Day Sensitive cells / millions Resistant cells / millions 1 480 2 5 90 8 10 5 140 Fig. 1.1 A patient in hospital was infected with a population of the bacte...
Question 1 Report
| Day | Sensitive cells / millions | Resistant cells / millions |
|---|---|---|
| 1 | 480 | 2 |
| 5 | 90 | 8 |
| 10 | 5 | 140 |
Fig. 1.1
A patient in hospital was infected with a population of the bacterium Klebsiella and was treated with the same antibiotic for 10 days. Doctors took samples of the bacteria on three days during the treatment and counted the bacterial cells that were sensitive to the antibiotic and those that were resistant to it. Although the antibiotic was given at the same dose throughout, the infection did not clear up. The results are shown in Fig. 1.1.
(a) State what is meant by the term natural selection. [2]
(b) Calculate the percentage of the total cells that were resistant to the antibiotic on Day 10. [2]
(c) Describe how the resistant bacteria came to make up the majority of the population by Day 10. [4]
(d) State the process that first produced the allele for antibiotic resistance in the population. [1]
(e) Suggest two ways in which hospitals can reduce the spread of antibiotic-resistant bacteria. [2]
(f) Explain why bacteria are able to evolve resistance much more quickly than mammals. [2]
Answer Details
This question tests natural selection using antibiotic resistance in Klebsiella. The antibiotic does not create resistance; it selects for bacteria that are already resistant.
(a) Natural selection is the process by which the organisms best adapted / with advantageous features survive [1] and reproduce, passing on their alleles to the next generation [1] [2].
(b) On Day 10 there are 5 sensitive and 140 resistant cells. \[\frac{140}{5 + 140} \times 100 = \frac{140}{145} \times 100 = 96.6\%\] Accept 96-97%. One mark for the method, one for the answer [2].
(c) Explain the rise of resistance in steps: some bacteria already carried a resistance allele [1]; the antibiotic killed the sensitive bacteria but the resistant ones survived [1]; the survivors reproduced / divided [1]; passing the resistance allele to their offspring, so the proportion of resistant cells increased over the days [1] [4]. The antibiotic is the selection pressure, not the cause of the mutation.
(d) The allele was first produced by (random) mutation [1].
(e) Any two hospital measures: hand washing / staff hygiene; isolating infected patients; only prescribing antibiotics when needed; completing the full course; disinfecting surfaces and equipment [2].
(f) Bacteria evolve resistance faster than mammals because they have a very short life cycle / reproduce rapidly [1]; so advantageous mutations spread through many generations in a short time [1] [2].
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