Tumour-dwelling microbes: A startling new frontier in cancer treatment

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Peter Strain

SALMONELLA bacteria are normally associated with violent food poisoning. But in 2019, Irit Balboul, a 71-year-old Canadian, volunteered to drink a liquid containing 1 billion live Salmonella typhimurium bacteria as a last resort against her pancreatic cancer, which had spread to other organs and given her just months to live.

The bacteria had been genetically engineered to trigger an immune attack on her cancer cells, while being less toxic than regular Salmonella to the rest of her body. Balboul was the first person in the world to trial them along with chemotherapy and her tumours shrank to just 10 per cent of their former size.

It may sound strange to use bacteria to treat cancer, but the approach harks back to the late 1800s, when William Coley, a New York surgeon, began injecting Streptococcus bacteria into the inoperable tumours of some of his patients, resulting in regressions he described as “nothing short of marvellous”. Despite this promise, the approach fell out of favour. More than a century later, however, we are witnessing Coley’s comeback.

It is now clear that many bacteria are naturally attracted to tumours, which are home to a rich microbial ecosystem. This tumour microbiome can influence how cancer progresses and responds to treatment. A clearer appreciation of this system is leading to the development of new microbial medicines targeting cancer, some of which are now in clinical trials – including the engineered Salmonella. These microbes can burrow deep into places that are hard to reach with existing treatments such as chemotherapy and can be equipped with extra, cancer-fighting weapons through genetic engineering, offering alternative ways to attack …

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