Try to make serial dilutions up to 1 to 0.00000000001mg/ml and use them one by one in toxicity bio-assays and expose bacterial colonies in culture. You will get two doses on ID50 (Inhibitory dose 50), another dose will be LC 50 that may kill 50% of bacterial colonies. It can be found by suspension culture and LB agar plate method.
Alamar Blue Assay, is the best one to use, although many people use MTT but I found MTT can react directly with the plant extracts so can give you false results, Also BRdU ELISA method is good.