I analyzed two chocolate samples for S. aureus using 3M Staph Express. After 24 hours my plates were filled with blue-green and black colonies, so I used the Express Disk. After 1-3 hours my plates had these large yellowish zones. For what I've read in the 3M interpretation guide, the zones have to be pink to be considered S. aureus, so I don't know what to make out of said result. I have linked a picture of the plates, hope it's clear enough.
Thanks in advance.
Dear Javier Amores
The picture you are showing does not refer to colonies of S.aureus. On 3M Staph normally only red violet colonies are considered other colonie types (blue dark etc ) are not Staphylococcus aureus colonies. Please do not expect to find colonies of S. aureus or any other strain in a food sample even if you suspect the presence of any strain you are searching. BUT please trust your result as it is. Your petri film do not contain S. aureus (negative).
Dear Javier Amores.
Although the plates used are specific to S aureus, Staphylococcus can also grow other, but should be reported as unquantifiable or not found in the lowest dilution that did.
Annex a photo where you can see colonies of S aureus and other Staphylococcus. They are samples of cheese
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