Hi, the solvent DMSO not inhibit growth of microorganisms ..I think (2.5%) not effect. But high concentration may increase permeability of cytoplasm membrane of microorganism and give false results.
When I was working in research about antimicrobial activity of vegetal compounds, we assessed activity of esential oils by well diffusion in agar plates inoculated with bacteria using a cotton swab. Oils were diluted in DMSO and water and 100% DMSO was used as control.
I never saw growth inhibition against E. coli, S.aureus, B. subtilis and P. aeruginosa but we never tried with diluted DMSO.
First of all you should test this yourself by conducting a pilot study on one gram positive and on gram negative bacterial strain using different concentrations of DMSO including with the concentration you are looking for your final experiment. you can do that in just a single experiment using two MH agar plates after lowing of overnight bacterial culture (Gram positive and Gram negative) on each plate respectively) making wells on plates, by labeling the wells accordingly and purring of DMSO to each well with respect to its concentration. you can check it out next day after overnight incubation. More over you can use propylene glycol as solvent for non polar compounds as it has found with no inhibitory effect in many peer reviewed publications.
What is the final percentage of DMSO will determine whether it will have any effect ok bacterial growth. For example, If you add 50 uL bacterial culture to 50 uL 100% DMSO, your bacterial should never grow.
Please kindly may you send some references to support that "I used 100% DMSO against S. aureus, E. coli, A. flavus, but there was no activity against any of the microbes." Thanks in advance.