Escherichia coli O.157:H7 and Escherichia coli O.157 (non-VTEC) give a false negative result for Escherichia coli on the CCA and TBX because the colonies do not appear characteristic.

Colonies of Escherichia coli appear on CCA (Chromogenic Coliform Agars) according to ISO 9308-1 as dark-blue to violet colonies. However, Escherichia coli O.157:H7 doesn’t do this and gives red colonies.
Colonies of Escherichia coli appear on TBX (Tryptone Bile X-glucuronide) according to ISO 16649-2 as blue colonies. However, Escherichia coli O.157:H7 doesn’t do this and gives colorless colonies.

The media ingredients cause causes of non-conforming results in appearance. Depending on the absence or lack of certain chromogenic agents for either β-galactosidase or β-glucuronidase, colonies will be colored a certain color. Some Escherichia coli (for instance, Escherichia coli O.157:H7) appear to be β-glucuronidase negative, but β-galactosidase positive and will therefore appear as coliform bacteria. Further identification tests should be performed to determine what kind of Escherichia coli this is.

For more information on chromogenic agents and colony color, visit  Comparison of four β-glucuronidase and β-galactosidase based commercial culture methods used to detect Escherichia coli and total coliforms in water

Biosisto has tested this with Certified Reference Materials CRM-STEC and CRM-EC.O157 (non-VTEC) on media of different suppliers. In the pictures below, you can see the results.

CCA of 6 different suppliers


TBX of 3 different suppliers


 

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  • Athanasios Boumpalos says: Reply

    Due to the significance of the presence of Escherichia coli O.157:H7 in food, in terms of public health, there have been developed ISO and AFNOR methods along with various chromogenic media and specific instrumental analytical methods.

    • Annemarie says: Reply

      Thanks you for your addition!

      This article informs you that you are not detecting all Escherichia coli strains when using CCA or TBX. If you want to detect STEC specifically, use ISO 13136 or an alternative that you can find at AFNOR.

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