What could cause brown discoloration to the HPC agar plates?

Document ID

Document ID TE12684

Published Date

Published Date 05/25/2022
Question
What could cause brown discoloration to the HPC agar plates?
Summary
Explanation of the most common cause of brown discoloration with HPC.
Answer
When testing for Hetrotrophic Plate Count (which can be done with the following products: 2811415, 2811145-90, 2280734, and 2406720), the plates can develop a brown color.

This discoloration is most commonly caused by the agar plate having what's commonly referred to as a "lawn" of bacterial growth. In these cases the bacterial growth can be so heavy that the complete plate is covered in colonies that cannot be distinguished from each other. The color of the colonies can sometimes me misinterpreted as a change in color of the agar.

If this happens, it would be recommended to streak the agar plate with unfiltered sample to see if individual colonies grow, this will confirm bacteria growth.

After it is confirmed, it would be recommended to dilute the sample. The target range after dilution is 20-200 colonies per plate. Sample dilution instructions can be found in US ENG Heterotrophic Bacteria, Membrane Filtration Method 8242, m-HPC_DOC316.53.01192

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