Culture Media Training Academy: Effect of Overweighing When Preparing Media

September 14, 2019

One reason for the potential failure of culture media performance is the effect of overweighing when the media is prepared. Find out how this can look, and how to avoid it.

Video Transcript

One reason for the potential failure a bacteriological performance of culture medium is the over-concentration of the formulation by overweighing.

Here we have trytone bile glucuronide agar, also known as TBGA or TBX.

The agar is for the identification and numeration of E. coli.

E. coli grows as blue colonies on this agar.

Here I have prepared a poor plate test where the sample is incorporated into the agar before it sets. If prepared correctly, you can see we have healthy bright blue colonies throughout the agar.

As we go across I have increased the concentration of the powder to simulate overweighing. You can see as we increase the concentration we start to see a suppression of the target organism.

As we go across you see reduction in number and color intensity of E. coli.

This occurs because by overweighing a selective agar such as this one, we effectively increase the selective pressure on the microorganism. In this case, the bile in the formulation reaches levels which ecoli are no longer tolerant to.

This causes suppression and even inhibition of the target organism leading to performance failure and potentially misreporting of a sample.

To prevent this, users should refer to manufacturer's preparation guidelines and keep good records of material weight.


Category: Solution Spotlights, Laboratory, University & Research, Microbiology