Proc. Natl Acad. Sci. USA 106, 15442–15447 (2009)

Cholera has affected humans for more than a hundred years, but how the bacterium that causes the disease, Vibrio cholerae, has evolved had not been described.

Rita Colwell of the University of Maryland in College Park and her collaborators compared the genomes of 23 strains of the bacteria isolated over the past 98 years. They found that the strains responsible for the current cholera pandemic, which started in 1961, are descendants of a single strain, and evolved mainly through gene transfer with other strains in the environment. The culprits behind the previous pandemic, in the early twentieth century, were from a different lineage altogether.

Because of the bacteria's rapidly evolving genome, the researchers say that cholera strains should be identified by gene content rather than by cell-surface protein marker.