Penicillin stops growing bacteria from cross-linking their cell walls, so they burst. It has nothing to act on in a virus. And every time an antibiotic is used, it selects the few bacteria that can survive it.
An antibiotic is a chemical that kills or stops the growth of bacteria without harming human cells. Penicillin acts on growing bacteria: it inhibits the transpeptidase enzymes that form cross-links between the peptidoglycan chains of new cell wall. The wall becomes weak, water enters the cell by osmosis, and the cell swells and bursts (osmotic lysis). Human cells have no cell wall or peptidoglycan, so they are not affected. Antibiotics do not affect viruses: viruses have no cell wall, no peptidoglycan, no ribosomes and no metabolism of their own, and they replicate inside host cells using the host's enzymes. Antibiotic resistance arises by chance mutation; each use of an antibiotic selects resistant bacteria, which multiply and pass on the resistance gene. The consequences are infections such as MRSA and MDR-TB that are hard or impossible to treat. The impact is reduced by prescribing antibiotics only when needed, finishing the course, hospital hygiene, less use in farming, changing the antibiotic used and developing new ones.
Fig. 1: penicillin blocks cross-linking of peptidoglycan in a growing wall, so water entering by osmosis bursts the cell
Section 01
How penicillin bursts a bacterium.
Leave the target on growing bacterium and press Add penicillin. Watch the blue penicillin molecules reach the yellow cross-links, the links disappear, water enter and the cell swell. Then press Reset, choose virus (HIV) and add penicillin again.
Peptidoglycan: the bacterial cell wall is made of long polysaccharide chains held together by short peptide cross-links, forming a strong net around the cell.
Growth needs new wall: as a bacterium grows and divides, enzymes called autolysins open small gaps in the wall and transpeptidases insert new chains and cross-link them.
Penicillin inhibits transpeptidase: it binds to the enzyme's active site, so new cross-links do not form while the gaps are still being made. The wall becomes weaker.
Osmotic lysis: the cytoplasm has a lower water potential than the surroundings, so water enters by osmosis. Without a strong wall to resist the pressure, the cell swells and bursts.
Selective: human cells have no cell wall and no peptidoglycan, so penicillin does not harm them. Bacteria that are not growing are not killed.
Examiner trap
Penicillin does not dissolve or digest the wall that is already there. It stops new cross-links forming, so it works only on bacteria that are growing. The cell is killed by water entering by osmosis, not by penicillin entering the cytoplasm.
Section 02
Why antibiotics do not touch viruses.
With the virus selected, the penicillin molecules drift around the particle and never bind to anything. That is the whole point.
Antibiotic target in bacteria
Present in a virus?
peptidoglycan cell wall (penicillin)
no: a virus has a protein coat (capsid), and HIV has a lipid envelope from the host
70S ribosomes (some other antibiotics)
no: viruses use the host cell's ribosomes
enzymes of bacterial metabolism
no: a virus has no metabolism of its own outside a host cell
Viruses replicate inside host cells using the host's own machinery, so there is nothing an antibiotic can act on. Viral infections such as HIV are treated with antiviral drugs, for example antiretrovirals that inhibit reverse transcriptase. Prescribing antibiotics for a cold or flu does not help the patient and adds to selection for resistance.
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.
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In the penicillin tab, what happens to the yellow cross-links after penicillin reaches the growing bacterium?
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What makes the bacterium in the lab burst at the end?
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When the target is switched to virus (HIV) and penicillin is added, what does the lab show?
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In the resistance tab with no steps ticked, what happens to the percentage of resistant bacteria over the 20 years?
Section 03
Selecting for resistance.
Press Run 20 years with every box unticked: 1% of the 200 bacteria on the dish start resistant (orange). Then tick the steps one by one and run it again.
Origin: a random mutation gives a bacterium a resistance gene, for example one coding for an enzyme (penicillinase, a beta-lactamase) that breaks down penicillin. The antibiotic does not cause the mutation.
Selection: when the antibiotic is used, susceptible bacteria die and resistant ones survive, reproduce by binary fission and pass the gene on (vertical transmission).
Spread between bacteria: resistance genes are often on plasmids, which can be passed to other bacteria, even of other species, by conjugation (horizontal transmission).
Consequences: infections such as MRSA (methicillin-resistant Staphylococcus aureus) and MDR-TB become hard or impossible to treat; patients stay in hospital longer; more expensive drugs with more side effects are needed; surgery, cancer treatment and transplants become riskier; more people die.
Reducing the impact: prescribe antibiotics only for bacterial infections that need them; patients finish the course; strict hospital hygiene (hand washing, screening and isolating patients); less antibiotic use in farming; change which antibiotic is used; develop new antibiotics.
Examiner trap
Bacteria do not become resistant because they "get used to" an antibiotic, and people do not become resistant: bacteria carry resistance genes that arose by chance mutation, and the antibiotic selects them.
Stage 2 · Exam
Exam-style questions
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Which describes how penicillin kills bacteria?
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Penicillin is most effective against bacteria that are dividing rapidly. Why?
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Which statement about the origin of antibiotic resistance is correct?
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Why does strict hand washing between patients help reduce the impact of antibiotic resistance in hospitals?
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Antibiotics and resistance, mastered.
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