The acquisition of a single gene transformed Y. Pestis from humble pathogen to Black Death

In a 2015 publication in Nature Communications, Wyndham Lathem et al described the inheritance of a plasmid, pPCP1, which afforded Yersinia Pestis - the causative agent of Plague - the ability to infiltrate and produce deadly plague in mammals. 

Y. Pestis

 Most of us are familiar enough with the old school-yard rhyme 'Ring around the Rosie,' to know that it is commonly believed to refer to the ghastly buboes that were symptomatic of Plague.  Less commonly known is the fact that the bacterium responsible for plague is actually capable of causing three different types of deadly plague; bubonic , septicemic, and pneumonic, the last of which is most deadly, causing nearly 100% fatality if untreated. 

The pPCP1 plasmid codes for a protease known as Pla. which is capable of performing a number of different functions vital to infiltrating and surviving in a host organism, including the break down of fibrous blood clots used by the immune system to sequester invading organisms.  

To determine pPCP1's effectiveness, the gene was inserted into strains ancestral to Y. Pestis who's genomes were known not to possess the plasmid. Progression and physiological effects were then compared to modern isolates of Y. Pestis referring to criteria such as c.f.u count, size of  pulmonary lesions, cytokine presence, as well as the expression of Pla.