New vaccine sneaks into body, then self-destructs

By Maggie Fox, Health and Science Editor

WASHINGTON, July 7 (Reuters) - A new type of vaccine that sneaks into the body and then self-destructs -- all without needles -- may offer a new way to protect against a range of diseases, U.S. researchers reported on Monday.

The researchers genetically engineered a type of Salmonella bacteria to carry a little piece of Streptococcus and dripped it into the mouths of mice.

Reporting in the Proceedings of the National Academy of Sciences, they said the vaccine protected the mice, and the Salmonella carrier blew itself up.

"We have developed a technique of biological containment where the microorganism self-destructs," Roy Curtiss of the Biodesign Institute at Arizona State University, who led the study, said in a telephone interview.

"Not only does the bacteria lyse (break open) and die and have no survival, but it can be used with an antigen," Curtiss added.

An antigen is a protein that can be recognized and attacked by the immune system.

Curtiss and colleagues used an antigen found in Streptococcus pneumoniae, which causes bacterial pneumonia. They put it into Salmonella, a bacteria that invades cells and then reproduces out of control until it bursts the cell.

The vaccine protected mice from infection, carrying the strep antigen into cells. Then, before the Salmonella could do any damage, it burst open.

Curtiss believes the approach could be used against not only bacteria, but viruses, fungal infections and parasites.

It might solve the problem of using so-called live vaccines, Curtiss said. Such vaccines are highly effective but even when the bacteria or virus used to make the vaccine is attenuated, or weakened, it can escape and mutate into a dangerous form to cause disease.

The live polio virus vaccine, which is given orally as drops, is one example of where this has happened.

Curtiss developed a way to weaken the live Salmonella bacteria so it cannot live for long outside a laboratory dish.

"We used a gene that is regulated by the presence of the sugar arabinose," he said. This can be supplied in growth media used in lab dishes but is not found in the bodies of animals, including humans.

The bacteria can be grown in a way that prevents them from making a cell wall -- so they cannot survive and replicate.

"You deliver these vaccines orally," Curtiss said. "Some will be excreted, but they will not survive."

He hopes to start human tests within a year.

"Last, this system could be modified to provide effective biological containment for genetically engineered bacteria used for a diversity of purposes in addition to vaccines," Curtiss and colleagues wrote. (Editing by Will Dunham and Eric Walsh)