Learning Objective

Explain the principle, procedure, and interpretation of the minimal inhibitory concentration (MIC) for antibiotic susceptibility testing.

Principle

• MIC measures the lowest concentration of an antibiotic that inhibits visible bacterial growth.
• It is a quantitative test to determine the effective inhibitory concentration for a specific bacterial isolate.
• MIC does not necessarily indicate bactericidal activity; killing is measured separately as the MBC (Minimal Bactericidal Concentration).

Procedure

• Prepare serial dilutions of an antibiotic in test tubes, wells of a microtiter plate, or automated system bottles.
• Inoculate each container with a standardized suspension of the patient’s bacterial isolate.
• Include a growth control with bacteria but no antibiotics to ensure viability.
• Incubate under appropriate conditions.
• Determine the lowest antibiotic concentration with no visible growth. This is the MIC.

Key Points

• MIC provides quantitative data for clinical decision-making.
• Often used to guide dosage and antibiotic selection.
• Standardized interpretation charts (e.g., CLSI) classify bacteria as susceptible, intermediate, or resistant based on MIC.

Activity