What proportion of the initial inoculum is typically killed to define the MBC?

What proportion of the initial inoculum is typically killed to define the MBC?

• A. 90%
• B. 95%
• C. 99%
• D. 99.9%

Answer: (D)

The key idea is how much killing defines the minimum bactericidal concentration. MBC is the lowest drug concentration that reduces the viable bacterial count by 99.9% after a standard incubation. That 99.9% reduction corresponds to a 3-log10 drop in colony-forming units, so if you started with 10^6 CFU/mL, you’d aim for about 10^3 CFU/mL remaining. To determine this, you expose bacteria to increasing drug concentrations, then subculture from the clear tubes onto drug-free agar; the smallest concentration that produces no growth defines the MBC. This strict 99.9% kill criterion distinguishes truly bactericidal activity from mere growth inhibition, ensuring that the antibiotic can nearly eliminate the initial population.

The key idea is how much killing defines the minimum bactericidal concentration. MBC is the lowest drug concentration that reduces the viable bacterial count by 99.9% after a standard incubation. That 99.9% reduction corresponds to a 3-log10 drop in colony-forming units, so if you started with 10^6 CFU/mL, you’d aim for about 10^3 CFU/mL remaining. To determine this, you expose bacteria to increasing drug concentrations, then subculture from the clear tubes onto drug-free agar; the smallest concentration that produces no growth defines the MBC. This strict 99.9% kill criterion distinguishes truly bactericidal activity from mere growth inhibition, ensuring that the antibiotic can nearly eliminate the initial population.