I am currently working on evaluating the antifungal activity of the cell-free supernatant from certain probiotic strains. For this, I need to use a positive control plate. In our laboratory, we have Nystatin available, but according to the manufacturer, it is not water-soluble. As we do not have access to any other antifungal agents, I would appreciate your guidance on how to prepare Potato Dextrose Agar (PDA) with Nystatin under these conditions.
Also, what will be the working concentration of Nystatin
As Nystatin is soluble in ethanol, I suggest preparing a concentrated solution in ethanol. After the PDA has come out of the sterilisation process, when it is about 70°C, add the required microlitres of Nystatin-ethanol to the bottle and then prepare the agar plates. At this temperature Nystatin is stable and the ethanol evaporates quickly.
Hope this has been useful.
Kind regards
Nystatin solutions can be prepared by dissolving 70 mg of nystatin into 2 mL of DMSO. For different purposes concentration may vary. Selective medium can have 25 mkg/ml of nystatin.
I would highly suggest making "solvent only" negative control plates just in case the ethanol or DMSO have any side effects on your strains and their growth.
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