So-called temperature-sensitive vaccines already exist for a few viruses. An influenza vaccine called FluMist, for instance, consists of a weakened flu virus that can't grow at 37˚C, the temperature inside the lungs, but that can reproduce in the slightly cooler nose and throat. As it does so, it triggers a protective immune response.

The strain used in FluMist was created by growing flu virus for many generations at gradually declining temperatures and letting it adapt. For bacterial pathogens, Francis Nano of the University of Victoria in Canada, took a very different tack: He turned to the psychrophilic, or cold-loving, bacteria from Earth's polar regions. These organisms have many enzymes that can function only at lower temperatures; turn up the heat and they die.

For a proof of principle, Nano used a bacterium called Francisella novicida, an innocuous cousin of a human pathogen called F. tularensis. (The latter is high up on the list of potential bioweapons because it causes incapacitating malaise and fever and would be easy to disseminate.) One at a time, the team swapped out nine so-called essential genes—involved, for instance, in DNA repair or cell division—in F. novicida for their counterparts from Arctic bacteria, such as Colwellia psychrerythraea, a marine microbe that lives in polar waters and ice. Francisella normally dies at 45˚C; introducing the cold-loving genes lowered that threshold by up to 12˚C, depending on the gene and the species it was borrowed from.

When the researchers injected these altered strains into the relatively cool tails of rats, they found that the microbes reproduced locally but didn't spread to the warmer spleen and lungs, where they would normally replicate as well. The key test was whether the strains could act as vaccines. F. novicida is lethal to mice, but when the researchers injected the temperature-sensitive strains, the animals didn't get sick—and they were protected from an otherwise fatal dose of the unaltered F. novicida given 3 weeks later.

Nano—who has patented the method and assigned the rights to his university—says he's particularly interested in developing a vaccine for TB, which kills more than a million people a year and for which only a very imperfect, old vaccine exists. As part of the study, the researchers showed they could make Mycobacterium smegmatis—a research stand-in for M. tuberculosis—temperature-sensitive, but they didn't test its potential as a vaccine. The team reports its findings online today in the Proceedings of the National Academy of Sciences.

"It's a unique and very innovative approach," says Michael Brennan, senior adviser for global affairs at Aeras Global TB Vaccine Foundation in Rockville, Maryland. But the method still has quite a way to go, he cautions. Because these vaccines are alive, safety is going to be a crucial issue, Brennan says. The researchers will have to show that it's impossible for the microbes to lose their temperature sensitivity and cause disease.

The method could have other uses as well, Nano notes. Right now, research on many pathogenic bacteria "is like working on the space shuttle" because of the strict containment measures that prevent researchers from becoming infected. Creating harmless strains of the same bacteria that are identical except in their maximum temperature could make such studies a lot easier, he says.

Autor: Martin Enserink
Original Date: July, 12 2010
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