

A blood culture is set up from a febrile patient with suspected bacteraemia. Two bottles are inoculated: one aerobic bottle and one thioglycollate broth bottle. After 48 hours, the aerobic bottle is clear. The thioglycollate broth shows turbidity at the bottom of the tube — discrete growth in the reduced anaerobic zone. Gram stain from the thioglycollate bottle reveals gram-positive rods. Clostridium perfringens bacteraemia — an organism that would never have grown in the aerobic bottle.

Thioglycollate broth is the most frequently used enrichment broth in diagnostic bacteriology because it does something no other standard medium does: it simultaneously supports organisms with completely different oxygen requirements in a single tube, stratifying them by position according to their oxygen needs.

Thioglycollate broth is the enrichment broth most frequently used in diagnostic bacteriology. This broth supports the growth of anaerobes, aerobes, microaerophilic, and fastidious microorganisms.  It contains many nutritive factors, including casein, yeast, beef extracts, and vitamins to enhance the growth of the most medically important bacteria. Other nutrient supplement includes; oxidation-reduction indicator (resazurin),  dextrose, vitamin K1, and hemin.

This medium also contains 0.075% agar to prevent convection currents from carrying atmospheric oxygen throughout the broth.  This agar supplement and the presence of reducing agent thioglycolic acid create an anaerobic environment deeper in the tube which allows anaerobic bacteria to grow.

Figure: Growth characteristics of various bacteria in thioglycollate broth

Composition of Thioglycollate broth

Ingredients per liter of deionized water:*

1. Pancreatic Digest of casein: 15 gm
2. Dextrose: 5.5 gm
3. Yeast extract: 5 gm
4. Sodium chloride: 2.5 gm
5. Sodium thioglycollate: 0.5 gm
6. L-Cystine: 0.5 gm
7. Resazurin: 1 mg
8. Agar 0.75 gm

Final pH 7.1 +/- 0.3 at 25°C.

Principles

1. Cystine and casein: They supply carbon and nitrogenous compounds,
2. Dextrose: It is added as another energy source
3. Yeast extract or papaic digest of soybean meal are added as growth enhancers.
4. Sodium chloride: It maintains osmotic equilibrium.
5. Sodium Thioglycollate: Sodium Thioglycollate is a reducing agent which maintains a low oxygen tension by removing molecular oxygen from the environment i.e., it creates anaerobic conditions when it reduces molecular oxygen to water. Peroxides, which may be lethal to many anaerobic organisms, are not formed under this condition.
6. Resazurin is an oxidation-reduction indicator that turns pink when increased oxidation has occurred, it is colorless when reduced.
7. Agar: The addition of a small amount of agar in the Thioglycollate medium aids in the initiation and growth of small inocula and anaerobes by impeding the diffusion of oxygen into the medium. It also retards the dispersion of CO2 and the reducing substance from the microenvironment surrounding the inoculum.

Certain additives can be incorporated into the Thioglycollate medium as desired:

1. Hemin is incorporated to supply X-factor for the stimulated growth of many fastidious organisms,
2. Vitamin K is a growth requirement for some gram-positive spore-formers and Bacteroides species.
3. The calcium carbonate chip is added to act as a buffer for the medium and prevents a buildup of toxic acid.

Preparation and Inoculation of Thioglycollate broth

Thioglycollate broth is best prepared from ready-to-use dehydrated powder, available from most suppliers of culture media.

• The medium is usually used at a concentration of 29.5g in every liter of distilled water (concentration may vary depending on the manufacturer).
• Prepare as instructed by the manufacturer. Dispense the well-mixed medium in 50 ml amounts in bottles fitted with screw caps that have a central hole and rubber liner. Sterilize by autoclaving (with caps loosened) at 121°C for 15 minutes.
• When cool, tighten the bottle caps. Cover each bottle top with a foil cap or other protective covering (previously soaked in 70% v/v ethanol).
• Label the bottles. Date the medium and give it a batch number. Store in a cool dark place.
• Shelf-life: Up to 2 years providing there is no change in the volume or appearance of the medium to suggest oxidation or contamination.
• Important: if at the time of use, more than a narrow band at the surface of the medium appears pink, this indicates oxidation, and the broth should not be used. It should be reheated by placing the bottle in a container of boiling water for 15 minutes (with the bottle cap loosened) to drive off the dissolved oxygen.

Inoculation

• Inoculate the medium following the aseptic technique.
• Written inoculation instructions should be issued with each bottle of the medium.
• Once inoculated, the broth should be incubated at 35-35°C as soon as possible.

Uses of Thioglycollate broth

1. Blood culture for suspected bacteraemia — especially anaerobic bacteraemia

Thioglycollate broth is used as the anaerobic blood culture bottle in laboratories without automated blood culture systems. The blood is diluted at least 1:10 with broth to prevent clotting and to dilute the blood's natural bactericidal substances (complement, lysozyme, antibodies).

Important: Sodium polyanethol sulphonate (SPS — the standard anticoagulant in most blood culture media) is inhibitory to anaerobic streptococci (Peptostreptococcus) and should NOT be added to thioglycollate broth when anaerobic infection is suspected. Use gelatin or another anticoagulant if needed, or use SPS-free broth.

