Time Delayed Growth
This time delayed growth system attempts to address a common problem in system designs that require small molecule signals to be passed between consecutive colonies on an agar plate. By growing colonies in a time delayed manner, each colony will be able to respond to the signals that are being secreted by cells upstream in the chain of communication. Projects that could benefit from this system are Davidson's Bacterial Hash Function and Brown's Bacterial Freeze Tag. To our knowledge, no solution to this problem has been presented in the literature, short of using microuidics or other liquid handling procedures, which are in many cases expensive and unfeasible.
I used the properties of ampicillin to my advantage to refine a time-delayed growth system. By placing a single beta-lactamase-secreting colony near multiple non-resistant colonies on an ampicillin plate, I quantified time-delayed growth of the non-resistant colonies as the beta-lactamase diffused through the agar (see figure below). Colonies farther from the resistant colony grew later than colonies that are nearby, since the beta lactamase takes longer to diffuse out to those colonies.
A movie of this growth can be seen here. If you cannot see the movie in your browser, you can download it and play it locally. The movie is a large file so it may take a while to download (150 MB).
I also tested and modeled delayed colony growth for three variables: ampicillin concentration, agar concentration, and growth temperature. The raw data for these tests can be found here, and include useful parameters that could be helpful in tuning this system to grow the colonies at different rates. Briefly, I found that increased ampicillin concentrations slow that rate of time-delayed colony growth, as do lower temperatures. Unexpectedly, higher agar concentration appeared to accelerate time-delayed growth. Changing the ampicillin plasmid copy number could also affect time-delayed growth, although this was not tested in the data shown here.