Antibiotics, resistance and infection controlEdexcel A-Level Biology A: Subtopic test

1. 1
   A student investigates the effect of three antibiotics on a bacterium that is harmless to humans. She spreads a lawn of the bacterium on nutrient agar, places paper discs soaked in antibiotic A, B or C on the surface, and adds a further disc soaked in a different liquid. After incubation she measures the diameter of each clear zone around the discs. The zone around antibiotic A has a diameter of 18 mm, around B 9 mm and around C there is no clear zone.

   (a)

   Which of the following is the most appropriate negative control for this investigation?

   [1 mark]

   • AA disc soaked in a higher concentration of antibiotic A
   • BA sterile disc soaked in sterile water
   • CAn agar plate with no bacteria and no disc
   • DA second disc of antibiotic B on the same plate

   (b)

   Using the measurement for antibiotic A, what is the area of the clear zone?

   [1 mark]

   • A28 mm²
   • B57 mm²
   • C254 mm²
   • D1018 mm²

   (c)

   Explain why the plates are incubated at 25 °C rather than 37 °C, and why the lid is taped on only in a few places rather than sealed completely.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A research team grows a species of bacteria in two flasks of nutrient broth. After 4 hours antibiotic X is added to flask 1 and antibiotic Y to flask 2. In flask 1 the number of living cells stays at about 2 × 10⁶ cells per cm³ for the next 12 hours and then rises rapidly when the antibiotic is washed out. In flask 2 the number of living cells falls to zero within 6 hours and stays at zero after the antibiotic is washed out.

   (a)

   Which type of antibiotic is antibiotic X, and what is the evidence?

   [1 mark]

   • ABacteriostatic, because the cells stopped reproducing but were not killed
   • BBactericidal, because the number of living cells stayed constant
   • CBacteriostatic, because the number of living cells fell to zero
   • DBactericidal, because the cells began to grow again when it was removed

   (b)

   A patient with a very low white blood cell count after chemotherapy has a serious bacterial infection. Which antibiotic type and reason are most appropriate?

   [1 mark]

   • ABacteriostatic, because it prevents resistance from developing
   • BBacteriostatic, because phagocytes will remove the remaining bacteria
   • CBactericidal, because it only stops the bacteria from reproducing
   • DBactericidal, because it kills the bacteria without relying on the patient's immune system

   (c)

   Explain why a bacteriostatic antibiotic can still cure an infection in a healthy person.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Mycobacterium tuberculosis is taken up by macrophages in the lungs, but it produces substances that stop the phagosome from fusing with lysosomes, so the bacteria survive and reproduce inside the macrophage. The influenza virus is also able to cause infection in the same person year after year, because the genes coding for its surface protein haemagglutinin mutate frequently.

   (a)

   Explain how the antigenic variation shown by the influenza virus allows it to evade the immune system.

   [3 marks]

   (b)

   Explain how the survival of M. tuberculosis inside macrophages and the antigenic variation of influenza both support the theory of an evolutionary race between pathogens and their hosts.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A surgical ward reports a rise in infections caused by meticillin-resistant Staphylococcus aureus (MRSA). An audit finds that many affected patients had received broad-spectrum antibiotics for minor infections, that staff did not always clean their hands between patients, and that several patients were moved between wards without being screened for MRSA.

   (a)

   Explain how antibiotic resistance in MRSA has arisen and spread on the ward.

   [6 marks]

   (b)

   Evaluate the extent to which changes to antibiotic prescribing and to hospital practice could reduce hospital acquired infections such as MRSA.

   [6 marks]

   Total for question 4: 12 marks

End of questions