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) 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) as blue colonies. However, Escherichia coli O.157:H7 doesn’t do this and gives colorless colonies.
Causes of non-conforming results in appearance are caused by the media ingredients. 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 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 colony 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
CCA of 6 different suppliers
TBX of 3 different suppliers