Toxicity of a chemical compound towards bacteria can be determined visually using modified broth micro-dilution methods: Iodonitrotetrazolium violet-formazan crystalline (INT) or with the yellow tetrazolium salt 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT).
Ref:
Lewies, A., Wentzel, J.F., Jordaan, A., Bezuidenhout, C., Du Plessis, L.H. 2017. Interactions of the antimicrobial peptide nisin Z with conventional antibiotics and the use of nanostructured lipid carriers to enhance antimicrobial activity. International journal of Pharmaceutics. 526:244-53
You can also conciser LIVE/DEAD™ BacLight™ Bacterial Viability Kit, for microscopy and flow cytometry