Accepting innovation: chromogenic media in reference methods

TBX Chromogenic Agar (Tryptone Bile X-Glucuronide) is based on Tryptone Bile Salts Agar medium, which is normally used to detect and enumerate Escherichia coli in foods. In addition to this base, TBX chromogenic agar adds a chromogenic agent, x-ß-D-glucuronide, which makes it possible to detect the presence of the enzyme glucuronidase, highly specific for E. coli.   

The color from the chromophore released by the agar makes the target colonies easier to identify. To release this color, the bacteria must absorb the chromogenic agent x-ß-D glucuronide and the intracellular glucuronidase enzyme activity breaks the bond between the chromophore and the glucuronide. 

The released chromophore is thus accumulated inside the cells, causing the colonies of Escherichia coli to present a distinctive blue-greenish color.    

This formulation is contemplated in the ISO 16649 standard, which describes a horizontal method for the enumeration of E. coli beta-glucuronidase positive in products intended for human consumption or animal feeding.