An antibiotic has to do something quite difficult: destroy a bacterium sitting inside your body without touching any of your own cells. The way it manages that also explains, in one step, why antibiotics are useless against a cold.
📘 What you need to know
Antibiotics are drugs that inhibit the growth of microorganisms. Most kill bacteria or stop them growing.
They work by blocking processes that occur in prokaryotic cells but which do not work the same way in eukaryotic cells — so the host is unharmed.
Targets include transcription, translation, DNA replication, ribosome function and cell wall formation.
Penicillin is produced by fungi in the genus Penicillium and interferes with the building of the bacterial cell wall.
In the wild, these secretions help the fungus compete by killing nearby saprotrophic bacteria.
Antibiotics can also be made synthetically in a laboratory.
Penicillin does not work on all bacteria — some have thicker cell walls that reduce permeability, or enzymes that break penicillin down.
Antibiotics are ineffective against viruses, because viruses are not cells and have no metabolism of their own. Antivirals target viral enzymes instead.
Hitting the bacterium and missing you
This is called selective toxicity, and it is the entire design problem of an antibiotic. The solution is to attack something that only prokaryotes do, or that prokaryotes do differently.
Cell wall formation is the ideal target: it is essential to the bacterium and completely absent from animal cells.
Penicillin in particular
Penicillin interferes with the building of the bacterial cell wall. A bacterium that cannot maintain its wall ruptures and dies when it tries to grow, because there is nothing to resist the pressure of the cell contents.
Penicillin is produced naturally by certain fungi in the genus Penicillium. Out in the wild, secreting antimicrobial chemicals helps the fungus compete, by killing nearby saprotrophic bacteria that would otherwise take its food. We simply borrowed a weapon that was already in use.
Not every bacterium is vulnerable. Penicillin does not work on all species — tuberculosis is a classic example. Some bacteria have thicker cell walls that reduce permeability so the drug cannot get in, and some produce enzymes that break penicillin down.
Why antibiotics do nothing to a virus
This comes up constantly, and the reasoning is worth learning as a chain rather than a fact.
Antivirals exist precisely because a few viruses carry their own enzymes. Where a virus brings nothing of its own, it is very hard to treat.
If a doctor refuses to prescribe antibiotics for a sore throat, this is why. Most sore throats are viral, so the antibiotic could not possibly help — and, as the next page explains, taking it anyway makes things worse for everyone.
Worked examples
WORKED EXAMPLE
Explain how antibiotics can kill bacteria inside a patient without damaging the patient’s own cells. [3]
The principleAntibiotics block specific processes that occur in prokaryotic cells.Give an examplePenicillin interferes with cell wall formation, so the bacterium ruptures and dies when it tries to grow.Why the host is safeHuman cells are eukaryotic and have no cell wall, and their ribosomes and enzymes differ, so the drug has no effect on them.Target something prokaryote-onlya named example is almost always worth one of the three marks
WORKED EXAMPLE
A patient with influenza is prescribed antibiotics. Explain why this will not help. [3]
What influenza isInfluenza is caused by a virus, which is a particle rather than a cell.Why the drug has nothing to attackViruses have no cell wall, no ribosomes and no metabolism, so none of the processes antibiotics block are present.And the host machinery pointThe virus replicates using the host cell’s own machinery, which antibiotics do not bind to.No prokaryotic processes present, so no effectmention antivirals as the alternative if the question asks what would help
💡 Exam tip
Learn the target list: cell wall formation, ribosome function, transcription, translation, DNA replication. Any two will usually do.
Say prokaryotic and eukaryotic rather than “bacteria cells” and “our cells”. The terms carry the mark.
For penicillin, say it interferes with cell wall building and the cell ruptures when it grows.
Give two reasons antibiotics fail against viruses: no cell structure to target, and viruses use host machinery.
Remember Penicillium is the fungus; penicillin is the drug. One letter apart, and both get asked for.
⚠ Common mix-up
Saying antibiotics kill all microorganisms. Most target bacteria specifically.
Writing that antibiotics kill viruses slowly. They have no effect on viruses at all.
Thinking penicillin works on every bacterium. Thicker walls and penicillin-breaking enzymes make some species resistant.
Confusing antibiotic with antibody. One is a drug; the other is a protein your body makes.
Saying antibiotics dissolve the cell wall. Penicillin prevents the wall being built — the damage shows up when the cell tries to grow.
Up next: Antibiotic Resistance — natural selection happening fast enough to watch, and why it is one of the biggest problems in modern medicine.
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