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

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

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.

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.

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.

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

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!

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

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

What does phase variation look like? It is a reversible process This process is reversible, whereby mutation can also switch genes back on again.

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.

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.

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.

How can phase variation benefit bacteria?

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

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

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

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.