The minimum concentration of antimicrobial agent at which resistant strains have a competitive advantage.

At the MSC, the benefit of bacterial growth exceeds the fitness cost of carrying the drug-resistance trait compared with a non-resistant strain. At concentrations greater than the MSC, there is a competitive advantage for having the resistance trait. The difference in fitness between susceptible and resistant organisms at sub-MIC values could be much lower than the MIC against some antibiotics.