Team:ETHZ Basel/Achievements/E lemming
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Video 2 shows another E. lemming which is swimming straight with frequent interruptions by tumblings when being in a constant light environment. When the blue light is switched on, the tumblings nearly completely disappear and the E. lemming is swimming straight over large distances. When the light is switched off, the tumbling disappears or the E. lemming alternatively stops movement at all. | Video 2 shows another E. lemming which is swimming straight with frequent interruptions by tumblings when being in a constant light environment. When the blue light is switched on, the tumblings nearly completely disappear and the E. lemming is swimming straight over large distances. When the light is switched off, the tumbling disappears or the E. lemming alternatively stops movement at all. | ||
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Revision as of 18:03, 27 October 2010
The E. lemming
What it needs to bring E. lemming alive
It needs an archeal photoreceptor that is fused to a bacterial chemotactic transducer. This was successfully demonstrated by Jung et al. in 2001, who fused the Natronobacterium pharaonis NpSRII (Np seven-transmembrane retinylidene photoreceptor sensory rhodopsins II) and their cognate transducer HtrII to the cytoplasmic domain of the chemotaxis transducer EcTsr of Escherichia coli. For more information visit our Archeal Light Receptor wiki page.
To make the nice videos shown below, the optimal chemotactic conditions, that were concluded from a series of different microscopy images, were applied. Escherichia coli K12 cells were grown at 30 °C in Lysogeny Broth to on OD of 1.0. IPTG for induction of gene expression and all-trans retinal for NpSRII folding were added to the media.
Experimental Results
We imaged several transfected E. coli cells with a 20× lens in a ≈50μm high flow channel. Approximately 5% of the cells reacted on the switch-on and -off of the blue light signal by changing significantly their swimming behavior. In Video 1 shows an E. lemming swimming in regular circles in a constant light environment. When switching the blue light on, it completely changes its motility after a 2-3s delay by swimming straight for several seconds. When the light is switched off, it returns to its original behavior after a similar delay (see paragraph "Characterization").
Video 2 shows another E. lemming which is swimming straight with frequent interruptions by tumblings when being in a constant light environment. When the blue light is switched on, the tumblings nearly completely disappear and the E. lemming is swimming straight over large distances. When the light is switched off, the tumbling disappears or the E. lemming alternatively stops movement at all.
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Characterization
To characterize the change of swimming behavior when switching on or off the blue light signal, we estimated the angle of the E. lemming for each frame of Video 1. This was done by obtaining the positions (xi, yi) of the E. lemming from our cell detection and tracking algorithm. The angle φi of frame i was then calculated by central differences:
tan(φi)=(yi+1-yi-1)/(xi+1-xi-1).
When plotting the angle over time (see Figure 1), one observes that during white light periods the angle is increasing with a nearly constant angular speed of about 27° per second (&asymp8° per frame). When switching on blue light, the angular speed decreases to nearly zero for several seconds after a delay between 2 and 3s.
For the first light pulse this decrease of angular speed lasted for about 10s until the return to pre-blue light behavior, for the second light pulse this effect only ended after the blue light was switched off again. In the latter, normal swimming behavior re-established after a delay of approximately 2.4s, which is nearly the same delay as the delay when switching the light on. Please note that the natural adaptation system of the chemotaxis pathway downstream of the light receptor is active in this mutant, such that the swimming behavior only changes directly after the blue light is switched on or off, but is not necessarily different between long periods of light on or off.
We furthermore noticed that the E. lemming seems to have the tendency to show a bigger tumble right before starting swimming straight when the blue light is switched on. However, if this behavior occurred by chance or if this is a general property of the E. lemming was yet not possible to show.