Agreed with Diane.  You need to determine the concentration of the plant extract.  If it was made from powder form this would calculated by mass of extract/ volume diluted with.  If made from a liquid you could determine weight from the molecular weight if the concentration is not given.  If actually extracted, you'd need to use an appropriate method to determine the concentration of the extract.

Weigh the vessel in which you will be collecting the extract. Suppose the weight of the vessel is 100 g. The vessel can be a round bottom flask (if you are gonna use a rotavapor), or a conical flask of 25-50mL capacity (if you intend to lyophilize the sample). Collect the plant extract in the vessel and dry it by lyophilizing or using rotavac. The methodology adopted depends on the solvent in which your plant extract is suspended. Dry the extract and then again weigh the vessel. The vessel would now weigh a little higher (ex. 100.01 g) The difference (0.01 g) is the weight of the extract. scrape out the extract and suspend it in 1 mL (or higher, depends on you) of a solvent such as ethyl acetate/hexane/methanol/DMSO, etc. You need to determine in which solvent does your extract dissolves the best. Since you intend to use for some antimicrobial test, better suspend it in DMSO as ethyl acetate and methanol themselves show antimicrobial activity, thereby resulting in false positives. 

Good luck!