Like any good military unit, infectious bacteria have access to numerous weapons and efficient communication systems. But like soldiers in the field, they're also susceptible to suicide bombers. Researchers have used the tools of synthetic biology to create an Escherichia coli cell that can infiltrate foreign bacteria and explode, killing off the pathogens along with itself.
The project, says bioengineer Chueh Loo Poh of Nanyang Technological University in Singapore, was "inspired by nature," particularly by quorum sensing, the ability of some bacteria to detect the number of microorganisms—either of their own species or others—in their environment. When pathogenic Pseudomonas aeruginosa sense other species impeding on their space and nutrients, they communicate with members of their own species using chemical signals and collectively start releasing a bacterial toxin called pyocin that kills off the competition. Together, these communication and defense capabilities allow P. aeruginosa to form tightly packed layers called biofilms, which can cause respiratory tract infections in humans and are particularly dangerous to cystic fibrosis patients.
Poh and chemical engineer Matthew Wook Chang, also at Nanyang Technological University, decided to turn P. aeruginosa's weapon system against itself, using E. coli as the carrier. The researchers tweaked the genes that allow P. aeruginosa to detect other members of its species and put this synthetic genetic code into E. coli's genome. They also gave E. coli a gene for making a modified pyocin that is toxic to P. aeruginosa. By linking the pyocin gene to the sensing genes, the researchers ensured that when the E. coli detected P. aeruginosa in the vicinity, it would fill itself with large amounts of pyocin and become a biological time bomb.
The researchers gave E. coli one last synthetic component: a "suicide gene" that is activated once the pyocin has had some time to build up, causing the cells to burst open and release their toxin. When Chang and Poh grew these synthetic E. coli in a dish with P. aeruginosa, the suicide bomber was able to kill 99% of the P. aeruginosa cells, the researchers report today in Molecular Systems Biology.
Justin Gallivan, a synthetic biologist at Emory University in Atlanta, says in an e-mail that the study "nicely illustrates" how synthetic bacteria can perform complex tasks. But he worries they may not be able to finish the job, because 1% of the infectious bacteria remained after the treatment—even when the researchers put four times as many E. coli as P. aeruginosa into the mix.
The system would also have to undergo a lot of work before it can be considered for use in humans—including, perhaps, replacing E. coli with another delivery system, says Richard Kitney, a synthetic biologist at Imperial College London. "Exposing people to E. coli is not a good thing," Kitney says, as the bacteria are toxic outside the gut. He adds that the team would also have to show that pyocin is effective at killing P. aeruginosa that have already formed a biofilm.
For their part, Chang and Poh say that they plan to test the suicidal bacteria in mice infected with P. aeruginosa. It's not clear, they say, whether pyocin is harmful to mammals, although some other natural bacterial toxins are currently approved for use as food preservatives. They also hope to tweak the synthetic system so that it can sense and respond to signaling molecules released by other species of pathogenic bacteria, such as those responsible for cholera.