IGCSE Biology 0610: Natural Selection and Antibiotic-Resistant Bacteria

Some poultry farms give their chickens a continuous low dose of an antibiotic to prevent bacterial infections. A vet studied the bacterium Escherichia coli living in the gut of these chickens and compared it with E. coli from flocks that had never been given the antibiotic.

In flocks never given the antibiotic, almost all E. coli were killed by the antibiotic in a laboratory test.
In flocks given the antibiotic continuously for several years, almost all E. coli survived the same laboratory test.

(a) Use the idea of natural selection to explain how the proportion of antibiotic-resistant E. coli in the treated flocks could have increased over time. In your answer, refer to variation within the bacterial population, the large number of offspring produced by bacteria, competition for survival in the presence of the antibiotic, and the passing on of alleles to offspring. [4]

(b) Explain why the change from a mostly non-resistant population to a mostly resistant population happened gradually, over many bacterial generations, rather than in a single generation. [2]

(c) A farm worker suggests that the bacteria "changed themselves" to resist the antibiotic because they were exposed to it. Explain why this suggestion is incorrect. [2]

Part (a): Explaining the increase in resistance using natural selection

Before the antibiotic was ever used, the E. coli population already showed variation: a small number of bacteria happened to carry an allele that made them resistant to the antibiotic, while most did not. Bacteria reproduce extremely quickly and produce very large numbers of offspring, so each generation there are many individuals available for selection to act on. Once the antibiotic is present continuously in the chickens’ gut, non-resistant bacteria are killed, while the resistant bacteria survive and face less competition for resources such as space and nutrients. Because the resistant bacteria survive and go on to reproduce, they pass their resistance alleles on to their offspring. Over successive generations, a larger and larger proportion of the population is descended from resistant ancestors, so the proportion of resistant bacteria in the population increases.

Part (b): Why the change happens gradually

Selection can only act on the bacteria present in a single generation: it removes non-resistant bacteria from that generation, but it cannot instantly convert every existing bacterium into a resistant one. Each generation, the surviving (mostly resistant) bacteria reproduce and pass on their alleles, so the proportion of resistant offspring rises a little compared with the generation before. Because bacteria go through very many generations over a period of years, these small generation-by-generation increases in the proportion of resistant bacteria accumulate into a large overall change, which is why the shift from a mostly non-resistant to a mostly resistant population takes place gradually rather than immediately.

Part (c): Why “the bacteria changed themselves” is incorrect

This suggestion wrongly implies that exposure to the antibiotic caused individual bacteria to change so that they could resist it, and that they changed because they were exposed. In reality, the mutation that gives resistance arises randomly, at any time, independent of whether the antibiotic is present. Some bacteria in the population already carried this mutation, purely by chance, before the antibiotic was introduced. The antibiotic did not create this resistance and did not teach any bacterium to resist it; its only role was to kill the non-resistant bacteria, which meant that the resistant bacteria that already existed survived, reproduced and became more common. The bacteria did not respond to the antibiotic by changing; the antibiotic simply selected between bacteria that already differed from one another.

Final answers

(a) Existing variation in resistance, large numbers of offspring, survival advantage for resistant bacteria under antibiotic pressure, and passing on of resistance alleles together explain the rising proportion of resistant E. coli.
(b) Selection acts generation by generation, so the proportion of resistant bacteria rises only slightly each time; these small rises accumulate over many bacterial generations.
(c) Resistance arose from a random mutation that existed before antibiotic exposure; the antibiotic only selected already-resistant bacteria for survival, it did not cause or teach resistance.