Antibiotics Quiz

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Which statement best describes the minimum inhibitory concentration, or MIC, of an antibiotic?

A The concentration of antibiotic achieved in the blood after a standard dose
B The lowest antibiotic concentration that prevents visible bacterial growth in vitro ✓
C The lowest antibiotic concentration that kills all bacteria in vitro
D The concentration of antibiotic that inhibits bacterial growth at the site of infection
E The concentration of antibiotic that prevents the emergence of resistant mutants in vitro

The answer is B: The lowest antibiotic concentration that prevents visible bacterial growth in vitro

Explanation

Why B is correct

See MIC and Breakpoints. Option B is correct. The minimum inhibitory concentration (MIC) is the lowest antibiotic concentration that prevents visible bacterial growth in vitro (in the laboratory, in a tube, plate or broth, rather than in a patient). Microbiology laboratories measure it and then compare the MIC with breakpoints: threshold values that take account of how the drug behaves in the human body (its pharmacokinetics, meaning how much drug reaches the blood and tissues over time). This lets the laboratory report the organism as susceptible (S), intermediate (I) or resistant (R) to the antibiotic at standard doses. A low MIC suggests that the organism is easily inhibited; a high MIC suggests a drug that is less likely to work at the usual dose. The MIC says that growth has stopped, but not that the bacteria are dead.

Definition: The MIC is the lowest antibiotic concentration that prevents visible bacterial growth in vitro.
Breakpoints: Thresholds that let the laboratory report the organism as susceptible, intermediate or resistant.
Interpretation: A low MIC suggests easy inhibition, while a high MIC suggests the drug is less likely to work at the usual dose.
Limit: It shows that growth has stopped, not that the bacteria are dead.

Why the other options are incorrect

A. The concentration of antibiotic achieved in the blood after a standard dose: The concentration reached in the blood after a dose is a pharmacokinetic measurement. It is compared with the MIC to decide whether a dose is likely to work, but it is not what the MIC means.
C. The lowest antibiotic concentration that kills all bacteria in vitro: This describes the minimum bactericidal concentration (MBC), which is a different laboratory measurement of killing rather than growth inhibition. The MIC only shows that growth has stopped, not that the bacteria have died.
D. The concentration of antibiotic that inhibits bacterial growth at the site of infection: MIC is an in vitro laboratory measure against a standardised inoculum, not the concentration achieved at an infection site. Tissue concentrations depend on pharmacokinetics and vary between patients.
E. The concentration of antibiotic that prevents the emergence of resistant mutants in vitro: This describes the mutant prevention concentration rather than the MIC. The MIC is the lowest concentration that prevents visible growth of the original bacterial inoculum, not the threshold for preventing resistant mutants.

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1
Which statement best describes the difference between bacteriostatic and bactericidal antibiotics?
2
A 23-year-old woman is prescribed trimethoprim for an uncomplicated urinary tract infection. Which of the following best describes the primary mechanism of action of trimethoprim in bacteria?

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