Microbiology and pathogens

Sample questions

Original practice questions written in the style of the exam.

1. Question 13 marks

   A bacterial culture was diluted by a factor of $10^{5}$. Then $0.1\,\text{cm}^3$ of the diluted culture was spread on an agar plate. After incubation, 48 colonies grew.

   Calculate the number of bacteria per $\text{cm}^3$ in the original culture. Give your answer in standard form.

   Mark scheme

   • Bacteria per $\text{cm}^3$ of diluted culture $= 48 \div 0.1 = 480$ (1)
   • Multiplies by the dilution factor: $480 \times 10^5$ (1)
2. Question 24 marks

   Explain how infection with HIV can lead to the development of AIDS.

   Mark scheme

   • HIV infects T helper cells (by binding to CD4 receptors) (1)
   • The virus replicates inside T helper cells and destroys them, so their number falls (1)
   • With few T helper cells, B cells and T killer cells are not activated, so the immune response is weak (1)
   • The person develops opportunistic infections (e.g. TB) that a healthy immune system would control: this is AIDS (1)