If you are unlucky enough to develop bacterial pneumonia, the bacterium in question will probably be Streptococcus pneumoniae (aka the ‘pneumococcus’), a nasty microbe that can also cause meningitis, septicaemia, otitis media and other serious conditions. The particular factor which enables pneumococcus to attack mammalian cells is a toxin called pneumolysin which is capable of disrupting cell mechanisms by creating pores in membranes. The World Health Organisation estimates that 1.6 million people die each year from pneumococcal infections. Cryo-EM, a rapidly advancing form of electron microscopy conducted at very low temperatures, had already revealed the circular structure, 30-50 molecules in size, which builds up on the membrane and creates the pore. This structure has been visualised at a resolution of about 28 angstroms (one angstrom is a ten-billionth of a metre or 10-10m) but that’s not sufficient to view a single molecule of pneumolysin and ascertain the mechanism by which it binds to the membrane in the first place. It is the possibility of disrupting or preventing that initial binding which holds a key to targeting pneumolysin and hence the pneumococcus.
In 2015 a team led by Professor Peter Andrew and Professor Russell Wallis from our Department of Respiratory Sciences used X-ray crystallography to study pneumolysin at an eye-watering resolution of just 1.98 angstroms and from this they were able to achieve the long-sought goal of determining its molecular structure.
The first stage of the process was to grow a crystal of pneumolysin. The toxin was extracted, purified and filtered then grown in a protein substrate. A second crystal was grown of just one of the molecule’s four parts or ‘domains’; Domain 4 was already known to have the membrane-binding mechanism.
Determining the molecular structure required a trip to Diamond Light Source, the UK's national synchrotron science facility. In this Oxfordshire particle accelerator the crystals were blasted with X-rays, producing distinctive diffraction patterns. Analysing those patterns is what led the Leicester team to their great breakthrough.