Antibiotics, resistance and infection controlEdexcel A-Level Biology A: Flashcards

Exam questions on Antibiotics, resistance and infection control

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.
   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
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.
   Explain why a bacteriostatic antibiotic can still cure an infection in a healthy person.2 marks
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.
   Explain how the antigenic variation shown by the influenza virus allows it to evade the immune system.3 marks