Capsular Polysaccharide and Pneumococcal Disease

This week looking at the capsular polysaccharide of Streptococcus pneumoniae. Want to know how I know? I wrote it :)

Insert stock photo of pneumo. Check.

It seems a little wrong to blog my own paper but in reality more people will read this blog entry than will read the paper itself, and that’s fine. Its relevance is very narrow and the work very preliminary but really it’s the drive behind the work that is important. So lets talk about Streptococcus pneumoniae (aka the pneumococcus or simply pneumo) for a second to set the scene.

Pneumo is a big deal. It is responsible for roughly a million deaths in children under 5 around the world every year and is such a problem in the developing world its one of the bugs that Bill and Melinda Gates have taken a personal dislike to.

This data is taken striaght from the WHO website. The only modification is I have added Streptococcus pneumoniae as a separate bar. Deaths caused by pneumo make up approximately 2/3 of all lower respiratory infections. I colour coded the bars for some reason for another post, I cant remember the key... #badblogger

Despite being such a huge killer it’s actually a very common bacteria. Every single person on Earth probably has it right now, so in that respect it’s not a great killer but at the same time, it doesn’t want to be. If you kill the host you destroy your home and food source so pneumo is happily carried in populations without causing disease, for the most part.

I have spoken about pneumo like there is only one kind of pneumo but that’s as silly as saying there is only one kind of human. An important common characteristic is that the outside of each pneumo, similarly to humans, doesn’t always look the same. In humans, differences in skin colour, texture, etc allows us to tell people apart, for pneumo however it’s the chemical composition of a structure called the capsule which encases the entire bacterium.

This capsule has, roughly, 90 different common variations and the distributions of these variations (referred to as serotypes) change depending on geographical region, age and community.

The important thing about the capsule is its job. The bacterium uses the capsule to hide from the host immune system, which is what generally triggers an immune response, and so allows the bacteria to persist asymptomatically in the naso-pharynx.

But as I hinted before pneumo doesn’t always sit happily in the naso-pharynx…

Generally during some other infection the pneumo will descend into the lungs and cause pneumonia, from which the bacteria derives its name. Approximately 30% of pneumonia patients will develop bacteremia as the bacteria use the lung damage to gain access to the blood. Once in the blood the bacteria can move all around the body and cause all sorts of disease but importantly they can cause meningitis (inflammation of the meningies in the brain) and this occurs in approximately 30% of bacteraemic patients.

Pneumo relies on its capsule to hide from the immune system during pneumonia, bacteraemia and meningitis (collectively called the invasive pneumococcal disease of IPD’s), but what if you take the capsule away? It turns out the immune system can ‘see’ the bacteria very easily without its capsule and so un-encapsulated bacteria can sit in the naso-pharynx but if they try to cause trouble the immune system can knock them out very easily.

This is where my work comes in.

Now it’s true that we can treat pneumo with antibiotics and we have two vaccines against pneumo but its still a massive problem. Something in the order of 60% of all clinical isolates show some level of antibiotic resistance with up to 10% showing resistance to 4 or more common antibiotics. The vaccines we have are also of limited use as one doesn’t work in children, where most of the disease occurs and the other doesn’t have enough ‘serotype coverage’ to be effective throughout the entire world.

To compensate for this my lab looks at alternative vaccines and that protective capsule layer. My work has really been to investigate how the biosynthesis of this layer is regulated and look for potential to break the regulation and in doing so prevent capsule formation and the invasive potential of the pneumococcus.

The regulation of capsule biosynthesis is achieved through a dynamic phosphorylation cycle that essentially acts like a switch.

This model was developed by my supervisor based on extensive mutagenesis work.

This is the current working model of the regulation. On the left we have the capsule being synthesised. The details are a little much for here but ‘C’ is a protein embedded in the membrane, it interacts with ‘D’, a tyrosine kinase. A tyrosine kinase is an enzyme that adds a chemical group called a phosphate to something else. In this system it actually adds that group to itself and when it does it forces itself to change shape. This altered shape in ‘D’ (represented on the right of the diagram) is thought to either change ‘C’s’ shape as well or the change is simply relayed through ‘C’ to the outside of the cell and production of capsule stops. Instead when ‘D’ is phosphorylated any complete capsule is stuck to the cell wall. As this process is not a one way switch we need to return to the left of the diagram and this is achieved due to the activity of ‘B’ which is able to remove phosphate groups from ‘D’.

For what was published in the paper we (or rather I) performed some mutagenesis on ‘C’ that resulted in it not being able to relay the change in shape of ‘D’. This resulted in switches jammed on to the left or the right but incapable of changing.

Importantly other mutants that have been constructed with an inability to switch capsule biosynthesis properly are unable to cause disease. Our hope however is to reach a point where we know enough about this system that instead of relying on a mutation in ‘C’ to give us a switching defect we might be able to design a drug or inhibitor that blocks ‘C’ activity in vivo. But that’s 5 – 10 years away.

I think I’ll just write this and my other experiments up into my thesis, then worry about the rest of the work that needs doing…

References

Byrne JP, Morona JK, Paton JC, & Morona R (2011). Identification of Streptococcus pneumoniae Cps2C Residues that Affect Capsular Polysaccharide Polymerisation, Cell Wall Ligation and Cps2D phosphorylation. Journal of bacteriology PMID: 21378192
Morona JK, Miller DC, Morona R, & Paton JC (2004). The effect that mutations in the conserved capsular polysaccharide biosynthesis genes cpsA, cpsB, and cpsD have on virulence of Streptococcus pneumoniae. The Journal of infectious diseases, 189 (10), 1905-13 PMID: 15122528