Pathogens

Discovery of new drugs to combat infection with Streptococcus bacteria

Group A Streptococcus is a type of harmful bacteria that can cause life-threatening infections in humans. In 2005, it was estimated that 18 million people suffered from a serious infection caused by this type of bacteria, leading to 517,000 deaths (1).

The surface of group A Streptococcus is coated with a chain of sugars called group A carbohydrate. This sugar chain is produced through the concerted action of multiple enzymes, as illustrated in the diagram below. Enzymes are proteins that help carry out the chemical reactions that are essential for life. It has been shown that these bacteria do not survive if enzyme A or enzyme B is removed.


With the eventual goal of developing drugs to kill Group A Streptococcus and therefore save lives, I am currently testing 30,000 compounds for their ability to block Enzymes A and B from working. The collection of unique compounds is called a compound library, with each individual compound being made by chemical reactions carried out by chemists. The ability of each compound to block these enzymes is tested using a technique called mass spectrometry.

In a nutshell, a mass spectrometer helps scientists identify molecules present within a mixture of molecules by weighing them. The weight of a molecule is called its mass, and most molecules have different masses. The mass spectrometer can also count the number of each molecule present. Imagine a packet of jelly beans. We can sort these jelly beans based on their colour, and then count how many of each colour we have. This is analogous to how a mass spectrometer works.

However, not all molecules have the same mass. To prevent accidentally misidentifying some of the molecules present in the mixture, tandem mass spectrometry is used where two mass spectrometers are joined together by a chamber. Inside the chamber, the molecules are broken up into pieces much like a puzzle. The molecules are first weighed in the first mass spectrometer, sent to the chamber to be broken up, and the resulting puzzle pieces are then weighed by the second mass spectrometer. It is highly unlikely that two different molecules of the same mass will break up into the same pieces.


To be able to study enzymes A and B, the enzymes are purified from bacteria and placed in a test tube along with the substrates, which are the molecules that the enzymes helps join together. In this case, the substrates are the lipid chain containing a blue sugar, and the individual grey sugars. This reaction using purified components is carried out on a plate that has 384 individual holes with the help of robots. Each hole has the same reaction components but a different compound from the library. As the reaction is carried out, sugar chains containing grey sugars will be produced. If the compound added blocks the activity of enzyme A and/or enzyme B, fewer sugar chains containing the grey sugars will be produced. This can be detected by the mass spectrometer since it can count how many of each type of sugar chain is present. A robot called RapidFire passes the liquid from each hole to the mass spectrometer one at a time.


If a molecule is found that blocks one or both of these enzymes, the next step will be to test them on live bacteria to see if it can kill them.

References

(1) Carapetis, J. R., Steer, A. C., Mulholland, E. K. & Weber, M. (2005) The global burden of group A streptococcal diseases. The Lancet. Infectious Diseases. 5 (11), 685-694.

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