Resistant organisms and antibiotic resistanceSpec 4.3
The emergence of resistant organisms is an example of natural selection that can be seen happening within a human lifetime. Bacteria reproduce very quickly, so changes in a population happen fast.
- In a population of bacteria, a mutation produces an allele that makes one bacterium resistant to an antibiotic. This is variation.
- The antibiotic is used. It kills the non-resistant bacteria, but the resistant bacterium survives. The antibiotic is the selection pressure.
- The resistant bacterium reproduces with less competition, and passes the resistance allele on to its offspring.
- Over time the population is mostly resistant bacteria, so the antibiotic no longer works against the infection.
This supports Darwin's theory because all the steps of natural selection can be observed: variation, a selection pressure, survival and reproduction of the better adapted, and inheritance of the useful allele.
Percentage of resistant bacteria (practice data)
In a lab, 300 bacteria were sampled before treatment and 12 were resistant to an antibiotic. After several rounds of treatment, 300 bacteria were sampled again and 192 were resistant. Calculate the percentage resistant at each time, and the change.
- Before: 12 ÷ 300 × 100 = 4%
- After: 192 ÷ 300 × 100 = 64%
- Change: 64 − 4 = 60 percentage points
Answer: 4% before, 64% after, an increase of 60 percentage points.
Quick check
What causes new alleles to appear in a population?
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Mutation.
Give the four main steps of natural selection in order.
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Variation, competition so some survive and reproduce, alleles passed on, proportion with the useful allele increases.
In antibiotic resistance, what is the selection pressure?
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The antibiotic.
Why do populations of bacteria change quickly?
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Bacteria reproduce very rapidly, so the resistance allele spreads in a short time.