Examining the Y. pestis strain tree from its Y. pseudotuberculosis base reveals modern strains and ancestral isolates with differences on the tree. Toward the bottom, some ancestral strains lack the plasmid pPCP1 that carries the pla gene, a major virulence determinant for the high-mortality pneumonic Y. pestis. In addition, the modern strains have a point mutant in Pla I259T. Researchers wanted to learn which factors may be involved in pneumonic disease.
First, several modern and ancestral strains were tested to learn which could infect mouse lungs. Both ancient strains like Angola and modern strains like CO92 were able to rapidly proliferate in mouse lungs. However, two ancient strains, Pestoides E and F, did not grow in the lungs, suggesting they lacked factors that allowed growth in this part of the body. The major difference between Angola and Pestoides E and F was that Angola had pPCP1 and the other two strains lacked it.
What happens when pla is removed from the modern strain CO92? CO92, CO92 Δpla and Pestoides F were introduced to mice intranasally, and subsequent infection followed. Both the mutant CO92 and Pestoides F strain were unable to replicate to high levels in the lungs, did not form big lesions and did not induce an influx of immune cells. In contrast, wildtype CO92 replicated to high levels in the lungs, forming large lesions, inducing inflammation and numerous infiltrating immune cells.
Researchers hypothesized that pPCP1 was necessary for bacterial growth in lungs. To test if transforming the Pestoides F strain with pPCP1 and thus, expressing pla, would work, both Pestoides F and CO92Δpla, which lacks pPCP1, were transformed with a kanamycin-resistant pPCP1 and the antibiotic resistance gene excised to prevent any issues with gene expression. In both Pestoides F and CO92Δpla, the pPCP1 was accepted and maintained, with the copy number slightly higher in Pestoides F compared to CO92 wildtype and CO92Δpla mutant. Tests also showed transcriptional regulation, expression and activity of pla in Pestoides F were similar to CO92 and other strains.
Was expresion of pla from pPCP1 in Pestoides F sufficient to cause pneumonic plague? When mice were infected intranasally with CO92, CO92Δpla or Pestoides F + pPCP1, Pestoides F + pPCP1 was able to replicate to high levels in the lungs and cause death similar to that seen with wildtype CO92. This modified Pestoides F strain that proliferated in the lungs induced large lesions, immune cell infiltration and inflammation, in contrast to Pestoides F lacking pPCP1 and similar to wildtype CO92 that carries pPCP1.
With the demonstration that an ancestral Y. pestis strain now able to cause pneumonic infection after acquiring pPCP1, what effect if any would the more modern I259T Pla have on lung virulence? Whether the modern CO92 or ancestral Pestoides F strain was expressing the T259 variant or the I259 variant, the number of colony forming units (cfu) in mouse lungs were the same, so the Pla variant did not seem to affect the pneumonic infection. However, there were fewer cfus in the spleen for I259 compared to T259, suggesting the Pla I259 variant may have another role to play during infection. In fact, when the two variants were tested using a subcutaneous mouse model, more closely mimicking bubonic plague, the I259T variant resulted in a more invasive, systemic infection compared to the more ancestral T259 Pla.
Zimbler et al. were able to convert an ancestral strain of Y. pestis that did not cause pneumonic infection to one that does after acquiring the plasmid pPCP1 and expressing pla. This experiment demonstrates that expression of pla is sufficient for causing pneumonic plague and offers insight into the evolution of Y. pestis.
Zimbler, D.L. et al. (2015) Early emergence of Yersinia pestis as a severe respiratory pathogen.
Nat. Commun. 6, 7487.
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