Brilliant Green Agar (BGA) is a specialized culture medium widely used in microbiology, particularly for isolating and identifying Salmonella species (except S. Typhi and S. Paratyphi). Its distinct composition and features make it an invaluable resource in clinical and food microbiology contexts.
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What is Brilliant Green Agar ?
BGA is a selective medium primarily used for the isolation of Salmonella species, except for S. Typhi and S. Paratyphi. It inhibits the growth of most gram-positive and gram-negative bacteria while allowing Salmonella species to grow.

Composition of BGA
Ingredients | (g/l) |
Peptone | 5.00 |
Tryptone | 5.00 |
Yeast extract | 3.00 |
Lactose | 10.00 |
Sucrose | 10.00 |
Sodium chloride | 5.00 |
Phenol red | 0.080 |
Brilliant green | 0.0125 |
Agar | 20.00 |
Final pH (at 25°C): 6.9±0.2
Principle of Brilliant Green Agar
Amino acids, carbon, nitrogen, vitamins, and minerals are all provided by peptones for the growth of organisms. Sodium chloride keeps the medium’s osmotic equilibrium intact. Vitamins, especially B-group vitamins, can be found in yeast extract.
The carbohydrates that can ferment are lactose and sucrose. The substrate for decarboxylase is lysine. The pH indicator is called phenol red. With the exception of Salmonella species, brilliant green is the selective agent that inhibits Gram-positive and the majority of Gram-negative bacteria. The solidifying agent is agar.
Preparation of BGA
Materials Needed
- Brilliant Green Agar powder: 58.09 grams
- Purified/distilled water: 1000 milliliters
- Autoclave
- Sterile Petri plates
- Stirring equipment
- Heating source (e.g., Bunsen burner, hot plate)
- Thermometer
- Protective gear (e.g., gloves, lab coat)
Procedure
- Weigh the Brilliant Green Agar powder (58.09 grams).
- Combine the 58.09 grams of BGA powder with 1000 milliliters of pure or distilled water.
- Stir thoroughly to ensure that the powder is equally distributed.
- Heat the mixture to boiling, then stir constantly until the medium is completely dissolved.
- Transfer the dissolved media to suitable containers.
- Autoclave at 121°C (15 psi) for 15 minutes. Avoid overheating.
- Allow the medium to cool to 45-50°C while monitoring the temperature with a thermometer.
- Mix the cooled medium thoroughly to ensure equal dispersion of all components.
- Pour the medium into sterilized Petri plates and cover the bottom to a depth of 4-5 millimeters.
- Allow the agar to solidify at room temperature.
- Label the Petri plates and keep them in a cold, dry location or refrigerate if not used right away.
Result Interpretation on BGA
Organisms | Growth |
Salmonella typhimurium | Good growth; red-colored |
Salmonella Abony | Good growth; pinkish white |
Salmonella Enteritidis | Good growth; pinkish white |
Salmonella Typhi | Poor-good; reddish pink |
Staphylococcus aureus | Inhibited |
Escherichia coli | Inhibited or no growth; yellowish-green |
Enterococcus faecalis | No growth to light growth; yellowish colonies with or without yellow halos |
Uses of BGA
- Salmonella spp. isolation from clinical samples: This includes stool, urine, and other body fluids.
- Food safety testing: Salmonella detection in food items.
- Environmental monitoring: Checking for Salmonella contamination in environmental samples and water.
- Diagnostic testing: Used in microbiology labs to detect the presence of Salmonella in food products or stool samples that may be contaminated.
Benefits
Selective: Effectively promotes the recovery of Salmonella by suppressing the development of non-target organisms.
Differential: By using color variations in the media, it enables a clear visual differentiation to be made between Salmonella and other bacteria.
Practical: Simplifies the procedure of identifying and isolating Salmonella from complicated samples.
Limitations
Some Salmonella strains are inhibited: On this medium, certain strains, such as S. Typhi and S. Paratyphi, may not grow well.
Potential false positives or negatives: Atypical reactions from certain bacteria may call for additional testing, such as serological or biochemical analysis.
Frequently Asked Questions (FAQ)
Can Brilliant Green Agar be used to detect all types of Salmonella?
Brilliant Green Agar works effectively for isolating most Salmonella species, although it does not detect Salmonella Typhi or Salmonella Paratyphi. These strains may not grow well on this medium because they are sensitive to the selection agents present. Other specialized medium may be necessary for these specific Salmonella strains.
Why is it important to avoid overheating Brilliant Green Agar during preparation?
Overheating Brilliant Green Agar during preparation can damage some of its delicate components, including the brilliant green color and other nutrients. This can diminish the medium’s efficacy in suppressing undesirable bacteria and differentiating Salmonella species, resulting in inconsistent outcomes. The autoclaving process must be carefully controlled to ensure the medium’s integrity.
Can Brilliant Green Agar be used directly with environmental samples?
Yes, Brilliant Green Agar can be used to identify Salmonella in environmental samples. However, pre-treating or enriching the samples increases the likelihood of isolating Salmonella. Enrichment processes may include using a selective broth before plating on BGA, which increases the amount of Salmonella cells while suppressing other bacteria.
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