Which antibiotic is bactericidal?

• A. Penicillin G
• B. Erythromycin
• C. Tetracycline
• D. Clindamycin

Answer: (A)

The key idea is the distinction between killing bacteria and simply stopping their growth. Penicillin G is bactericidal because it disrupts the bacterial cell wall. It blocks the enzymes that cross-link peptidoglycan, which weakens the cell wall and, in actively dividing bacteria, leads to osmotic lysis and death.

The other antibiotics shown are typically bacteriostatic—they stop bacteria from growing by inhibiting protein synthesis rather than causing immediate cell-killing. Erythromycin and clindamycin bind to the 50S ribosomal subunit and block peptide chain elongation (translocation), halting production of proteins. Tetracycline binds the 30S subunit and prevents attachment of aminoacyl-tRNA, also halting protein synthesis. While there are exceptions at high concentrations or against certain organisms, they are generally taught as bacteriostatic.