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3.6.8Additional lessons

Antibiotics — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 77 of 142

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Part 1 of 5How Antibiotics Work

3.6.8 Antibiotics
Definition
  • Antibiotics are chemicals that kill bacteria or stop them from growing without seriously harming human cells.
Cell wall synthesis
  • Penicillin prevents bacteria from building their cell walls, so water enters by osmosis and the cell bursts.
Protein synthesis
  • Some antibiotics block bacterial ribosomes, which differ from the ribosomes of eukaryotic cells.
Selective toxicity
  • The drug targets processes that bacteria have and human cells lack, so it harms the bacterium but not the patient.

Part 2 of 5Why Antibiotics Do Not Work on Viruses

No metabolism
  • Viruses have no metabolism of their own, so they have nothing for antibiotics to block.
Inside host cells
  • Viruses live inside human cells where antibiotics cannot attack them without harming the host.
No cell wall
  • Viruses lack cell walls and ribosomes, so antibiotics that target these structures are ineffective.
Antivirals
  • Different drugs, called antivirals, are needed to treat viral infections.

Part 3 of 5Discovery of Penicillin

1
In 1928 Alexander Fleming noticed that bacterial growth on an agar plate was prevented around a contaminating mould.
2
The mould, a Penicillium species, was releasing a substance that stopped bacteria from growing.
3
Howard Florey and Ernst Chain later purified penicillin and tested it on infected mice, with those treated surviving.
4
Trials in humans followed, and mass production made penicillin widely available.
5
The case shows how an unexpected observation can lead to an important discovery, which is an example of chance in science.

Part 4 of 5Using Antibiotics Well

Using Antibiotics Well
Complete the course
  • Stopping early may leave surviving bacteria that can multiply again and become resistant.
Match the pathogen
  • Antibiotics should be used only for bacterial infections.
Testing
  • Controlled trials show whether a new antibiotic is safe and effective.

Part 5 of 5Exam Checklist & Tips

3.6.8 – Key Points for the Exam

  • Antibiotic A substance that kills or inhibits the growth of bacteria.
  • Penicillin An antibiotic from the mould Penicillium that blocks cell wall synthesis.
  • Selective toxicity Harming the pathogen but not the host.
  • Antiviral A drug that treats viral infection.

Common Mistakes

  • Saying antibiotics cure colds or influenza, which are caused by viruses.
  • Stating that penicillin kills by dissolving the bacterial cell membrane, when it prevents cell wall formation.
  • Thinking that antibiotics make the patient immune.
  • Writing that Fleming made penicillin into a medicine by himself; Florey and Chain developed it.

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