1. Phase variation in Neisseria meningitidis
First discuss that phase variation occurs in Neisseria and go on to explain…

2. Recap: genes and proteins
1 codon = 1 amino acid G A G A C T T A C
DNA C T C T G A A T G C U C U G A A U G
RNA
DNA is read in sets of 3 bases (codons) which make up genes. These genes are transcribed into RNA which is then translated into amino acids which make up functional proteins.
Proteins

3. Recap: immunity
Molecules on the surface of bacteria are called antigens
B cells
The immune system recognises antigens on the surface of bacteria which stimulates the production of antibodies specific to that protein. These antibodies then coat the bacteria, targeting it for killing by the immune system.

4. Recap: immunity
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
B cells
The immune system recognises antigens on the surface of bacteria which stimulates the production of antibodies specific to that protein. These antibodies then coat the bacteria, targeting it for killing by the immune system.

5. What is Meningitis?
Infection of the membranes around the brain and spinal cord
Neisseria meningitidis is the major cause of bacteria meningitis world wide
Meningitis is primarily caused by a bacteria called Neisseria meningitidis. This bacteria causes meningitis when it infects the otherwise sterile membranes around the brain and spinal cord.

6. Neisseria meningitidis
Found in the throats of healthy people.
Only causes disease in rare cases.
N. meningitidis has several key virulence determinants
Neisseria meningitis is a gram negative pathogen. It colonises the throats of healthy people asymptomatically but in sever cases can infect the meninges and cause meningitis.
N. meningitidis has adapted ways to evade the immune system, which is where phase variation ties in.
Microscope image of N.meningitidis

7. All are subject to a process known as phase variation!
Virulence determinants of N. meningitidis
Opa
Is an adhesion molecule that allows N. meningitidis to stick to stick to human epithelial cells.
HpuA
HmbR
Are both haemoglobin receptors responsible for acquiring iron.
What do they have in common?
All are subject to a process known as phase variation!

8. Phase variation = switching genes ON or OFF.
Caused by reversible mutation
Off On
Phase variation is the switching on and off of genes in response to mutation. The mechanism for this will be explained later.
These mutations are reversible, meaning genes can be switched on and off interchangeably.
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)
Every time a cell divides there is a random chance they will mutate and switch on/off phase variable genes.
These mutations are then passed down to daughter cells
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
This process is reversible, whereby mutation can also switch genes back on again.

11. What does phase variation look like?
Many bacteria have more than one phase variable gene
Both genes can mutate at the same time
Many bacteria have multiple phase variable genes that can be switched on/off in tandem giving a wide variety of phenotypes
Mutation isn’t this quick in real bacteria.

12. What is the effect of phase variation on populations of cells?
All of the different cells have the same DNA but have different antigens expressed.
The combination of different phase variable genes can give a population that has arisen from a single cell, which has different proteins expressed on their surface.

13. How does phase variation happen? -Slippery DNA
One mechanism of phase variation is through slippery or hypermutatable DNA.
Hyper mutatable DNA is far more likely to get insertion/deletion mutations than other sections of DNA
Repeat nucleotide tracts (eg. Poly G above) are hypermutatable
Insertions/ deletions can cause frame shifts which truncate the expressed proteins and put them in the off position.
Insertions deletions can just as easily restore the frame and switch gene expression back on.

14. How can phase variation benefit bacteria?

15. How can phase variation benefit bacteria?
Phase varying surface protein expression can help bacteria to evade immune effectors.

16. Phase variation is a key factor in bacteria’s ability to cause disease

17. Summary
Phase variation is the reversible switching on or off of gene expression
Phase variation is caused by mutations in hyper-mutatable DNA sequences
Bacteria have many phase variable genes that contribute to their ability to cause disease

18. Simulating phase variation
The attached simulator and work sheets will help you to visualise phase variation in real time.
This will also help you to consider the roles of phase variation in disease.