2. Determination of oxygen requirements

Thioglycollate broth is the standard medium for determining whether an unknown isolate is an obligate aerobe, facultative anaerobe, microaerophile, aerotolerant anaerobe, or obligate anaerobe — by observing where in the tube growth occurs relative to the pink resazurin band.

3. Enrichment of fastidious organisms from clinical specimens

Thioglycollate supports the growth of fastidious organisms (with hemin and vitamin K supplementation) that may be present in small numbers in clinical specimens — particularly from normally sterile sites (CSF, joint fluid, peritoneal fluid).

4. Sterility testing

Thioglycollate broth is one of the two standard media specified by the United States Pharmacopeia (USP) for sterility testing of pharmaceutical products, biologics, and medical devices — along with Soybean-Casein Digest (SCD) broth. It detects anaerobes and facultative anaerobes; SCD detects aerobes and fungi.

5. Transport and short-term preservation of anaerobes

Specimens can be inoculated directly into thioglycollate broth at the collection site for short-term transport to the laboratory, providing a reduced environment that maintains anaerobe viability during transit.

Growth Characteristics of various bacteria in thioglycollate broth

A. Gram-negative, facultatively anaerobic bacilli (i.e., those that can grow in the presence or absence of oxygen) generally produce diffuse, even growth throughout the broth.

B. Gram-positive cocci frequently grow as discrete “puffballs”.

C. Strict aerobic bacteria (i.e., require oxygen for growth), such as Pseudomonas spp., tend to grow toward the surface of the broth,

D. Strictly anaerobic bacteria (i.e., those that cannot grow in the presence of oxygen) grow at the bottom of the broth.

Reading results in Thioglycollate broth

Figure: oxygen requirements of bacteria and media

A: For isolating bacteria  from Blood Culture

Examine daily (up to 14 days) for visible signs of bacterial growth such as turbidity above the layer of red cells, colonies growing on the surface of the red cells (cotton balls), hemolysis, gas bubbles, and clots. Remember, not only anaerobes but also other bacteria having diverse oxygen requirements grow in thioglycollate broth. When there are visible signs of bacterial growth, subculture the broth and examine a toluidine blue stained smear for bacteria.

B: For finding oxygen requirements of various bacteria

Results are read after 48 hours of incubation of the inoculated test tubes. When oxygen diffuses near the top of the broth a pink band develops (Remember: Oxidation-Reduction indicator Resazurin is pink when oxidized and colorless when reduced). The absence of pink in the rest of the tube indicates the absence of oxygen and a suitable environment for strict anaerobes. The growth of the organisms in different regions of the test tubes will depend on the oxygen requirement characteristics of those organisms:

1. Strict aerobes will grow only in the pink band,
2. Microaerophiles will grow near the bottom of the band where the concentration of oxygen is lower.
3. Both facultative anaerobes and aerotolerant anaerobes will grow throughout the tube. Facultative anaerobes grow throughout the medium but most densely near the top where oxygen is available — they can grow without oxygen but prefer it. Aerotolerant anaerobes grow evenly throughout the tube — they neither require nor are harmed by oxygen, so they distribute uniformly regardless of the oxygen gradient.
4. The aerotolerant grows equally well throughout the tube. Usually, this test should be read at about 48 hrs although some slower growing microbes may take more incubation time.

Interpreting the Resazurin Pink Band

The pink band position is the key to reading thioglycollate results correctly:

Normal tube appearance: Pink band at the top 10–20% of the tube; colourless (reduced) for the remainder. This gradient is maintained by the 0.075% agar (which prevents convection currents from mixing the oxygen in) and the thioglycollate reducing agent (which continuously scavenges oxygen in the lower portion).

If more than 20–30% of the tube is pink: The broth has been oxidised — oxygen has penetrated too deeply. This broth must not be used for anaerobic culture. Reheat by placing in boiling water for 15 minutes (cap loosened) to drive off dissolved oxygen, then cool before use.

Growth pattern summary:

Troubleshooting Thioglycollate Broth

How to Remember: Thioglycollate Broth

"Pink = oxygen = aerobic zone; colourless = reduced = anaerobic zone": The resazurin indicator is the map of the tube. Pink at the top = oxygen present. Colourless below = anaerobic. Organisms grow where their oxygen requirement is met.

The five growth patterns — from top to bottom:

1. Top only (pink band) = Obligate aerobe (Pseudomonas)
2. Just below pink = Microaerophile (Campylobacter)
3. Throughout, denser at top = Facultative anaerobe (E. coli)
4. Throughout, evenly = Aerotolerant anaerobe (Streptococcus pyogenes)
5. Bottom only = Obligate anaerobe (Bacteroides, Clostridium)

The puffball clue: Gram-positive cocci grow as discrete "puffballs" in thioglycollate — round, fluffy clumps rather than diffuse turbidity. This appearance prompts Gram stain before subculture.

The SPS rule: No SPS in thioglycollate when anaerobic streptococci are suspected — SPS inhibits them.

Key Exam Facts in One Table

References and further readings:

1. Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 14th ed. Elsevier; 2023.
2. Cheesbrough M. District Laboratory Practice in Tropical Countries, Part 2. 2nd ed. Cambridge University Press; 2006.
3. Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2020.
4. Garcia LS. Clinical Microbiology Procedures Handbook. 4th ed. ASM Press; 2016.
5. United States Pharmacopeia. <71> Sterility Tests. USP-NF. Rockville, MD: USP; 2023.