outline how penicillin acts on bacteria and why antibiotics do not affect viruses

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Antibiotics

Antibiotics

Learning Objective

Outline how penicillin acts on bacteria and explain why antibiotics do not affect viruses.

1. How Penicillin Works on Bacteria

Penicillin belongs to the β‑lactam class of antibiotics. Its primary target is the bacterial cell wall.

  1. Target enzyme – Penicillin‑Binding Proteins (PBPs): These are transpeptidases that catalyse the cross‑linking of peptidoglycan strands during cell‑wall synthesis.

  2. Binding mechanism: The β‑lactam ring of penicillin mimics the D‑alanine‑D‑alanine terminus of the peptidoglycan precursor. It forms a covalent acyl‑enzyme complex with the active site serine of PBPs, permanently inactivating the enzyme.

  3. Resulting effect:

    • Inhibition of peptidoglycan cross‑linking.

    • Weakening of the cell wall.

    • Osmotic lysis of the bacterium, especially during active growth and division.

Suggested diagram: Penicillin binding to a penicillin‑binding protein and preventing peptidoglycan cross‑linking.

2. Why Antibiotics Do Not Affect \cdot iruses

Viruses differ fundamentally from bacteria in structure and replication strategy, making most antibiotics ineffective against them.

FeatureBacteriaVirusesRelevance to Antibiotics
Cellular organizationProkaryotic cells with membrane, cytoplasm, ribosomes, and cell wallAcellular particles; consist of nucleic acid + protein coat (and sometimes envelope)Antibiotics target cellular processes (e.g., cell‑wall synthesis, protein synthesis) absent in viruses.
Genetic materialDNA chromosome in cytoplasmDNA or RNA genome, often single‑stranded, packaged in capsidAntibiotics that interfere with DNA replication (e.g., quinolones) act on bacterial enzymes not present in viruses.
MetabolismIndependent metabolism; synthesises its own proteins and nucleotidesMetabolically inert; relies on host cell machinery for replicationMetabolic inhibitors (e.g., sulfonamides) have no target in viruses.
ReproductionBinary fission (cell division)Replication inside host cell using host enzymesAntibiotics that disrupt cell division (e.g., penicillin) are irrelevant to viruses.

3. Summary Points

  • Penicillin blocks the transpeptidase activity of PBPs, preventing peptidoglycan cross‑linking and causing bacterial lysis.

  • Viruses lack a cell wall, ribosomes, and independent metabolic pathways; therefore, the molecular targets of most antibiotics are absent.

  • Effective antiviral agents must target virus‑specific processes (e.g., viral polymerases, proteases, entry mechanisms), not the bacterial targets used by antibiotics.