Modern cancer treatments have become remarkably effective, but they all face the same problem. However precisely a tumour is identified, destroying it without damaging healthy tissue remains difficult. Surgery removes healthy tissue along with the tumour. Radiotherapy can affect nearby cells. Chemotherapy attacks healthy fast-growing cells as well as cancerous ones.
Researchers have spent decades looking for ways around that problem. One possibility comes from an unexpected source: bacteria.
Certain bacteria can survive only where oxygen levels are extremely low. That makes them almost impossible to establish in healthy tissue, but well suited to the interior of many solid tumours. As a tumour grows, its blood supply becomes increasingly disorganised, leaving large regions starved of oxygen. Spores carried through the bloodstream remain inactive almost everywhere else before becoming active inside the tumour itself.
At first glance, this looks like an elegant solution. The bacteria naturally target cancer while leaving healthy tissue alone. The reality is more complicated.
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One of the bacteria attracting attention is Clostridium sporogenes, a harmless soil bacterium. It readily colonises the oxygen-starved centre of a tumour, but struggles to survive closer to the outer edge where oxygen levels are higher. The result is that while the middle of the tumour can be damaged, living cancer cells remain around the outside and continue to grow.
Researchers at the University of Waterloo decided to tackle that limitation as a design problem rather than a biological one. Instead of searching for another bacterium, they redesigned the one they already had.
The first step was to increase the bacterium’s tolerance to oxygen by introducing an additional gene. That allowed it to survive further towards the edge of the tumour. But solving one problem immediately created another. A bacterium capable of surviving in oxygen-rich tissue also has the potential to become active where it is not wanted.
The answer came from another natural bacterial behaviour known as quorum sensing. Many bacteria constantly release signalling molecules that allow them to detect how many neighbouring cells are nearby. Once enough bacteria have gathered together, their behaviour changes. The researchers used that communication system as a biological control circuit, switching on the enhanced oxygen tolerance only after the bacteria had established themselves inside a tumour.
It is this control system that makes the work particularly interesting from an engineering perspective. Rather than simply modifying the bacterium, the researchers designed a system in which different functions are activated only under specific conditions. The same approach could eventually be used to release drugs, stimulate the immune system or trigger other therapeutic responses only after the bacteria have reached their target.
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The work remains at an early stage, but it reflects a much broader change taking place in medicine. Synthetic biology is allowing researchers to design living systems in much the same way engineers design machines, building in sensing, control and programmed responses to solve increasingly complex problems. In the future, some of the most sophisticated medical technologies may not be mechanical devices at all, but engineered organisms carrying out precisely the task they have been designed to perform.