Hello Juande,

A microbiological challenge test is done to determine what will happen to a final package product if it were to be contaminated by a microbe. In the context of testing preservatives: first you inoculate your product with a specific microbe, get aliquots at specific intervals, recover surviving microbes, and then compute for the reduction or increase of surviving microbes.

For cosmetic products, you can further check this online resource: http://www.qacslab.com/challenge-testing-cosmetics.php

I suggest you can use skin bacteria for your purposes.

Best,

Arizaldo