A 60-year-old woman has been hospitalized for three weeks with widely metastatic ovarian adenocarcinoma, and she becomes septic with vancomycin-resistant enterococcus. What is the mechanism of vancomycin resistance in this organism?
So, vancomycin resistance in enterococci, specifically VRE, is usually due to a change in the cell wall precursor. I remember that the resistance mechanism involves altering the D-Ala-D-Ala to D-Ala-D-Lac, which has a lower affinity for vancomycin. This is mediated by the VanA gene cluster, which encodes enzymes that modify the peptidoglycan. The VanA system is the most common in clinical isolates.
The options aren't provided, but based on common distractors, the correct answer would be the modification of the D-Ala-D-Ala terminus. Other possible wrong answers might include other resistance mechanisms like efflux pumps, enzymatic inactivation, or target site modification unrelated to the D-Ala-D-Ala change. For example, some might confuse it with beta-lactam resistance mechanisms, which involve beta-lactamase production or penicillin-binding proteins. But VRE specifically uses the D-Ala-D-Lac modification.
Clinical pearl: VRE resistance is a significant concern in hospitals, especially among immunocompromised patients. The key takeaway is that the resistance mechanism is structural alteration of the cell wall precursor, reducing vancomycin's binding affinity. This is different from other resistance mechanisms in other bacteria, so it's important to remember the specific pathway here.
**Core Concept**
Vancomycin-resistant enterococci (VRE) evade vancomycin by altering their cell wall precursor structure. Vancomycin typically inhibits peptidoglycan cross-linking by binding to D-Ala-D-Ala termini in Gram-positive bacteria. Resistance arises via a **D-Ala-D-Lac substitution**, reducing drug affinity.
**Why the Correct Answer is Right**
VRE, particularly *Enterococcus faecalis*/*faecium*, acquires resistance via the **VanA gene cluster**, which encodes enzymes that replace D-Ala at the terminal position of peptidoglycan precursors with D-lactic acid (D-Ala-D-Lac). This modification decreases vancomycin’s binding affinity by ~1,000-fold, rendering the antibiotic ineffective. The VanA system is plasmid-mediated and horizontally transferable, contributing to its spread in healthcare settings.
**Why Each Wrong Option is Incorrect**
**Option A:** *Beta-lactamase production* is incorrect because VRE does not inactivate vancomycin through enzymatic degradation. Beta-lactamases target beta-lactam antibiotics like penicillins.
**Option B:** *Efflux pump overexpression* is incorrect; vancomycin resistance in enterococci is not mediated by efflux mechanisms.
**Option C:** *Ribosomal modification* is incorrect; vancomycin targets cell wall synthesis, not ribosomes.
**Clinical Pearl / High-Yield Fact**
Always associate **VanA-mediated D-Ala-D-Lac substitution** with VRE resistance. This mechanism is