It is used to compare a selective medium (zb. XLD) with a non- selective medium (zb. TSA).
XLD should grow Salmonella well and there should be no-more than 0.5 log10 difference in the plate counts compared to a non-selective medium like TSA.
A selective medium must be selective, and should inhibit the growth of non-pathogens. A reduction of 2 log10 in plated count on the selective medium when compared to a nonselective medium has been suggested as a standard.
This is covered in an ISO standard on microbiological methods. I think the number is ISO6358. Pd.
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