Notes on Mechanism of Action of Tetracyclines, Aminoglycosides, and Macrolides, and MCQ for NEET PG, GPAT and NORCET Exams
1. TETRACYCLINES
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Target: 30S ribosomal subunit of bacteria.
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Action:
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Tetracyclines bind reversibly to the 30S subunit.
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They block the attachment of aminoacyl-tRNA to the mRNA-ribosome complex.
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This prevents addition of new amino acids to the growing peptide chain → Stops protein synthesis.
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Result:
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Bacteriostatic (inhibits bacterial growth, does not kill directly).
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2. AMINOGLYCOSIDES
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Target: 30S ribosomal subunit (different binding site compared to tetracyclines).
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Action:
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Bind irreversibly to 30S ribosome.
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Cause:
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Misreading of mRNA → Wrong amino acids inserted → Abnormal proteins.
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Early termination of protein synthesis.
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Formation of nonfunctional proteins.
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Additional:
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Disrupt bacterial cell membrane integrity.
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Result:
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Bactericidal (kills bacteria directly).
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Important: Aminoglycosides require oxygen for uptake — not effective against anaerobes.
3. MACROLIDES
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Target: 50S ribosomal subunit of bacteria.
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Action:
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Macrolides bind reversibly to the 50S subunit.
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They inhibit translocation — prevent movement of the peptidyl-tRNA from the A site to the P site.
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As a result, protein elongation is blocked.
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Result:
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Bacteriostatic, but bactericidal at high concentrations.
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Summary Table
| Class | Target Subunit | Binding Type | Effect on Protein Synthesis | Bacteriostatic/Bactericidal |
|---|---|---|---|---|
| Tetracyclines | 30S | Reversible | Block tRNA binding | Bacteriostatic |
| Aminoglycosides | 30S | Irreversible | Misreading + membrane damage | Bactericidal |
| Macrolides | 50S | Reversible | Inhibit translocation | Bacteriostatic (mostly) |
MCQs on Mechanism of Action
1. Tetracyclines inhibit bacterial protein synthesis by:
A) Causing misreading of mRNA
B) Blocking translocation of peptidyl tRNA
C) Preventing binding of aminoacyl-tRNA to ribosome
D) Inhibiting DNA gyrase
➔ Answer: C) Preventing binding of aminoacyl-tRNA to ribosome
2. Aminoglycosides exhibit bactericidal action because they:
A) Reversibly bind to 30S ribosomal subunit
B) Cause misreading of mRNA and disrupt cell membrane
C) Inhibit cell wall synthesis
D) Block tRNA binding reversibly
➔ Answer: B) Cause misreading of mRNA and disrupt cell membrane
3. Which ribosomal subunit is the target for macrolides?
A) 30S
B) 40S
C) 50S
D) 60S
➔ Answer: C) 50S
4. Which of the following antibiotics requires oxygen for uptake into bacteria?
A) Tetracyclines
B) Aminoglycosides
C) Macrolides
D) Chloramphenicol
➔ Answer: B) Aminoglycosides
5. Which of the following drugs inhibits the translocation step in protein synthesis?
A) Doxycycline
B) Gentamicin
C) Erythromycin
D) Streptomycin
➔ Answer: C) Erythromycin