The feature to watch for is an increasing steepness to the slope of selection curves as the drug concentration gets high. You see that this is true for the two examples shown (panel A is erythromycin treatment of S. aureus (solid) and M. smegmatis (open); panel B is penicillin treatment of the same two organisms). We emphasize that increasing steepness does not always occur and that for many cases MPC is not below the maximum serum level. Thus being able to dose above MPC may be an unusual situation. That would mean that combination therapy would be required for most pathogens.