1. Meningitis and Phase Variation

2. Recap: genes and proteins G TC AG CT A G CT A T AT A G C DNA Proteins UCCUGAAUG RNA

3.  Molecules on the surface of bacteria are called antigens Recap: immunity B cells

4.  Molecules on the surface of bacteria are called antigens  Cells of the immune system recognise antigens and produce antibodies  Antibodies bind to the bacteria’s antigen and kill them Recap: immunity B cells

5. What is Meningitis  Infection of the membranes around the brain and spinal cord  Main cause is a bacteria called Neisseria meningitidis

6.  Fever What are the symptoms?  Headache  Neck stiffness

7. Neisseria meningitidis  Found in the throats of healthy people.  Only causes disease in rare cases. Microscope image of N.meningitidis  Must have a way for its antigens to go unnoticed until it causes disease

8. Phase variation = switching genes ON or OFF. Caused by reversible mutation On Off Mutation

9. What does phase variation look like?  One cell will divide into two daughter cells  With every division, there is a chance they will mutate (mutation rate)  This mutation will be passed down to the daughter cells  Mutation is random.  Like rolling a dice

10. What does phase variation look like?  It is a reversible process

11. What does phase variation look like?  Many bacteria have more than one phase variable gene  Both genes can mutate at the same time  Mutation isn’t this quick in real bacteria.

12. How does this make the population look? All of the different cells have the same DNA but have different antigens expressed.

13. How does phase variation happen? -Slippery DNA

14. How can phase variation benefit bacteria?

15. Switching some genes off can help the bacteria to escape the immune system.

16.  There are lots of different genes that can be phase variable  Some examples in N. meningitidis are genes involved in:  Taking up nutrients such as iron  Sticking to the host cells What genes can be phase variable? Many of these genes are involved in causing disease