The agent Erysipelothrix rhusiopathiae is best treated with which antibiotic?

• A. Tetracycline
• B. Erythromycin
• C. Vancomycin
• D. Penicillin

Answer: (D)

Penicillin is the best first-line treatment because Erysipelothrix rhusiopathiae is reliably susceptible to beta-lactam antibiotics that inhibit bacterial cell wall synthesis. Penicillin works by binding penicillin-binding proteins and blocking cross-linking of peptidoglycan, which compromises the cell wall and leads to bacterial death. This mechanism is particularly effective for this Gram-positive organism, giving a consistent and rapid clinical response.

Other antibiotics can be active in some cases, but their effectiveness is more variable. Tetracyclines and macrolides may work for certain strains, but resistance and inconsistent susceptibility limit their reliability. Vancomycin is effective against Gram-positives but is typically reserved for severe infections or when penicillin cannot be used, due to cost, IV administration, and broader coverage that isn’t necessary for this pathogen.