Research and Techniques
We have a number of projects underway in our lab at the moment, and this section will be expanded as the projects progress. All of our methods make use of high-speed imaging in a microscope to tell us new things about biological samples. In particular, we have been looking at the swimming behaviour of E. coli bacteria. These microorganisms are arguably the best characterised bacteria, thanks in no small part to the work of Howard Berg's lab (see the Links page, left). The image below (click the image for a movie, compressed using the Xvid codec) shows a 'physicist's view' of a swimming bacterium. The cell body is a sphero-cylinder, around 2μm in length and 1μm in across. At the back of the cell there is a bundle of 4-6 helical filaments, about 10μm in length, that rotates driving the cell forward. As the flagellar bundle at the back rotates, it causes the cell body to rotate in the other direction. There are three main ingredients that are required to produce a physical model of swimming for a cell like this: The swimming speed V, the body rotation speed Ω and the flagellar rotation speed ω.
We recently showed how a method based on light scattering (differential dynamic microscopy or DDM) can be used as a powerful tool to measure V (L.G. Wilson et al., Phys. Rev. Lett. 106 018101, 2011). The overwhelming majority of measurements in the existing literature rely on tracking the images of individual cells, with varying degrees of automation. Our approach is different in that we measure changes in the local density of cells, and use that to infer the distribution of swimming speeds for the population. In fact, we also measure the fraction of the population that's swimming, and the diffusivity of the non-motile part at the same time. Intriguingly, these last two properties are related...
The image to the left shows around a quarter of a typical image from one of our videos, taken under phase contrast illumination; the scale bar represents 20μm. The bacteria are the little black specks scattered around the frame; there are maybe 50 in this picture. We collect a few thousand images, at around 200 frames per second, and then analyse the sequences 'offline' later on. As the data collection takes as little as 15 seconds, and typically measures the swimming speeds of around 5000 bacteria, we are really taking a representative 'snapshot' of the motility of a population of cells. This opens the door to studying the population-level dynamics of a bacterial suspension with unprecedented resolution in both time and space.