I will suggest looking at the following open access publication: Statistical characterisation of bacterial wild-type MIC value distributions and the determination of epidemiological cut-off values: https://doi.org/10.1111/j.1469-0691.2006.01377.x
Pharmacodynamics (PD) relates concentration of an antibacterial to effect, specifically to its antimicrobial effect, and its goal is to find a dose of drug that has a high probability of therapeutic success while also trying to balance it with acceptable toxicity.
Therefore, what matters most is the antimicrobial susceptibility of the target bacterial population as a whole. In that respect MIC50 and MIC90 values provide only a limited insight into which isolates might represent the wild-type population and which isolates harbour acquired resistance.
Another suggested reading is:
Antimicrobial pharmacodynamics: critical interactions of ‘bug and drug’, Nature Reviews Microbiology: http://dx.doi.org/10.1038/nrmicro862