NEW YORK (GenomeWeb) – A study by researchers at Northwestern University sheds new light on how the plague bacterium Yersinia pestis evolved from a less pathogenic ancestor to a vicious killer.
According to the study, published in Nature Communications today, the acquisition of a single protease gene early in the evolution of the bug enabled it to infect the lungs and cause pneumonic plague. Later on, a small change in the same protease gene that altered a single amino acid allowed Y. pestis to spread beyond the lungs and cause the invasive infection that's the hallmark of bubonic plague.
The findings outline how other new respiratory pathogens may emerge in the future, by gaining just a small amount of new genetic material, the authors wrote.
Y. pestis only evolved from Y. pseudotuberculosis within the last 10,000 years, through the gain and loss of several genes. While Y. pseudotuberculosis causes only mild gastrointestinal disease and is transmitted through contaminated water and soil, the plague bacterium leads to deadly and virulent disease — it can cause bubonic, septicemic, and pneumonic plague — and is transmitted by fleabites and aerosols. Pneumonic plague, the deadliest of the three forms, kills almost 100 percent of the time if untreated and results in severe pneumonia.
It was already known that one requirement for Y. pestis to cause pneumonic plague is to possess the protease Pla, which is encoded by a plasmid, pPCP1, that Y. pestis picked up along the way as it evolved from Y. pseudotuberculosis. The amino acid sequence of Pla is completely conserved in all modern strains of Y. pestis, but ancestral strains differ in a single amino acid — an isoleucine instead of a threonine at position 259 — that makes the protease less efficient.
What was previously unknown was when during its evolution Y. pestis gained the ability to cause pneumonic plague. In their study, the researchers, led by Wyndham Lathem of the department of microbiology-immunology at Northwestern, showed that when ancestral Y. pestis lineages acquired the qPCP1 plasmid — and with it, the Pla protease — they became capable of infecting the lungs, and no other genetic changes were needed for them to cause pneumonic plague.
The single amino acid change in the protease enabled the bacterium to spread beyond the lungs and cause systemic disease, "suggesting that Y. pestis was primed to cause a fulminant pneumonia before its ability to efficiently cause invasive infections," the authors wrote.
In addition, the protease mutation "may have been one of the defining events that enabled pandemic spread of Y. pestis," they noted.
For their study, they infected mice via the nose with ancestral plague strains — Angola, Pestoides A, E, and F — as well as with the modern strains CO92 and KIM, and found that Pestoides E and F, which do not carry the pPCP1 plasmid, cannot grow rapidly in the lungs, whereas the other strains do.
Next, they infected mice via the nasal route with wild type CO92, CO92 lacking the Pla protease, and Pestoides F. The latter two killed mice more slowly, were unable to grow to high levels in the lungs, and led to a smaller host immune response, demonstrating that they are unable to cause primary pneumonic plague.
After that, the researchers introduced the pPCP1 plasmid — which carries the modern Pla gene that contains the threonine substitution — into Pestoides F and showed that it expressed active Pla protease at similar levels as the CO92 strain.
When they introduced this modified strain into mice through the nose, the bacteria grew to high levels in the lungs and killed the animals within days, similar to wild type CO92. This showed that acquiring the pPCP1 plasmid was sufficient to transform an early strain of Y. pestis into one that causes pneumonic plague.
Finally, they tested what difference the amino acid change in the Pla protease makes. They introduced the older, isoleucine-containing protease into both Pestoides F and modern CO92 and found no significant difference in their ability to cause pneumonic plague, compared to the same strains containing the modern protease gene.
However, when they infected mice subcutaneously with the four strains, the ones carrying the modern, threonine-containing protease grew to higher levels in the spleen than the ones carrying the ancestral protease, indicating that the amino acid change "was important for Y. pestis to cause a disseminated, invasive infection from the site of inoculation, such as occurs during bubonic plague," the authors wrote.