Arctic genes kill bacteria

Genes from cold-loving bacteria may one day be used to create live bacterial vaccines for common pathogens such as __Salmonella__ and the tuberculosis bacterium, according to a linkurl:study;http://www.pnas.org/content/early/2010/06/24/1004119107.abstract published yesterday (July 12) in the __Proceedings of the National Academy of Sciences__.

Image:flickr/DrShapero

By swapping genes that are essential for survival in pathogenic bacteria with those of their counterparts in cold-adapted bacteria from the arctic, microbiologist linkurl:Francis Nano;http://web.uvic.ca/~fnano/ at the University of Victoria in Canada and his colleagues succeeded in rendering them temperature-sensitive -- meaning, they died when exposed to normal temperatures. Moreover, when these new strains were injected into mice, they were unable to proliferate into the warmer internal organs of the animals but remained near the skin. Afterward, these animals did not fall ill from wild-type strains, suggesting the modified bacteria primed their immune cells against them, thus immunizing them. This paper stresses the importance of looking at the many adaptations of wild microbes for their uses in medicine, said linkurl:Matt Wallenstein,;http://warnercnr.colostate.edu/~mawallen/ who studies microbial diversity in Arctic soils at Colorado State University and was not involved in this study. "This is a really good example of the untapped potential of microbial diversity." Nano had been studying proteins made by cold-loving bacteria when he observed that they tended to die between 19 and 33 degrees Celsius, but sometimes as low as 12 degrees. "It seems obvious that an essential protein was denaturing at the higher temperature and causing them to die," he explained. Coincidentally, he had been carrying out a separate research project on pathogenic bacteria, in particular, the gram-negative __Francisella tularensis__, cause of tularemia. He then got the idea of transferring the genes encoding those temperature-sensitive proteins into pathogens to see if they died at lower temperatures as well. So the authors chose a set of genes, mostly coming from the Arctic bacterium __Colwellia psychrerythraea,__ and swapped them for their homologs in a __Francisella__ subspecies __F. novicida__, which is highly virulent in mice but not in humans. From those, they narrowed the list down to five genes which did not mutate into a temperature-resistant version after several generations. They then proceeded to infect a culture of mouse macrophages with the strains of __F. novicida__ carrying the temperature-sensitive genes. They spotted several genes that allowed __F. novicida__ to grow normally inside the macrophages at 30 degrees Celsius, but could not withstand a seven degree increase in temperature. Then, for the ultimate test, they injected the tails of mice with the temperature-sensitive __F. novicida__ strains. During a typical infection in the skin, immune cells called follicular dendritic cells destroy the microbes and present the antigens to the immune system by traveling first to the local lymph nodes under the skin, then to the armpit, groin, neck, and finally into internal organs such as the spleen and liver, coaxing the body to develop a cell-mediated immune response to fight off further attack. But the modified bacteria did not reach the internal organs. A temperature difference between the skin and the body core of up to eight degrees Celsius meant the temperature-sensitive bacteria couldn't venture far from the surface. Indeed, Nano and his team found that a few days after infection, the bacteria persisted in the skin but "they never made it alive to the internal organs." Borrowing from concepts used to develop temperature-sensitive viral vaccines, the authors hypothesized that these non-threatening superficial infections could provide immunity from the wild __F. novicida__. When they tested this out, they found that mice that had been previously infected with the sensitive bacteria didn't get sick from the wild strains. To test whether some of the cold-adapted genes had the same effects on other types of bacteria, Nano inserted Arctic versions of a DNA ligase into strains of __Salmonella enterica__ and __Mycobacterium smegmatis__ (a research surrogate for __M. tuberculosis__), and found that they too became sensitive to temperature. All three microbes are intracellular parasites that infect macrophages and require T-cell immunity to clear them. __M. smegmatis__ is a gram-positive bacterium, as opposed to __F. novicida__ and __S. enterica__. "We did this as a demonstration to show that the same gene works on a wide variety of bacteria," Nano said. "We think one of the best applications would be to make a better tuberculosis vaccine." (Indeed, a provisional patent application has been filed that covers the technology.) This novel method of rendering bacteria temperature-sensitive could prove useful not only in the production of live bacterial vaccines, but also for making vaccines from heat-killed pathogens that can still induce an immune response. "When you do a mild heating you don't denature the proteins. So their proteins stay in their normal state and are better in inducing a proper immune response."B.N. Duplantis, et al., "Essential genes from Arctic bacteria used to construct stable, temperature-sensitive bacterial vaccines," PNAS, doi: 10.1073/pnas.1004119107, 2010.
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