Hi Sir

Simple and easy way to determine antimicrobial activity, first start with Agar Well Diffusion method, Agar disc Diffusion Method and Minimal inhibitory concentration against selected microorganism.

Check these publications on the subject:

Hi Sir,

In continuation to what Mr. Rajashekar has suggested, You can go for the higest possible concentration and from it follow double dilution technique. This will help you in both determinig MIC and the potentiality of the compound at different doses...

You can screen the activity of the extract with various type of microbes (eg gram +. gram -) on disc diffusion assay. Then if there is a clear inhibition zone, then you can proceed to determine its MIC and MLC values via broth dilution assay. These are the basic experiments that you can do. 

Start with 1mg of extract per disc for disc diffusion assay. 

Antimicrobial activity of any plant extract is verified on agar gel plates by serial dilution method keeping a standard ref.disc. However, researchers need to observe and note that  crude extract experiments many a times mislead results due to presence of many known organic acids present  and also due to seasonal and geo-botanical variability of herbs and plants  collected.

Most of the crude extracts show activity at in vitro level in the range of 10 microgram/ml to 1000 microgram/ml. You can make various dilutions in the suggested range and then can test using broth dilution assay or disc diffusion/ well diffusion assay. For disc diffusion assay, you will have to impregnate sterile paper discs with your herbal extracts.