Overview

To create pixels from bacteria that can turn ‘on’ and ‘off’, we need some mechanism that has the ability to enable and disable fluorescence inside the bacteria. A lot of different approaches were considered, including using bio-bricks, etc. Unfortunately bio-bricks are out of scope for this project due to time constraints, so a solution was needed that is based on already available results.

Green Fluorescent Protein Fluorimetry

A solution was found during consultations with Prof. Huub de Groot, Prof. Pál Ormos and Prof. Péter Maróy – that to use bacteria that already has the Green Fluorescent Protein, so that the level of fluorescence can be changed by changing the pH value of the surroundings of the bacteria.

This can be done, based on [Wilks], by using E-Coli bacteria that has TorA-Green Fluorescent Protein mutant 3* (TorA-GFPmut3*) added to it.

The basic effect is that E-Coli can sustain a wide range of pH values (between 4.5 to 9), as it has to survive in people’s stomachs. And, the level of fluorescence for the GFPmut3* strain of GFP changes linearly with the pH value surrounding it. Thus, if the bacteria has the GFPmut3* strain, the change in pH value will make the level of the bacteria’s fluorescence change. This effect works on a fast timescale, typically in less than 4 seconds.

Of course, the E-Coli has a mechanism to keep its internal pH level at around 7, even if the pH value changes outside the bacteria. This would also keep the level of fluorescence generally constant. Thus, one has to make the bacteria disable this functionality. (Question is, would a bacteria survive longer periods with an internal pH value of sah 5.5?)

In practice

Using the above mechanism, one can turn a bacteria pixel ‘off’ by changing the pH value around the bacteria to 5.5. To turn off the bacteria’s ability to raise its internal pH value back to 7, one has to add benzoate or sodium acetate (see [Wilks]). To turn the bacteria ‘on’, one simply either raises the pH value to 7, or re-enables the internal pH regulation mechanism.

References

• [Wilks] Jessica C. Wilks and Joan L. Slonczewski: pH of the Cytoplasm and Periplasm of Escherichia coli: Rapid Measurement by Green Fluorescent Protein Fluorimetry, Journal of Bacteriology, Aug. 2007, p. 5601–5607