Antimicrobial Drug Concentration and Bacterial Growth Test

What does it mean if a subculture of an MIC test grows in an MBC test?

Choose the correct statement based on the data:

a. The concentration of the drug was bactericidal

b. The concentration of the drug was bacteriostatic

Answer:

The presence of bacterial growth in an MBC test following an MIC test indicates that the antimicrobial drug concentration was not bactericidal but bacteriostatic at the MIC level.

If a subculture from a minimal inhibitory concentration (MIC) test shows growth in a subsequent minimal bactericidal concentration (MBC) test, it means that the concentration of the drug was not bactericidal at the MIC level. The MIC measures the lowest concentration of an antimicrobial drug that inhibits visible bacterial growth. After determination of the MIC, samples from broths that showed no growth (potentially bacteriostatic effect) are subcultured onto agar plates without the antibiotic to measure the MBC.

The MBC is the lowest concentration that kills ≥99.9% of the starting inoculum. For a drug to be considered bactericidal, the bacterial cells from the MIC assay should not grow in the MBC assay, confirming the drug at that concentration has a bactericidal action. Therefore, growth in an MBC test signifies that the concentration was not high enough to kill the bacteria, hence the drug acted in a bacteriostatic manner at the MIC level.