We put the bacterium through its paces
Acinetobacter baylyi, a naturally competent bacterium, was chosen to be the experimental biosensor – a disease-detecting cell.
Our team modified the A. baylyi genome to contain long sequences of DNA to mirror the DNA found in a human cancer gene we were interested in capturing. These “complementary” DNA sequences functioned as sticky landing pads – when specific tumour DNA was taken up by the bacteria, it was more likely to integrate into the bacterial genome.
It was important to integrate – hold in place – the tumour DNA. In doing so, we could activate other integrated genes, in this case an antibiotic resistance gene, as a signal for the cancer being detected.
The signal would work as follows: if bacteria could be grown on antibiotic-laden culture plates, their antibiotic resistance gene was active. Therefore they had detected the cancer.
We conducted a series of experiments in which our new bacterial biosensors and tumour cells were brought together in increasingly complex systems.
Initially, we simply marinated the biosensor with purified tumour DNA. That is, we presented our biosensor with the exact DNA it was built to detect – and it worked. Next, we grew the biosensor alongside living tumour cells. Again, it detected the tumour DNA.
Ultimately, we delivered the biosensor into live mice that either did or did not have tumours. In a mouse model of colorectal cancer, we inject mouse colorectal cancer cells into the colon, using mouse colonoscopy.
Over several weeks, the mice that were injected with cancer cells develop tumours, while the mice that were not injected serve as the healthy comparison group. Our biosensor perfectly discriminated between mice with and without colorectal cancer.
CATCH’s promising start – but more testing is needed
After these encouraging results, we engineered the bacteria even further. The biosensor can now tell apart single base pair changes within the tumour DNA, allowing for finely tuned precision in how it detects and targets the genes. We have named this technology CATCH: cellular assay for targeted, CRISPR-discriminated horizontal gene transfer.
CATCH holds great promise. This technology uses cell-free DNA as a new input for synthetic biological circuits, and thus for the detection of a range of different diseases, particularly infections and cancers.
However, it is not yet ready to be used in the clinic. We’re actively working on the next steps – to increase the efficiency of DNA detection, to more critically evaluate the performance of this biosensor compared to other diagnostic tests, and, of course, to ensure patient and environmental safety.
The most exciting aspect of cellular healthcare, however, is not in the mere detection of disease. A laboratory can do that.
But what a laboratory cannot do is pair the detection of disease (a diagnosis) with the cells actually responding to the disease with an appropriate treatment.
This means biosensors can be programmed so that a disease signal – in this case, a specific sequence of cell-free DNA – could trigger a specific biological therapy, directly at the spot where the disease is detected in real time.
