Understanding antibiotic resistance
Understanding antibiotic resistance

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Understanding antibiotic resistance

3.2 Evolving resistance to antibiotics

How do bacteria evolve resistance to antibiotics? Activity 3 will help you to think about how evolution and natural selection contribute to the spread of antibiotic resistance.

Activity 3 Evolution, natural selection and antibiotic resistance

Timing: Allow about 10 minutes

Darwin’s theory was that evolution by natural selection would occur if the following conditions were met:

  • There is a struggle for existence. Survival is limited by environmental constraints, so that there is competition, and not all individuals will survive to produce offspring.
  • There is variation between individuals. Individuals with advantageous traits will have a greater probability of survival under these conditions and are therefore more likely to reproduce.
  • The characteristics, or traits, of an individual are inherited. Advantageous traits that promote survival will be inherited by the next generation so that these traits become increasingly common in the population.

Now answer the following questions about the evolution of antibiotic resistance.

1 Which of the environmental conditions below might lead to the evolution of antibiotic resistance?


(a) low nutrient supply


(b) presence of antibiotics


(c) low oxygen availability


(d) all of the above

The correct answer is d.


Your answer is correct. Any of these could conditions could create a struggle for existence that could lead to the evolution of antibiotic resistance.

2 Would antibiotic resistance be an advantageous, or a disadvantageous, trait for bacteria growing in the presence of antibiotics?


(a) Antibiotic resistance would be advantageous.


(b) Antibiotic resistance would be disadvantageous.


(c) Antibiotic resistance would have no effect on the survival of bacteria growing in the presence of antibiotics.

The correct answer is a.


Your answer is correct. Antibiotic resistance would be an advantageous trait in the presence of antibiotics because resistant bacteria in the population will have a survival advantage over sensitive bacteria. However, in the absence of antibiotics, resistance can sometimes be disadvantageous because it can result in slower growth.

3 How would the trait of antibiotic resistance be inherited by the next generation?


(a) horizontal gene transfer


(b) transformation


(c) binary fission


(d) genetic mutation

The correct answer is c.


Your answer is correct. Antibiotic resistance genes, acquired via genetic mutation or horizontal gene transfer, can be inherited by subsequent generations through binary fission.

Within a bacterial population, some bacteria will be sensitive to antibiotic treatment while others will have acquired resistance to antibiotics, via either genetic mutation or horizontal gene transfer. In the presence of antibiotics, the resistant bacteria have a survival advantage over the sensitive bacteria and are more likely to survive and reproduce. Because bacteria reproduce so quickly, resistant bacteria can quickly dominate the population (Figure 10).

Described image
Figure 10 Natural selection for antibiotic resistance. When susceptible bacteria, shown in blue, are treated with antibiotics they all die. However, in the presence of antibiotics resistant bacteria, shown in green, survive and replicate by binary fission. This new bacterial population is now completely insensitive to treatment with the same antibiotic.

Of course, changes to the bacteria’s environment made by us can affect the evolution of antibiotic resistance. You will return to this theme when you look at how our use of antibiotics contributes to the rise of antibiotic resistance bacteria in Week 5.


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