In its natural form, Clostridium novyi (C novyi) is found in the soil and, in certain cases, can cause tissue-damaging infection in cattle, sheep and humans.
The microbe thrives only in oxygen-poor environments, which makes it a targeted means of destroying oxygen-starved cells in tumours that are difficult to treat with chemotherapy and radiation, researchers said.
Researchers at the Johns Hopkins Kimmel Cancer Centre removed one of the bacteria's toxin-producing genes to make it safer for therapeutic use.
They tested direct-tumour injection of the C novyi-NT spores in 16 pet dogs that were being treated for naturally occurring tumours.
Six of the dogs had an anti-tumour response 21 days after their first treatment.
Three of the six showed complete eradication of their tumours, and the length of the longest diameter of the tumour shrunk by at least 30 per cent in the three other dogs.
Most of the dogs experienced side effects typical of a bacterial infection, such as fever and tumour abscesses and inflammation.
In a Phase I clinical trial of C novyi-NT spores conducted at MD Anderson Cancer Center, a patient with an advanced soft tissue tumour in the abdomen received the spore injection directly into a metastatic tumour in her arm.
The treatment significantly reduced the tumour in and around the bone.
"She had a very vigorous inflammatory response and abscess formation," according to researcher Nicholas Roberts.
"But at the moment, we haven't treated enough people to be sure if the spectrum of responses that we see in dogs will truly recapitulate what we see in people," said Roberts.
"One advantage of using bacteria to treat cancer is that you can modify these bacteria relatively easily, to equip them with other therapeutic agents, or make them less toxic as we have done here," said Shibin Zhou, associate professor of oncology at the Cancer Centre.
Verena Staedtke, a Johns Hopkins neuro-oncology fellow, first tested the spore injection in rats with implanted brain tumours called gliomas.
Microscopic evaluation of the tumours showed that the treatment killed tumour cells but spared healthy cells just a few micrometres away.
The treatment also prolonged the rats' survival, with treated rats surviving an average of 33 days after the tumour was implanted, compared with an average of 18 days in rats that did not receive the C noyvi-NT spore injection.
The research was published in the journal Science Translational Medicine.