For a bloodstream isolate, the MIC of vancomycin is 2 mg/L (within susceptible range) but the MBC is >32 mg/L. The ratio MBC/MIC exceeds 32. This phenomenon is termed:

• A Inoculum effect
• B Heteroresistance
• C Antagonism
• D Tolerance ✓

Explanation

Tolerance is defined when the MBC/MIC ratio exceeds 32, meaning the organism is inhibited at low concentrations but requires very high concentrations to be killed. This is clinically relevant in endocarditis and deep-seated infections where bactericidal activity is needed. Heteroresistance refers to a subpopulation with higher MIC within an otherwise susceptible isolate. The inoculum effect describes a shift in MIC when tested at high bacterial density.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.