Hi Rene,

In my experience, several antimicrobial compounds are not soluble in water and therefore you need to use various solvents such as DMSO, DMF, ethanol...

When doing this, the idea is to dissolve the compound in a very concentrated form, like 100x the working concentration. You can then use this ultra concentrated, yet dissolved stock to generate your testing dilutions. In this case, the dilutions can be made ( using either water, growth media, etc). This will give you a good range of compound concentrations while limiting the exposure to solvents needed to dissolve in the first place.

If you give a few more details about how you are setting up your dilutions, what the drug is, perhaps others will have more information to add.

Good luck!