Jos Arents just got back to me with the good news that he has found a pH sensitive fluorescent protein that has a kanamycin marker instead of ampicillin, and this he can combine this DNA with the one containing a light-dependent proton pump, so as to create bacteria that would change the level of fluorescence as a reaction to some light.
Thus the basic setup would be the following:
function1: light -> pH value change in the cell function2: change in pH value -> change in level of fluorescence
The original GFP in question comes from “ratiometric pHluorin”, and was used already in Bacillus Subtilis – let’s hope it works again in E. Coli as well. Jos will try to transform it into their already existing UT5600/b-caro/proteorhodopsin E. Coli strain, which already contains the light-dependent proton pump.
As for why a different marker is needed: DNA transformation is usually a process of low success rate. Thus, one needs to get rid of over 95-99% of all bacteria that grows, as only the remaining 1-5% will contain the DNA that you wanted to add. The way this is done that the intended DNA comes bundled with resistance to an otherwise toxic material, like some sort of anti-biotic. And thus, to select keep only your bacteria, you grow them in an environment full of poison for them. But as the ‘good’ ones are resistent – voilá, they and only they will survive.