A clinical microbiology lab receives a blood culture growing MRSA. The isolate reports vancomycin MIC of 1.5 mcg/mL (still technically 'susceptible'). The patient has persistent bacteremia despite 72 hours of vancomycin therapy. The infectious disease consultant recommends switching to daptomycin. What is the mechanism underlying this 'vancomycin heteroresistance' phenomenon?

• A Subpopulations of MRSA have acquired vancomycin-resistant genes from enterococci encoding D-Ala-D-Lac terminal residues
• B Vancomycin's large molecular size prevents adequate penetration of the thickened cell wall of hVISA strains, allowing a resistant subpopulation to survive ✓
• C MRSA accumulates excessive cell wall peptidoglycan precursors creating a 'cloaking' effect that sequesters vancomycin before it reaches its target
• D Persistent MRSA biofilm on the catheter continuously reseeds the bloodstream with organisms that are inherently resistant due to reduced metabolic activity

Explanation

Heteroresistant VISA (hVISA) strains have a thickened, altered cell wall with excess peptidoglycan that acts as a vancomycin 'sink' — the drug binds D-Ala-D-Ala residues in the outer layers of the thick cell wall and never reaches the transpeptidase sites at the cell membrane. The main susceptible population is killed but a small subpopulation (1 in 10^5 to 10^6 cells) with the thickened-wall phenotype survives. This is distinct from VRE-type resistance (D-Ala-D-Lac), which is the mechanism in full vancomycin-resistant S. aureus (VRSA). Daptomycin disrupts the cell membrane through a calcium-dependent mechanism, circumventing the cell-wall target entirely.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.