NCEA Level 3 · Year 13 · Biology · 3 minutes

How bacteria beat antibiotics

Random mutation, selection pressure, and genes that spread sideways.

Key terms

Mutation: A random mistake made when DNA is copied, which can sometimes let a bacterium survive an antibiotic.
Selection pressure: A factor, such as an antibiotic, that kills susceptible bacteria so resistant ones survive and reproduce.
Resistance allele: A version of a gene that lets bacteria survive an antibiotic and becomes more common over generations.
Plasmid: A small ring of DNA that bacteria can pass to each other, carrying resistance genes.
Horizontal gene transfer: Passing genes sideways between bacteria, even different species, rather than from parent to offspring.

Transcript

In nineteen twenty eight, Alexander Fleming came back from holiday to a messy lab, and a mouldy dish of bacteria. Around the mould, the bacteria wouldn't grow. He had found penicillin. So why are bacteria beating it today?

Turning mould into medicine took a team at Oxford, and a world war. American factories grew penicillin in giant tanks, and by the D Day landings, about two point three million doses were ready. Infections that once killed soldiers could now be treated.

But in his Nobel lecture in nineteen forty five, Fleming gave a warning. Take too little of the drug, he said, and you could make your microbes resistant. He was right, and natural selection explains why.

Start with a population of bacteria. Every time they copy their DNA, there's a tiny chance of a mistake: a mutation. By pure chance, one mutation might let a bacterium survive the antibiotic. Key point: the antibiotic doesn't cause that mutation. It was already there.

Now add the antibiotic. That's the selection pressure. Susceptible bacteria die, but the resistant one survives. With less competition, it reproduces, maybe every twenty minutes, and passes the resistance gene to its offspring. Over many generations, the frequency of the resistance allele increases, until most of the population is resistant.

And bacteria have another trick. They can pass resistance genes sideways, on small rings of DNA called plasmids. That's horizontal gene transfer, and it can even move resistance between different species.

In twenty twenty one, resistant bacteria directly caused over a million deaths worldwide. Every unnecessary dose can select for resistant bacteria. And antibiotics don't work on viruses like colds and flu. So only take them when they're prescribed, exactly as directed, and never share or save leftovers.

So, for your exam: one, random mutation creates variation. Two, the antibiotic is the selection pressure, so resistant bacteria survive and reproduce. Three, resistance genes spread down the generations, and sideways on plasmids. Fleming saw it coming. Now you can explain it.

Sources

Every fact in the video, and where it comes from.

1
en.wikipedia.org/wiki/Discovery_of_penicillin
- In 1928 at St Mary's Hospital, London, Alexander Fleming returned from holiday to find a Penicillium mould contaminating a Staphylococcus culture plate, with a clear zone around the mould where bacteria did not grow.
2
nobelprize.org/prizes/medicine/1945/fleming/biographical
- In 1928 at St Mary's Hospital, London, Alexander Fleming returned from holiday to find a Penicillium mould contaminating a Staphylococcus culture plate, with a clear zone around the mould where bacteria did not grow.
3
nobelprize.org/prizes/medicine/1945/summary
- Howard Florey and Ernst Chain's team at Oxford turned penicillin into a usable drug around 1940–41; Fleming, Florey and Chain shared the 1945 Nobel Prize in Physiology or Medicine.
4
nationalgeographic.com/magazine/article/how-world-war-ii-put-penicill…
- During the Second World War, US manufacturers mass-produced penicillin by deep-tank fermentation; about 2.3 million doses were prepared for the D-Day landings in June 1944.
5
acs.org/education/whatischemistry/landmarks/flemingpen…
- During the Second World War, US manufacturers mass-produced penicillin by deep-tank fermentation; about 2.3 million doses were prepared for the D-Day landings in June 1944.
6
nobelprize.org/prizes/medicine/1945/fleming/lecture
- In his Nobel lecture (11 December 1945) Fleming warned that someone who underdoses himself could, by exposing his microbes to non-lethal quantities of the drug, make them resistant.
7
evolution.berkeley.edu/teach-resources/antibiotic-resistance
- Resistance arises through random mutations (or acquired genes); the antibiotic does not cause the mutation but acts as a selection pressure: susceptible bacteria die, resistant bacteria survive, reproduce and pass on the resistance gene, so its frequency in the population rises.
8
cdc.gov/antimicrobial-resistance/about/index.html
- Resistance arises through random mutations (or acquired genes); the antibiotic does not cause the mutation but acts as a selection pressure: susceptible bacteria die, resistant bacteria survive, reproduce and pass on the resistance gene, so its frequency in the population rises.
9
theory.labster.com/bacterial-growth
- Under ideal conditions some bacteria such as E. coli can divide about every 20 minutes.
10
cdc.gov/antimicrobial-resistance/causes/index.html
- Bacteria can share resistance genes with other bacteria, including other species, through horizontal gene transfer, e.g. plasmids passed by conjugation.
11
pmc.ncbi.nlm.nih.gov/articles/PMC4759269
- Bacteria can share resistance genes with other bacteria, including other species, through horizontal gene transfer, e.g. plasmids passed by conjugation.
12
healthify.nz/medicines-a-z/a/antibiotics
- Antibiotics don't work against viruses such as colds and flu; take antibiotics only when prescribed for you, as directed, and don't use leftovers or someone else's.
13
who.int/news-room/fact-sheets/detail/antimicrobial-res…
- Misuse and overuse of antimicrobials are the main drivers of drug-resistant pathogens.
14
ox.ac.uk/news/2024-09-17-antibiotic-resistance-has-clai…
- Bacterial antimicrobial resistance directly caused about 1.14 million deaths worldwide in 2021 (GRAM study, The Lancet, 2024); at least one million a year since 1990.

Image and sound credits

Alexander Fleming in his laboratory, St Mary's Hospital, London (IWM D17801, D17802, TR 1468): Ministry of Information official photographers, Imperial War Museums, public domain, via Wikimedia Commons
Howard Florey and Ernst Chain at Oxford; Penicillium flasks, 1943–44 (IWM D17803, D17806, D16958): Ministry of Information Photo Division, Imperial War Museums, public domain, via Wikimedia Commons
USDA Northern Regional Research Laboratory, Peoria, c. 1943: US Department of Agriculture, Agricultural Research Service, public domain, via Wikimedia Commons
1957 penicillin fermenter, Science Museum, London: Matt Brown, CC BY 2.0, via Wikimedia Commons
“Penicillin saves soldiers' lives” poster: US National Archives (NAID 515170), public domain
MRSA colourised scanning electron micrographs: National Institute of Allergy and Infectious Diseases (NIAID), CC BY 2.0, via Wikimedia Commons
Culture plate, bacteria, plasmid and virus diagrams: drawn in code
Paper textures: ambientCG (CC0) · Torn paper edges generated with AI
Sound effects: Freesound contributors (CC0)
Music: generated with ACE-Step · Voice: Gemini TTS

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