## Intrinsic Resistance of Enterococci **Key Point:** Enterococci possess inherently low-affinity penicillin-binding proteins (PBPs), which confer intrinsic resistance to cephalosporins and most β-lactams (except ampicillin and penicillin at high doses). ### Mechanism of Resistance Enterococci have altered PBP structures that prevent optimal binding of cephalosporin molecules, making these agents ineffective even at high concentrations. This is a chromosomal, constitutive resistance mechanism present in all enterococci. ### Antibiotic Susceptibility Profile | Antibiotic Class | Susceptibility | Reason | |---|---|---| | Penicillins (high-dose) | Susceptible | Adequate PBP binding | | Ampicillin | Susceptible | Standard therapy | | Cephalosporins | Resistant | Altered PBP affinity | | Vancomycin | Susceptible | Cell wall inhibition via different mechanism | | Aminoglycosides (alone) | Resistant | Poor cell wall penetration | | Fluoroquinolones | Variable | Depends on species and strain | **High-Yield:** Enterococci are **NOT susceptible** to cephalosporins — this is a classic NEET PG trap. Do not use cephalosporins for enterococcal infections. **Clinical Pearl:** For serious enterococcal infections (e.g., endocarditis), ampicillin or penicillin is combined with gentamicin to achieve synergy. Vancomycin is used in ampicillin-resistant strains. [cite:Jawetz, Melnick & Adelberg's Medical Microbiology Ch 19]
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