Team:Glasgow/Project/Measurements

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Floating Cells Measurement System

In order to characterise the floatation behavior of gas vesicle-filled E.coli, and confirm and/or revise the existing model, we would have to make some measurements. The preparation for this began before the gas vesicles were produced in the lab, so that when they were made, the characterisation process would be more efficient.

We decided to utilise the optical properties of the gas vesicles – they are known to scatter light. We devised an experimental set up that would light up and image the cells in suspension. When the cells float, their distribution in the fluid will change, thus changing the proportion of light that gets through at a given height. By tracking these changes over time, we would gain information on the speed of floatation, and perhaps how the cells distribute themselves – we should be able to see any clumping or filament formation.

Experimental Set-up

insert labelled picture of experimental set up

Experiment 1: Red Silicone Beads

For the first experiment, we would be tracking the sedimentation of red silicone beads through water. With a very similar size (1.1um) and density (1100kg/m^3) to E.coli, we felt these would be an acceptable substitute for the cells. Please see the Protocols page for a full experimental method.

Initially, the plan was to take an image every 10 minutes, but it very quickly became clear that there would be no changes in this time frame – at least no changes big enough to register. We changed the time between images to be 1 hour. In the end, this was also far too short, and even after 3 – 4 days there had been little change in the image brightness (again, nothing measurable). A quick calculation had put a preliminary estimate of 150 hours to travel the 3cm, but we had assumed changes would be visible in the time before this.
One reason for this could be the shear number of beads we had used: our engineering adviser agreed that the concentration we'd used was far too high and we'd be unlikely to see anything.

In order to be able to optimise the experiment, we would need a set-up which ran far quicker - otherwise, it could take days to get even the camera settings right! With this in mind, we procured some different beads for experiment 2.

Experiment 2: Yellow Glass Beads

For ease of repeatability, we needed to find a quicker experiment to run. Using glass beads 5um in diameter, and of density 200kg/m^3, the system could be tested multiple times per day. Preliminary calculations supported out hypothesis.
Again, please see relevant Protocols entry for full experimental method.

This set up was much better, in a number of ways. Though the solution was less opaque than the previous, changes were still visible thanks to further changes in the camera setting. We had noticed in the previous experiment that the overall brightness had varied between images, making comparison tricky. This was due to the gain settings on the camera, which were still active. Turned off, we were able to see the true brightness each time. Also, the beads did sink very quickly – the majority of them had sunk in ~10 minutes, meaning the experiment could be run multiple times.