If you do not find black colonies, or fewer colonies,  on the agar plate by the analyses of Clostridium perfringens or Sulfite-reducing bacteria, it can have several causes.
Checking the below items in your laboratory to solve the issue.

Is the media fit for purpose?
Did you perform a media performance test according to ISO 11133, and is the media fit for the purpose?

Food analyses:   
Media performance test TSC (SC)

Media performance test ISA (IS or ‘TS’)

Water analyses:
Media performance test TSC

Media performance test Sulfite Iron – Tryptose Sulfite (TS)
Did you perform a membrane filter validation according to ISO 7704, and is the filter fit for the purpose?

If the media and or membrane filters are not fit for purpose, try to use a different batch of media or filter. Or try another media of membrane filter supplier.

Was there an overlay used?
Cultures that are not overlaid with agar will have no or poor black colonies.

Are the incubation conditions correct?
The analysis must be performed under strictly anaerobic conditions.
You can check the anaerobic jar with an anaerobic indicator strip.

CRM-CPE.00007MH (Food analyses)  – TSC (SC) with and without an overlay                            

TSC (SC) agar with and without overlay

CRM-CPE.00007BL (Water analyses) – TSC with and without an overlay

CRM-CPE.00007BL (Water analyses) – Sulfite Iron-Trypose Sulfite (TS) with and without an overlay


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