Live biosensors
Millions of people take probiotics to improve digestion. What if these bacteria could detect intestinal diseases? A new study by the Wyss Institute of Harvard University and Harvard Medical School has shown an effective non-invasive diagnostic method using new bacterial biosensors that can recognize and report the presence of various signs of disease in the intestine.
The essence of this work is to identify the genetic elements of bacteria that respond to various intestinal signals.
When E.coli bacteria are exposed to a certain biological signal, the trigger element embedded in their DNA puts the memory element in the "on" state, making it easy to identify then the bacteria that "remember" the presence of the signal. Source: Wyss Institute.
The discovery is based on a previously created genetic scheme consisting of a "memory element" derived from a virus and a synthetic "trigger element", which together can detect and register the presence of a certain stimulus – an initially deactivated version of the antibiotic tetracycline. The synthetic chain was integrated into the E.coli genome. Modified bacteria were injected into live mice, and then they were given tetracycline. The antibiotic caused a trigger element in the bacterial circuit to activate a memory element that "clicked" like a switch and remained "on" for a week so that the bacteria would "remember" the presence of tetracycline. The "on" signal was then easily read by analyzing animal excrement.
The team then demonstrated that the circuit could be configured to detect and report tetrathionate (a natural molecule that indicates the presence of inflammation) in the intestines of live mice for up to six months after administration. Thus, they showed that the system can be used to monitor signals useful for the diagnosis of intestinal diseases in the long term.
But tetrathionate is just one molecule. In order to carry out bacterial-based diagnostics, researchers needed a way to quickly test various potential trigger elements.
To do this, they changed the genetic contour by inserting a gene for resistance to the antibiotic spectinomycin. The gene is activated when the memory element goes into an "on" state, allowing bacteria to survive when exposed to an antibiotic. To test the updated scheme for a wide range of molecular signals, the researchers created a library of different strains of E. coli, each of which contained a memory element and a unique trigger element in its genome. This library of bacterial strains was then injected into the intestines of live mice to see if any of the trigger elements were activated by substances in the intestines of mice. Cultivation of bacteria from excrement in a medium containing spectinomycin showed that the number of strains increased, which means that their memory elements were turned on during passage through the digestive system of mice. The two strains showed consistent activation even when the mice were in isolation, meaning they were activated by conditions inside the intestine and can be used as sensors of gut-specific signals.
The researchers repeated the experiment using a smaller library of E.coli strains whose trigger elements were genetic sequences presumably associated with inflammation. Ten of them were activated during passage through the body of mice, one particular strain showed a stronger response in mice with inflammation compared to healthy mice.
Additional functions of the system include the ability to record signals that occur in the intestine either permanently or temporarily. Sensitivity can also be adjusted using sequences of synthetic ribosome binding sites embedded in trigger elements that can control the rate at which promoters "turn on" memory in response to a signal. These capabilities allow fine-tuning of bacterial biosensors to detect specific conditions in the intestine over a long period of time.
The authors write that they are able to bring the technology to a level where it will be enough for a patient to take a capsule with modified bacteria to diagnose intestinal diseases – just as millions of people now take capsules with probiotics.
Article by A. Naydich et al. Synthetic Gene Circuits Enable Systems-Level Biosensor Trigger Discovery at the Host-Microbe Interface is published in the journal mSystems.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on the materials of the Wyss Institute: Take Two E. coli and Call Me in the Morning.