Learning Objective

Explain the principle, procedure, and clinical significance of determining the minimal bactericidal concentration (MBC) of an antibiotic.

Principle

• The MBC measures the lowest concentration of an antibiotic that kills bacteria, rather than just inhibiting growth.
• It is determined by subculturing from MIC dilution tubes onto antibiotic-free solid media to assess bacterial viability.

Activity

Procedure

• Start with the MIC dilution series for the bacterial isolate.
• Subculture a small inoculum from each MIC tube onto solid agar without antibiotics.
• Incubate to allow surviving bacteria to grow.
• Identify the lowest concentration that shows no colony growth on the agar. This is the MBC.

Key Points / Clinical Relevance

• MBC is always ≥ MIC, as killing requires at least the inhibitory concentration.
• Important for treating immunocompromised patients who may not be able to clear bacteria that are merely inhibited.
• Helps determine whether an antibiotic is bactericidal (kills bacteria) or bacteriostatic (inhibits growth).

Activity