Antibiotics – Explanations & Mechanisms

Understanding Antibiotics and Their Mechanisms

This page offers explanations and insights into how different antibiotics work, organized by their mechanism of action. Use this information to reinforce learning from your quiz or table.

1. Inhibitors of Cell Wall Synthesis

These antibiotics prevent bacteria from forming peptidoglycan cross-links, weakening the cell wall and leading to lysis. Only effective against actively dividing cells.

• Penicillin & Cephalosporins – β-lactam antibiotics that inhibit transpeptidase enzymes.
• Vancomycin – Binds to D-Ala-D-Ala residues, preventing polymer cross-linking.

2. Disruptors of Cell Membranes

Target the integrity of bacterial membranes, particularly in Gram-negative bacteria.

• Polymyxins – Bind LPS and disrupt outer membranes.
• Daptomycin – Inserts into Gram-positive membranes, causing depolarization.

3. Protein Synthesis Inhibitors

Block bacterial ribosomes (which differ from eukaryotic ones), halting protein production.

• 30S Subunit Inhibitors – Tetracyclines (block tRNA binding), Aminoglycosides (cause misreading of mRNA).
• 50S Subunit Inhibitors – Macrolides, Chloramphenicol, Lincosamides (prevent translocation or peptide bond formation).

4. DNA/RNA Synthesis Inhibitors

Interfere with nucleic acid replication or transcription.

• Fluoroquinolones – Inhibit DNA gyrase and topoisomerase IV.
• Rifampicin – Blocks bacterial RNA polymerase.

5. Metabolic Pathway Inhibitors

Act as antimetabolites, typically blocking folic acid synthesis which bacteria must produce themselves.

• Sulfonamides – Compete with PABA for DHPS enzyme.
• Trimethoprim – Inhibits dihydrofolate reductase (DHFR).

6. Anti-Mycobacterial Agents

Specifically target *Mycobacterium* species (e.g., TB).

• Isoniazid – Inhibits synthesis of mycolic acids.
• Ethambutol – Inhibits arabinogalactan synthesis in the cell wall.

Note: Human cells are spared by most antibiotics because our cells do not contain peptidoglycan or 70S ribosomes, making antibiotics selectively toxic.

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