A drug is any substance that changes the chemical reactions in your body. Antibiotics are the drugs that cure bacterial infections such as pneumonia and infected wounds, but they are useless against viruses, and every unnecessary dose helps resistant bacteria such as MRSA to spread.
A drug is any substance taken into the body that modifies or affects chemical reactions in the body. Antibiotics are drugs used to treat bacterial infections: they kill bacteria (or stop them growing) but they do not affect viruses. Some bacteria are resistant to antibiotics, which reduces how effective antibiotics are. Extended: using antibiotics only when they are essential limits the development of resistant bacteria such as MRSA.
Fig. 1: a few bacteria in a population are resistant; the antibiotic kills the sensitive ones, so the resistant survivors multiply and become most of the population
Section 01
What counts as a drug.
In biology the word drug is wider than the medicines in a pharmacy. Any substance taken into the body that modifies or affects the chemical reactions in the body is a drug, whether it is a medicine, a stimulant or a poison.
Definition: a drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Medicinal drugs: taken to treat or prevent disease or to relieve symptoms, for example antibiotics for bacterial infections and painkillers such as paracetamol.
Other drugs: caffeine, nicotine and alcohol also change reactions in the body, so they are drugs too, even though they are not taken as medicines.
Antibiotics: drugs used to treat infections caused by bacteria. Penicillin was the first to be widely used; it was discovered in a mould.
Examiner trap
Do not define a drug as "a medicine" or "an illegal substance". The syllabus definition is about effect: any substance taken into the body that modifies or affects chemical reactions in the body.
Section 02
Antibiotics cure bacterial infections only.
The dish holds a bacterial infection of 300 bacteria: most are sensitive (blue) and 4 are resistant (red). Give a full course of antibiotic and watch the counts, then start a new infection and stop the course early. Finally switch the chip to a viral infection and try the antibiotic again.
Use: antibiotics treat bacterial infections such as pneumonia, tuberculosis and infected cuts. They kill bacteria or stop them reproducing, and the immune system clears the rest.
Viruses are not affected: a virus reproduces inside the body's own cells and has none of the structures an antibiotic attacks (no cell wall, no bacterial ribosomes). Antibiotics do not cure colds, flu or COVID-19.
Resistant bacteria: some bacteria are resistant to an antibiotic, so it does not kill them. This reduces the effectiveness of antibiotics: an infection by a resistant strain may not respond to treatment.
Finish the course: in the sim the full course clears the infection; stopping early leaves survivors, all of the resistant ones among them, which multiply with no competition.
Examiner trap
Antibiotics kill bacteria, not viruses, and they do not "kill germs" in general. Taking an antibiotic for a cold does nothing to the virus but still gives resistant bacteria in the body an advantage.
Section 03
How resistance spreads, and how to slow it.
Extended. This colony of 150 bacteria contains one bacterium (orange) that a random mutation has made resistant. Press Add antibiotic, then Let them grow, and repeat. Watch the resistant percentage climb.
Selection: each dose kills the sensitive bacteria. The resistant ones survive, reproduce and pass on resistance, so after a few rounds most of the population is a resistant strain.
Every use counts: each time an antibiotic is used, resistant bacteria get an advantage. The more often antibiotics are used, the faster resistant strains build up.
Use only when essential: do not prescribe antibiotics for viral infections or for mild infections that the body can fight off; take the full prescribed course; avoid routine use in farm animals. Fewer uses means fewer chances for resistant bacteria to be selected.
MRSA: methicillin-resistant Staphylococcus aureus, a strain of a common bacterium that is resistant to many antibiotics. It causes infections that are hard to treat, especially in hospitals.
Examiner trap
The antibiotic does not cause the mutation, and bacteria do not "get used to" or become immune to the drug. The resistance mutation arises at random; the antibiotic only kills the bacteria without it, so the resistant ones are left to multiply.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the antibiotics tab, a full course of antibiotic is given for the bacterial infection. Which bacteria are killed first?
Quick check+10 XP
In the antibiotics tab, what happens after you choose Stop the course early?
Quick check+10 XP
When the chip is switched to a viral infection, why does the antibiotic have no effect?
Quick check+10 XP
In the resistance tab, after several rounds of Add antibiotic then Let them grow, the colony is mostly:
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which is the best definition of a drug?
Exam style+20 XP
A patient with influenza (a viral disease) asks for antibiotics. Why should the doctor refuse?
Exam style+20 XP
Why does the presence of resistant bacteria reduce the effectiveness of an antibiotic?
Exam style+20 XP
Extended: which practice best limits the development of resistant bacteria such as MRSA?
Skill unlocked
Drugs and antibiotics, mastered.
This skill is now recorded on your profile. Keep the chain going.