DMF or DMSO itself has antimicrobial properties. So, before dissolving need to maintain a minimal concentration of DMF or DMSO.  

No don't try this, be sure to dissolve your extracts in solvant konwing to have no antimicrobial properties 

As suggested by Akm F Haque, the concentration of the diluent should be as minimum as possible for biological assays. 

So what is the minimum concentration of DMSO to be used

IIn all cases i make a negative control using the solvent

And how to explain that all my results were negative when i used ethanol although i'm sure the extract has antimicrobial activity 

Dear Samar,

kindly follow the question shared below which was asked long back on research gate for your further querries.

What is the min DMSO concentration to dissolve unknown drugs?

Thanks to you all

I think that depends on the plant extract concentration. Generally for antimicrobial test researchers are using 2% of DMSO. If it's unknown drug then you have to try it from lower % up to 2% to see its solubility. Hope it will help you. Best of luck

I dont think that you can use upto 2% because 2% DMSO is able to kill the cells as well. In my opinion, if you use DMSO to dissolve your plant extracts  then the final concentration of it should be maximum 0.1% or lower than that. I also use DMSO for some of my cell based work and never used more than 0.1% in its final concentration.