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Cambridge IGCSE Biology · 0610

Chapter 15: Drugs

Definition of a drug

Drug
Any substance taken into the body that modifies or affects chemical reactions in the body.

Different types of drugs are used to treat various diseases.

Antibiotics

Use
Antibiotics are used specifically for the treatment of bacterial infections.
Mechanism of action
  • Some antibiotics kill bacteria by destroying their cell wall, which causes the cell to burst.
  • Other antibiotics work by inhibiting the growth of the bacteria.
Antibiotics and viruses
Antibiotics do not affect viruses. This is because viruses do not have the same structure as bacteria and do not grow or reproduce in the same way.

Exam Traps

  • Avoid saying antibiotics boost antibody production — they act directly on bacteria.

Antibiotic resistance

Problem
Some bacteria have become resistant to antibiotics, which significantly reduces their effectiveness in treating infections.
Natural selection
Bacterial strains become resistant through a process of natural selection:
  1. A mutation occurs in a bacterial cell, making it resistant to a specific antibiotic.
  2. When that antibiotic is used, the resistant cell survives while the non-resistant cells are killed.
  3. The surviving resistant cell reproduces, passing on the resistance to its offspring and creating a population of resistant bacteria.
MRSA
Resistance leads to serious infections in hospitals, such as MRSA (methicillin-resistant Staphylococcus aureus).

Exam Traps

  • Do not say the antibiotic causes the mutation — mutations occur randomly; antibiotics select survivors.
  • Avoid describing resistance as a change in the human immune system — it is a bacterial population change.

Slowing the development of resistance

To limit the development of resistant bacteria like MRSA, it is essential to:

  • Use antibiotics only when essential for serious infections.
  • Always complete the full course of antibiotics to ensure that all bacteria are killed, preventing any from surviving to develop resistance.
Three-stage flowchart of antibiotic resistance development showing mutant survival, antibiotic killing normal bacteria, and resistant population growth
Diagram 1: Development of antibiotic resistance. A three-stage flowchart shows (1) a population of bacteria with one resistant mutant, (2) the application of antibiotics killing the normal bacteria, and (3) the resistant bacterium multiplying to form a new, resistant population.

Exam Traps

  • Avoid using antibiotics for mild viral illnesses — unnecessary use speeds resistance development.

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