The General Idea
In order to help guard against infections of the gut, we wish to engineer a strain of E. coli capable of killing bacterial pathogens. White blood cells (neutrophils in particular) are already very efficient at killing bacteria. They do this by engulfing the bacteria and exposing it to a bombardment of reactive oxygen species. These include superoxide, hydrogen peroxide and hydrochlorous acid. The reactive oxygen species kill the bacteria by shredding any biological molecule they come in contact with by way of their potent oxidizing properties. However neutrophils are not able to migrate to the large intestinal lumen where pathogens can reside. Because bacteria are well adapted to live in the gut, this project’s goal is to engineer a strain of E. coli to seek out and kill invading bacterial pathogens by means of a sudden burst of hydrogen peroxide.
Bacteria are able to communicate between individuals of the same species by way of quorum sensing. Small molecules serve as the signal between individual cells. Gram negative bacteria use acylhomoserine lactones (AHL), which can freely diffuse across the cell membrane. The quorum sensing machinery relies in two enzymes, LuxI, an AHL producer, and LuxR, an AHL-dependent transcriptional activator. Figure 1 illustrates how the system works. In isolation, each bacterium constitutively produces a small amount of AHL, which quickly diffuses into the surrounds. If other bacteria of the same species are also nearby, the AHL will diffuse across their membrane where it will bind to LuxR. LuxR activates transcription of several genes, including luxI. A positive feedback loop is created, in which more AHL induces more LuxI, which in turn produces more AHL. Each species of gram negative bacteria produces a unique AHL, requiring unique LuxI and LuxR proteins, and so avoids crosstalk between species. A group of bacteria can thus toggle between an “off” state and an “on” state by using quorum sensing.
Our engineered strain will not participate directly in quorum sensing, but instead will eavesdrop on the conversation. It will be engineered to constitutively express a LuxR able to detect a species AHL. In this way, our engineered strain will be able to be tuned to specifically respond to a variety of bacterial pathogens. Once the AHL is bound, LuxR will activate a set of genes which will lead to the overproduction of hydrogen peroxide, killing the invading cell.