The plague-causing bacterium Yersinia pestis first evolved the ability to cause often fatal, pneumonic disease and then developed its highly infectious nature, a study in this week's Nature Communications reveals. The transitions were underpinned by relatively simple molecular changes, suggesting that other respiratory pathogens could possibly emerge via a similar route.
Sometime in the last 10,000 years, the bacterium Yersinia pseudotuberculosis, which causes mild gastrointestinal problems, evolved into Yersinia pestis, which causes different types of plague including bubonic, septicemic and the most deadly and infectious form, pneumonic plague. Wyndham Lathem and colleagues now use 'ancestral' isolates of Y. pestis, representing intermediate lineages between the two species, to retrace the evolution of the bacteria and show how this transition occurred.
Ancient strains of Y. pestis acquired a single gene that gave them the ability to cause pneumonic plague. Then later, a single amino acid change in the protein encoded by that gene was enough to make more modern strains of the bacteria highly infectious. The result was a perfect storm; a bacterium that is not only deadly, but that has the potential for pandemic spread.