You should be able to use the same solvents you used to make the extract in the first place in order to prepare the disks. The acetic acid will mostly evaporate. You can prepare a control disk with the same solvent to make sure the residual solvent is not inhibiting bacterial growth. However, if there are completely water-insoluble substances on the disk, then they will not diffuse well into the agar, potentially resulting in a false-negative (assuming they are active).

Hi

Please read this article at following link, may be helpful

Regards

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3506936/

Hi Adam, I tried with Acetic acid and even used as a control but in negative control as well I am getting inhibition zone. They should be completely soluble but I am seeing some visible particles of the extract in the solvents like Ethanol, DMSO,Water (for Withania somnifera).