Minimum Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC), and Post Antibiotic Effect (PAE) with examples and MCQ GPAT, NEET PG

Definitions with Examples

1. Minimum Inhibitory Concentration (MIC):

Definition:
MIC is the lowest concentration of an antimicrobial agent that inhibits visible growth of a microorganism after overnight incubation.

Example:
If an E. coli strain stops growing at 2 µg/mL of ampicillin but grows at 1 µg/mL, then the MIC is 2 µg/mL.

2. Minimum Bactericidal Concentration (MBC):

Definition:
MBC is the lowest concentration of an antimicrobial agent required to kill 99.9% of the bacterial population.

Example:
If the bacteria are inhibited at 2 µg/mL (MIC), but only killed at 8 µg/mL, then the MBC is 8 µg/mL.

3. Post-Antibiotic Effect (PAE):

Definition:
PAE is the persistent suppression of bacterial growth even after the antibiotic has been removed or its concentration has fallen below the MIC.

Example:
After exposure to gentamicin for 1 hour, bacterial growth remains suppressed for several hours even after the drug is removed—this is the PAE.

Minimum Inhibitory Concentration (MIC)

Definition:

The Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antimicrobial agent that inhibits the visible growth of a microorganism after a specified period, usually overnight incubation (16–20 hours).

Purpose:

• To determine the lowest effective dose of an antimicrobial.

• Helps in selecting the most potent antibiotic for treatment.

Methods to Determine MIC:

1. Broth Dilution Method:

   • Serial dilutions of antibiotic are prepared in a nutrient broth.

   • A standardized amount of bacteria is added.

   • After incubation, the lowest concentration with no visible growth is recorded as the MIC.

2. Agar Dilution Method:

   • Antibiotics are mixed into agar plates at different concentrations.

   • A standard number of bacteria are spotted on the surface.

   • Plates are incubated and the MIC is the lowest concentration with no bacterial colonies.

3. E-test (Epsilometer test):

   • A plastic strip with a gradient of antibiotic concentration is placed on an inoculated agar plate.

   • MIC is read where the inhibition ellipse intersects the strip.

Clinical Significance:

• MIC helps to classify bacteria as:

  • Sensitive (S)

  • Intermediate (I)

  • Resistant (R)

• MIC is used to guide antibiotic dosing regimens and formulary decisions.

Example:

If a strain of Staphylococcus aureus shows:

• Growth at 0.25 µg/mL,

• But no growth at 0.5 µg/mL,
  Then MIC = 0.5 µg/mL

2. Minimum Bactericidal Concentration (MBC)

Definition:

The Minimum Bactericidal Concentration (MBC) is the lowest concentration of an antibacterial agent that results in 99.9% killing of the initial bacterial population.

How it Differs from MIC:

• MIC prevents growth but may not kill the bacteria.

• MBC ensures killing, and is usually higher than MIC.

Determination:

• After finding MIC, the tubes showing no visible growth are subcultured onto antibiotic-free agar.

• The lowest antibiotic concentration that results in no colony growth is the MBC.

Interpretation:

• If MBC is close to MIC → Antibiotic is bactericidal

• If MBC is much higher than MIC → Antibiotic is bacteriostatic

Example:

If MIC = 2 µg/mL
But killing occurs only at 8 µg/mL,
Then MBC = 8 µg/mL

3. Post-Antibiotic Effect (PAE)

Definition:

The Post-Antibiotic Effect (PAE) is the persistent suppression of bacterial growth after a short exposure to an antibiotic, even after the drug concentration falls below the MIC or is removed entirely.

How PAE Works:

After being exposed to an antibiotic for a short time:

• Bacteria may not resume growth immediately even when the drug is gone.

• This delayed regrowth time is called the PAE duration.

Mechanisms of PAE:

1. Slow recovery from non-lethal damage.

2. Persistence of drug-target interaction.

3. Post-exposure stress responses in bacteria.

Measurement:

• Compare the time it takes for antibiotic-exposed and non-exposed bacterial cultures to increase 10-fold in viable count.

• PAE = T (treated) – C (control)

Clinical Importance:

• Allows for less frequent dosing of antibiotics (especially with long PAE).

• Important in antibiotics like aminoglycosides and fluoroquinolones, which are administered once-daily due to long PAE.

Example:

Gentamicin shows a long PAE against Gram-negative bacteria like E. coli, allowing once-daily dosing.

Comparison Table:

Summary Points:

• MIC helps identify the lowest dose to stop bacterial growth.

• MBC determines the dose needed to kill the bacteria.

• PAE shows how long bacterial growth remains suppressed after the antibiotic is removed.

• All three are vital tools in choosing and optimizing antibiotic therapy.

• What does MIC stand for?
  a) Maximum Inhibition Count
  b) Minimum Inhibition Capacity
  c) Minimum Inhibitory Concentration
  d) Maximum Infection Concentration
  Answer: c

• Which value indicates bacterial death, not just inhibition?
  a) MIC
  b) PAE
  c) MBC
  d) EC50
  Answer: c

• What is the relationship between MIC and MBC for bactericidal drugs?
  a) MIC is always higher
  b) MBC is equal or slightly higher than MIC
  c) MIC is unrelated to MBC
  d) MBC is always lower than MIC
  Answer: b

• The term ‘post-antibiotic effect’ refers to:
  a) Bacteria becoming resistant
  b) Continued suppression of bacteria after antibiotic removal
  c) Immediate killing of bacteria
  d) Drug degradation
  Answer: b

• Which antibiotic typically shows a strong PAE against Gram-negative bacteria?
  a) Penicillin
  b) Gentamicin
  c) Tetracycline
  d) Vancomycin
  Answer: b

• If the MIC of a drug against a bacterium is 1 µg/mL, what happens at 0.5 µg/mL?
  a) Bacteria are killed
  b) Bacteria multiply
  c) Growth is inhibited
  d) DNA is destroyed
  Answer: b

• MBC is determined by:
  a) Checking zone of inhibition
  b) Subculturing on antibiotic-free media
  c) ELISA test
  d) PCR test
  Answer: b

• Which of the following best describes a bacteriostatic drug?
  a) MBC = MIC
  b) MBC >> MIC
  c) MIC > MBC
  d) No MIC or MBC
  Answer: b

• PAE is influenced by:
  a) Duration of exposure
  b) Type of microorganism
  c) Concentration of antibiotic
  d) All of the above
  Answer: d

• Which test is commonly used to determine MIC?
  a) ELISA
  b) Disk diffusion
  c) Broth dilution
  d) Western blot
  Answer: c