Injecting a common bacteria into oxygen-free regions at the center of cancerous tumors causes the tumors to shrink dramatically.

Clostridium novyi, a type of bacteria that lurks in soil and feces, can cause a range of mild illnesses when it finds an oxygen-free area to grow and multiply. The area at the center of a cancerous tumor can offer the bacteria just the right environment, spurring research into whether C. novyi could be used to fight cancer.

Johns Hopkins researchers genetically modified the bacteria to make it less toxic. Then they injected it directly into tumors growing inside live patients — 16 canine and one human — who were treated at Houston’s MD Anderson Cancer Center. The researchers reported promising results in a paper recently published in Science Translational Medicine.

The human patient’s tumor that was treated with bacteria shrank, while other tumors in the patient’s body continued to grow.

Three of the dogs were cured of cancer completely, and three others saw their tumors shrink by at least 30 percent. In some of the other cases, the tumors turned out not to have oxygen-free regions for the bacteria to infect.

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The injected bacteria sought out and infected anaerobic cells (which do not require oxygen) deep inside the tumors. Once inside those cells, the bacteria made enzymes that damaged cell walls and proteins. And because the body recognized the bacteria as an intruder, its presence jumpstarted the body’s own cancer-fighting immune response.