The TBX plate count method (ISO 16649-2), referred to as “alternative method”, was compared with the EU prescribed ISO 16649-3.

PRINCIPLE OF THE METHOD: TBX (Tryptone Bile X-Glucuronide agar) is a selective chromogenic agar medium used as a pour-plate for the enumeration of Escherichia coli, as described in ISO 16649-2:2001. The chromogenic substance in TBX is 5-bromo-4-chloro-3-indolyl-beta-D-glucuronide (X-glucuronide). 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. A 1:1 dilution of shellfish meat and internal fluid in sterile peptone physiological salt solution is analysed by pour-plating 10 ml into 5 TBX plates (2 ml in each plate).

REFERENCE METHOD:ISO 16649-3:2005 Microbiology of food and animal feeding stuffs - Horizontal method for the enumeration of ß-glucuronidase-positive E. coli - part 3: Most probable number technique using 5-bromo-4-chloro-3-indolyl-ß-D-glucuronide (the Donovan MPN method),

SCOPE: Bivalve molluscs