Bacteria have evolved complex mechanisms called quorum sensing systems that provide for cell-to-cell communication, an adaptation that allows them to wait until their population grows large enough before mounting an attack on a host or competing for nutrients.
Lianhong Sun, a chemical engineer at the University of Massachusetts Amherst, has engineered one of these systems to create genetic switches that could lower the cost of producing therapeutic proteins and pharmaceuticals.
The switches will enhance the process of industrial fermentation, which uses colonies of organisms such as bacteria to produce a wide variety of chemicals including insulin, human growth hormone and antibiotics. “The genetic switches work quickly, are easy to control and do not require expensive chemicals to start the process,” says Sun. “They are also nontoxic, and can be introduced into a wide variety of organisms that are already in use including microbes and plant and animal cells.”
Sun and graduate students Pavan Kambam and Daniel Sayut started with the quorum sensing system of Vibrio fischeri, a marine bacterium that colonizes the light organs of certain fish and squid to generate bioluminescence. Vibrio fischeri produces a signal molecule that is sensed by other members of the colony. When the colony becomes large and dense enough, the signal molecule reaches a level that triggers a chemical chain of events, eventually causing a certain gene to produce the bioluminescent protein. The entire colony acts as a unit, and this rapid, all-or-nothing response makes quorum sensing systems suitable for use as genetic switches.
The switches were constructed inside E. coli bacteria using a gene that produces a fluorescent protein and components from the quorum sensing system of Vibrio fischeri. The switches were activated by adding the signal molecule and the amount of protein produced was measured by looking at the fluorescence of the surrounding media. The switches proved to be easy to activate, responding when the signal molecule reached a level of about six molecules per bacterial cell, and the reaction could be easily controlled by adjusting the amount of the signal molecule that was added.
Sun and his team then set out to create a variety of switches that would respond to even lower levels of the signal molecule. The genetic material of wild Vibrio fischeri colonies was replicated using a method that is prone to errors to create a library of mutant proteins, a process known as directed evolution. Screening of over 40,000 V. fischeri colonies found several mutants that were 10 to 2.5 times more sensitive than their wild counterparts.
Sun used the mutant proteins to create a variety of switches that each respond to specific levels of the signal molecule and can be used at a specific cell density. “A manufacturer can decide to produce a protein at a specific cell density and choose a quorum sensing system to match,” says Sun.
Sun is currently seeking a patent for the genetic switches. Interested parties can contact the UMass Amherst Office of Commercial Ventures and Intellectual Property for additional information. Sun's ongoing research involves the directed evolution of a different protein from Vibrio fischeri which may be applicable to the development of new antibiotics as well as the construction of genetic switches.