If you are trying to dissolve an fat-soluble substance for anti-microbial assay you can use Tween 80 for that...I don't think non polar solvents like hexane ect. are non-toxic to bacterial/fungal cells
DMSO can dissolve a lot of polar and non-polar compounds and is quite miscible. It is routinely used in yeast transformation, hence is relatively non-toxic
I work with nonpolar microbial secondary metabolites with antimicrobial activities. The nonpolar antimicrobials were solubilized in 1% DMSO and/or 1% Tween 80.
The goal of the assay is start preparing a stock 10x the initial concentration of microbial bioassay by first placing the DMSO or Tween, mix to compound and then add water or cultive middle of bioassay to obtain the final concentration.