The following results are obtained from a broth dilution test for microbial susceptibility of a bacterial infection in a patient: Antibiotic Growth in culture + drug Transfer to drug-free

Question

Antibiotic Growth in culture + drug Transfer to drug-free culture concentration

50 μg/ml No growth No growth

25 μg/ml No growth , Growth

12.5 μg/ml No growth , Growth

6.25 μg/ml Growth , Growth

What is the minimum inhibitory concentration (MIC) for this bacterial pathogen? Explain how you

arrived at your answer.

What is the minimum bactericidal concentration (MBC) for this bacterial pathogen? Explain how you

arrived at your answer

Answer 1

The minimum inhibitory concentration (MIC) is 12.5 μg/ml. This is the lowest concentration at which there is no visible growth of the bacteria in the presence of the antibiotic.

1. At 12.5 μg/ml, there is no visible growth in the culture.
2. At 6.25 μg/ml, there is growth.

Therefore, the MIC = 12.5 μg/ml, since it’s the lowest concentration where growth is inhibited.

The minimum bactericidal concentration (MBC) is 50 μg/ml. This is the lowest concentration at which there is no bacterial regrowth after transfer to drug-free media, indicating the bacteria were killed rather than just inhibited.

1. At 50 μg/ml, there’s no growth in the culture, and no regrowth after transfer ➜ bacteria were killed, not just inhibited.
2. At 25 μg/ml, there’s no growth in drug, but growth after transfer ➜ bacteria were inhibited but not killed.
3. So, 25 μg/ml did not kill the bacteria, only inhibited them.

Therefore, the MBC = 50 μg/ml, since this is the lowest concentration where the bacteria are not only inhibited but also killed (no regrowth after transfer).