The last common ancestor of the Chlamydiales group of bacteria lived about 700 million years ago inside a eukaryotic host cell, according to a new study. This primeval chlamydia encoded many virulence factors now found in modern pathogenic chlamydiae as well as in Salmonella and Escherichia coli, according to the authors of the paper in Science published on April 8, 2004.

Matthias Horn and colleagues at the University of Vienna used the genome sequence of the group's recently discovered closest living relative—an endosymbiotic chlamydia living in amoebae—to reconstruct the genetic makeup of the last common ancestor. Among the species' virulence factors, now found in a wide range of bacterial pathogens, was a type III secretion system—a mechanism by which bacteria can inject their proteins into host cells in order to force the host cells do something to the bacteria's advantage.

Horn showed that the sequence of the symbiotic chlamydia had...

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