10.2 Antibiotics

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Outline how penicillin acts on bacteria and why antibiotics do not affect viruses.
  2. Discuss the consequences of antibiotic resistance and the steps that can be taken to reduce its impact.

1. What Are Antibiotics?

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Antibiotics are chemical substances (drugs) used to treat bacterial infections. They work by either:

  • Killing bacteria — these are called bactericidal antibiotics (think: "cidal" = killing, like "homicide")
  • Stopping bacteria from growing — these are called bacteriostatic antibiotics (think: "static" = not moving/not growing)

Some antibiotics are naturally produced by living organisms. For example, penicillin is made by a type of fungus called Penicillium. Others are made synthetically (artificially) in a laboratory.

2. How Does Penicillin Work?

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Penicillin targets a very specific part of a bacterium — its cell wall.

What is the bacterial cell wall?

Bacteria have a rigid outer layer called a cell wall. This wall is made of a strong mesh-like material called peptidoglycan (say: pep-ti-do-gly-can). Think of peptidoglycan like a chain-link fence — it holds everything together and gives the cell wall its strength. The "links" in this fence are called cross-links.

What happens during normal bacterial growth?

When a bacterium grows, it must expand its cell wall to make room for the bigger cell. Here is the process step by step:

  1. The bacterium releases special enzymes called autolysins. These create small holes (gaps) in the existing cell wall so it can stretch and expand.
  2. New peptidoglycan molecules are added to fill in the gaps.
  3. An enzyme called transpeptidase joins the new peptidoglycan pieces together by forming cross-links — just like re-connecting the links in a fence.
  4. The cell wall becomes strong again.

How does penicillin disrupt this process?

Penicillin blocks transpeptidase. It does this by acting as a competitive inhibitor — this means penicillin has a shape very similar to the enzyme's normal target, so it "sits" in the enzyme's active site and prevents the enzyme from doing its job. (Think of it like a wrong key blocking a lock.)

Here is what happens when penicillin is present:

  1. Autolysins still create holes in the cell wall — the bacterium keeps trying to grow.
  2. New peptidoglycan is added, but because transpeptidase is blocked by penicillin, no cross-links can form.
  3. The cell wall becomes weaker and weaker with every growth attempt — it has holes but no proper reinforcement.
  4. Bacteria live in watery environments. Water enters the bacterial cell by osmosis (water moves from outside to inside because the concentration inside is higher). This creates enormous pressure inside the cell.
  5. The weakened cell wall cannot handle this pressure and bursts — the bacterium dies. This bursting is called lysis.

⚠️ Important: Penicillin only works on growing bacteria. Once a bacterium has finished growing, autolysins stop making holes and transpeptidase is no longer active — so penicillin has nothing to block.

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