Spores of the Clostridium sporogenes bacterium can grow within tumours because there is no oxygen.
UK and Dutch scientists have been able to genetically engineer an enzyme into the bacteria to activate a cancer drug.
Experts said it would be some time before the potential benefits of the work - presented to the Society of Microbiology - were known.
The work is being presented to the society's autumn conference at the University of York.
The spores grow only within solid tumours, such as breast, brain and prostate tumours and not in other tissue in the body, where oxygen is present.
