362 — Dynamic density shaping of photokinetic E. coli
Proteorhodopsins are proton pump proteins found in Pacific marine bacteria. Because they can use light energy to increase an ion gradient across a membrane, they’re effectively a bacterial “solar panel,” generating cellular power from light. Organisms powered predominantly by proteorhodopsins swim faster in bright light, and lower in the energy-dearth of dim light. And because bacteria aggregate when moving slowly, bacterial density changes in relation to the local light brightness. In effect, bacteria semi-randomly flee from bright light and congregate in the dark.
The researchers leveraged this phenomenon by adding the proteorhodopsin (PR) via plasmid transformation to E. coli colonies. The E. coli were then exposed to light fields: specifically, Mona Lisa-shaped light fields. The bacteria tended to accumulate in the dark areas due to the feedback loop explained above, and so when the light was removed after a few minutes, the bacteria remained in the configuration to which they were exposed: The dish now looked like a bacterial Mona Lisa.
There are some great videos of this work here, including an amazing video of a portrait of bacterial Einstein morphing into a bacterial Darwin. The authors explain that this research is useful for more than just painting with bacteria: This technology means that we can direct the movement of bacteria over nontrivial distances using light. This has applications in drug delivery (imagine an army of E. coli transporting drugs along a light gradient) or the movement of other cargoes.