I need to carry out antibacterial activity testing by disc diffusion or agar well diffusion followed by MIC by agar microdilution. The sample I have is a nanocomposit which doesn't dissolve in any of the solvents such as water, methanol, DMSO, ethyl acetate etc. In such cases how do I carryout these tests? Preliminary study has shown that if I use the suspension directly, antibacterial acrivity is observed. But since the substance has not dissolved in the solvent how can the concentration of sample be expressed?
One approach would be to make a suspension of the material in molten agar, so that it is evenly distributed within the agar, then pour it into a plate to solidify. Each plate (or each well of a multi-well plate) would contain a different concentration. The MIC would then be determined as the lowest concentration on which there was no growth.
Another approach would be to do the test in a liquid broth suspension, with the test substance maintained in suspension by rapid shaking throughout the test period.
If you really want to do a classic antibiotic testing then I concur with Adam B Shapiro although I would recommend doing the assay as a MIC 50 assay in liquid culture. I worry that with plates if the substance is not at all diffusing then the interpretation of the results may be problematic.
However I am not sure if this is really the right approach at all for a material that is likely to be used on a surface. At least I assume that is the intention otherwise why develop and insoluble agent. There should be a literature on developing antimicrobial surfaces and those would likely have procedures.
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