Designer bacteria have been genetically engineered to bolster the body's immune response against tumours.
Bacteria can mobilise the immune system
Researchers in Leeds added a gene to a strain of bacteria that is harmless despite being related to the bugs that cause tuberculosis.
The extra gene makes the bacteria produce a molecule that boosts the immune system's ability to identify and kill cancer cells.
The research is published in the International Journal of Cancer.
In mice, the modified bacteria worked much better against tumours in the bladder than existing treatment.
The researchers say that, although the research is still at an early stage, this enhanced bacterial strain could become a safer and more effective therapy for bladder cancer.
Current treatments for bladder cancer include BCG, the bacteria best known as a vaccine against tuberculosis.
A neutralised strain of the bacteria is injected close to the tumours, and is thought to stimulate the local immune system, which then kills both cancer cells and the bacteria.
However, BCG does not work in about a third of patients with bladder cancer - despite being the most effective immunotherapy yet developed against any form of cancer. It can also cause significant unwanted side effects.
Researchers in the Cancer Research UK Clinical Centre at St James's University Hospital, Leeds, aimed to improve both the safety and effectiveness of the treatment.
They took harmless bacteria - Mycobacterium smegmatis - and added a gene to produce a protein called tumour necrosis factor alpha (TNFa).
TNFa is a part of the immune system, with a crucial role in killing unwanted cells, including cancer cells, and is produced in response to BCG.
In mice with bladder cancer, the new treatment led to a reduction in the size and number of tumours.
Eight out of 10 of the animals had no tumours by the end of the treatment.
Professor Peter Selby, Director of the Clinical Centre, said: "BCG works by enhancing the immune response.
"We've shown that by modifying a related bacterium, we can boost the immune system in such a way that it kills cancer cells even more effectively.
"Our report suggests important avenues for future clinical research.
"The safety of these bacteria would need to be formally verified before they could be tried in people, but the prospects are exciting."
Professor John Toy, Cancer Research UK's Medical Director, says: "Using the patient's own immune system to fight cancer is becoming an increasingly promising approach.
"This research combines genetics with immunotherapy to offer the hope of a new way to treat bladder cancer more safely and effectively."
Each year bladder cancer affects more than 12,500 people in the UK and accounts for nearly 5,000 deaths.
It affects about two and a half times as many men as women, most cases developing after the age of 50.