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Genetic clues to Sodalis deepens knowledge of bacterial diseases

December 15, 2005

By sequencing the genome of the symbiotic bacterium Sodalis, which lives off the major disease-transmitting insect, the tsetse fly, researchers at Yale School of Medicine have come a step closer to understanding how microbial pathogens cause disease.

Led by Serap Aksoy, professor in the Department of Epidemiology and Public Health at Yale School of Medicine, the team was highly interested in Sodalis because of its close relation to human bacterial pathogens like E.Coli, Salmonella and Yersinia.

Published online December 15 in Genome Research, the study looked at the genome in Sodalis, the second of three kinds of bacteria known to aid the tsetse fly in feeding off the blood of its host. The tsetse fly is one of many eukaryotes (animals) that live in association with symbiotic microbes or bacteria. These animals depend on the microbes for vital nutrients they can't otherwise produce.

"We have been able to develop the first system to grow Sodalis in vitro in the laboratory," said Aksoy. "We were then able to genetically modify the symbiotic bacteria and put it back into the tsetse fly to manipulate host traits or functions. Information on the blueprint of this organism now gives us a better handle on this applied strategy."

When the team sequenced the bacterial genome, they found the hallmarks of an organism transitioning from a free-living state to a symbiotic state. "It's a relatively young association between the tsetse host and the bacterium," said Aksoy. "Surprisingly, Sodalis has many of the same features of pathogenic bacteria, although it is obviously a beneficial organism that poses no harm to the tsetse host. We can now investigate how these potentially pathogenic features function in a beneficial relationship. It has changed our view of host pathogen characteristics."

Aksoy and her team also found that the genome itself, in terms of physical structure and size, looks similar to free-living bacteria. As relationships become symbiotic-more and more dependent on the host-bacterial genomes usually begin to shrink, but in the case of Sodalis, it is the same size as the free-living bacteria. It has the largest number of pseudogenes, products with no function, of any bacteria to date, again indicating its recent transition to a symbiotic lifestyle.

"If we get rid of these symbiotic bacteria, the flies become sterile, so understanding what they provide to the flies is very important from a vector control point of view," Aksoy said. "The sequence will be extremely helpful and will expand functional studies. We have increased understanding of how Sodalis transitions from a free-living to a symbiotic state."

Yale University