CHROMOGENIC E. COLI AGAR (CHROMOGENIC TRYPTONE BILE GLUCURONIDE AGAR) (TBX AGAR)

TBX Agar is based on Tryptone Bile Agar. Tryptone Bile Agar was originally formulated to improve on earlier methods used to detect Escherichia coli in foods in terms of speed, reliability, better recovery from frozen samples and the detection of poor lactose fermenters.
TBX medium builds on these advantages through the addition of a chromogenic agent – X-glucuronide – which detects glucuronidase activity. This is the same enzyme detected by MUG reagent, and has been shown to be highly specific for E. coli. However, approximately 3-4% of E. coli are glucuronidase negative, notably E. coli O157 strains.
Most E. coli strains can be differentiated from other coliforms by the presence of the enzyme glucuronidase. The chromogen in TBX Agar 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 splits the bond between the chromophore and the glucuronide. The released chromophore is coloured and builds up within the cells, causing E. coli colonies to be coloured blue/green.

for enumeration of Escherichia coli from food samples, animal feed and water samples