This TBX Chromogenic Medium (Tryptone Bile X-glucuronide Agar) is a dehydrated powder formulation designed for the rapid detection and enumeration of Escherichia coli in food, water, milk, and environmental samples. The medium utilizes the chromogenic substrate X-β-D-glucuronide (BCIG), which is hydrolyzed by the β-glucuronidase enzyme produced by E. coli, releasing a chromogen that dyes the colonies blue-green .
This color reaction allows E. coli to be distinguished from other coliforms and accompanying flora, which typically appear colorless or yellow. The selective properties of the medium, combined with the chromogenic reaction, enable presumptive E. coli identification within 18–24 hours, significantly faster than traditional multi-step enrichment and confirmation procedures.
The 36.6g bottle format is suitable for laboratories preparing their own plates and is ideal for routine food safety testing, water quality monitoring, and hygiene verification.
Intended Use
- Rapid detection and enumeration of Escherichia coli in food and beverage samples.
- Water and wastewater microbiological testing.
- Milk and dairy product testing.
- Environmental monitoring and hygiene verification.
- Research and educational laboratory applications.
Composition (per Liter, as Dehydrated Basis)
- Special Nutrients: 21.4 g
- Agar: 15.0 g
- Chromogenic Reagent: 0.2 g
- Final pH: 6.8 ± 0.2 at 25°C
Interpretation of Results
- Blue-green colonies: Presumptive Escherichia coli (β-glucuronidase positive) .
- Colorless colonies: Other coliforms or non-target organisms.
- Yellow or no growth: Non-target organisms; Gram-positive bacteria are generally inhibited.
Quality Control (Growth Performance)
Validated at 36 ± 1°C for 18–24 hours:
- Escherichia coli ATCC 25922 (30–100 CFU): Blue-green colonies, good growth.
- Enterobacter aerogenes ATCC 13048 (30–100 CFU): Colorless colonies.
- Enterobacter cloacae ATCC 13047 (30–100 CFU): Colorless colonies.
- Salmonella typhimurium ATCC 14028 (30–100 CFU): Colorless colonies.
- Listeria monocytogenes ATCC 19114 (10⁴–10⁵ CFU): Inhibited; if growth occurs, colonies are colorless.
- Staphylococcus aureus ATCC 25923 (10⁴–10⁵ CFU): Inhibited; if growth occurs, colonies are colorless.
Storage and Shelf Life
- Format: 36.6 g/bottle, dehydrated powder.
- Shelf Life: 2 years from date of manufacture.
- Storage: Store in a dry, dark place at 2–8°C. Keep tightly closed to prevent moisture absorption. Protect from light.
Q: What is TBX medium used for?
A: TBX (Tryptone Bile X-glucuronide) Agar is a chromogenic medium used for the rapid detection and enumeration of Escherichia coli in food, water, and environmental samples. E. coli produces blue-green colonies through the action of β-glucuronidase on the chromogenic substrate .
Q: How quickly can results be obtained?
A: Presumptive E. coli colonies are visible after incubation at 36 ± 1°C for 18–24 hours, allowing rapid turnaround compared to traditional methods.
Q: Why do E. coli colonies appear blue-green?
A: E. coli produces the enzyme β-glucuronidase, which cleaves the chromogenic substrate X-β-D-glucuronide. The released chromogen is absorbed by the bacterial cells, producing the characteristic blue-green color .
Q: What if colonies appear colorless?
A: Colorless colonies are likely other coliforms or non-target organisms that do not produce β-glucuronidase. Only blue-green colonies are counted as presumptive E. coli.
Q: How should I prepare the medium?
A: Weigh 36.6 g of powder, add to 1,000 mL of distilled water, heat to boiling with agitation for no more than 1 minute until dissolved, autoclave at 121°C for 15 minutes, and cool to 50–55°C before pouring plates .
Q: What is the shelf life and storage condition for the dehydrated powder?
A: The shelf life is 2 years from the date of manufacture when stored at 2–8°C in a dry, dark place. Keep the bottle tightly closed to prevent moisture uptake.
