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                            

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

Biosisto strongly advises storing the CRM at a temperature of -80˚C. Only when using this storage temperature, the concentration on the Certificate is valid and the CRM is guaranteed stability.

You can download the Certificate of Analysis at https://www.biosistochart.com/login

If you are logged in, you can find the Certificate under the menu ‘Chart Setup’.

1. Choose ‘Batch certificates’
2. Search for product and batch

If you do not (yet) have an account, the Certificate can be found under the button ‘View Batch Certificates’ or the button ‘Certificates’ on the start page.

1. Go to the product and type and search the batch number or product name
2. Search for product and batch

All certificates are always available electronically.

Only when the method is precisely the same as described on the Certificate can you compare the results with de property value on the Certificate of Analysis. Another technique, media, or incubation temperature, can give other effects and can provide another result.

Checklist for root cause analyses:

1. Media – the most common cause of differences

Check if the used media and possibly used filters are fit your purpose. Therefore, performance testing of culture media according to ISO 11133 and performance testing of membrane filters according to ISO 7704 can be performed.

Search on the BiosistoStandard which kind of controls must be used as a minimum.

2. Incubation

Check the incubation temperature and time if they are correct.

3. Used equipment

Check if the used equipment is fit for the purpose. For example, is the pipette calibrated.

4. frequency

How often does this problem occur? Is this structural or is it an incident issue?

5. Comparison with other labs

How do your results compare with other laboratories? An easy way to compare your results with other laboratories is to use the BiosistoChart. Ask for a free account for the basic functions

If you cannot find a cause for the deviating CRM value as a result of the above questions, please do not hesitate to contact Biosisto.

Biosisto has five different types of CRM Salmonella in stock. The choice depends on what the requirements of the laboratory are and what the laboratory prefers.

In the table below, you will find the differences per Salmonella.

Find more information in the related link below.