It hides 'good bacteria' from the immune system, enabling them to reach tumors undetected.

The masked probiotic could revolutionize therapy. In experiments, it wiped out diseased cells in mice.

It is hoped the technique will work even better in humans, who are 250 times more sensitive to bacteria.

Biomedical engineer Professor Tal Danino said, "What is really exciting about this work is we are able to dynamically control the system.

"We can regulate the time bacteria survive in human blood, and increase the maximum tolerable dose.

"We also showed our system opens up a new bacteria delivery strategy in which we can inject bacteria to one accessible tumor, and have them controllably migrate to distal tumors such as metastases, cancer cells that spread to other parts of the body."

Infection fighting T cells sometimes fails to distinguish friend from foe. The genetically engineered "microbial encapsulation system" solves this.

In nature, bugs protect themselves from attack with a coating of sugary polymers known as capsular polysaccharides, or CAP.

Tetsuhiro Harimoto, a PhD student in the lab, said, "We have shown proof of concept in mouse models.

"But given that humans are 250 times more sensitive to bacterial toxins than mice, we expect our results may have an even bigger effect on human patients than on mice."