Among the listed antibiotics, which is bactericidal?

• A. Tobramycin
• B. Chloramphenicol
• C. Tetracycline
• D. Erythromycin

Answer: (A)

Bactericidal agents kill bacteria rather than merely inhibiting their growth. Tobramycin, an aminoglycoside, is bactericidal because it irreversibly binds the 30S ribosomal subunit, causing misreading of the mRNA and incorporation of faulty proteins, which leads to rapid bacterial cell death. Its killing is concentration-dependent, so higher drug levels produce faster and more extensive killing. The other drugs listed are typically bacteriostatic: chloramphenicol and erythromycin bind the 50S subunit to block peptide elongation, and tetracycline binds the 30S to prevent tRNA from entering the ribosome; these actions halt growth but do not directly kill the bacteria in most circumstances.