QUALITY CONTROL
Escherichia coli ATCC 25922 : good growth; blue-green coloured colonies,
Citrobacter freundii ATCC 43864: growth; white to green beige colonies,
Enterococcus faecalis ATCC 29212: no growth.
Coliform bacteria are a common indicator of water quality and food hygiene. The
presence of these bacteria indicates potential fecal contamination and increases the
risk of infections associated with consumption of contaminated food and water.


COMPOSITION
Tryptone ..................................................................................... 20.0 g
Bile Salts No. 3 ............................................................................ 1.5 g
Agar-agar ................................................................................... 15.0 g
X-glucuronide ........................................................................... 0.075 g
Distilled water ad 1 litre
Adjust pH to 7.2 ± 0.2 @ 25°C,
Sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C and pour the medium
into sterile Petri dishes.
DESCRIPTION
TBX agar is a selective, chromogenic medium for the detection and enumeration of Escherichia coli in food and animal feed.  
Formula meets the  ISO 16649  and ISO 11133:2014. requirements.
TBX AGAR (TRYPTONE BILE X-GLUCURONIDE)
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PRINCIPLE
Most strains of
Escherichia coli can be differentiated from other coliforms by the presence of the enzyme glucuronidase. The
chromogen in TBX medium is 5-bromo-4-chloro-3-indolyl-beta-D-glucuronide (X-glucuronide) and is targeted by this enzyme.
E. coli
cells are able to absorb this complex intact and intracellular glucuronidase cleaves the bond between the chromophore and the
glucuronide. The released chromophore is colored and accumulates inside the cells, causing
E. coli colonies to be colored
blue-green.
Growth of the associated Gram-positive flora is largely inhibited by the bile salts composition and the high incubation temperature of
44 °C.
PROCEDURE
The medium is suitable for “Colony-count technique” using membranes or using
poured plate method and for “Detection and most probable number (MPN)”
technique. See the ISO 16649-1 , 16649-2 and 16649-3.
REFERENCES
1. Thermo Fischer Scientific. Tryptone Bile X-Glucuronide Medium (TBX)  leaflet. http://www.oxoid.com/UK/blue/prod_detail/prod_detail.
asp, accessed 07.2023.
2. Merck Millipore. Chromocult TBX (Tryptone Bile X‑glucuronide) Agar leaflet. www.merckmillipore.com/RO/ro/product/TBX-Tryptone-
Bile-X-glucuronide-agar,MDA_CHEM-116122, accessed 07.2023.
Beta-glucuronidase positive colonies on TBX agar
(
Escherichia coli)