Question

How has the bacterium Y. pestis evolved to become virulent (based on your assigned literature and other literature)? [15 points]

Answer 1

The genus Yersinia consists of 15 species of which three are pathognomic to mammals these are Yersinia pseudotuberculosis,Y.pestis,Y enterolytica.This Y pestis is the organism causing plague also known as black death which is blamed for killing third of the population of Europe in middle ages.It still is present in some remote countries which is circulating in zoonotic reservoirs .Now moreover there is increased incidence of evolution of MDRS Ypestis this strain is resistant to multiple drugs thus making it even more difficult to treat.Now the question comes how Y pestis which is a pathogen of rodents evolved to become such a dangerous threat to mankind ?.

Now Y pestis is said to be evolved from the enteropathogen Y pseudotuberculosis as these two organisms are genetically identical but in fact these two organism produce 2 different diseases.So such a change in Y pestis to be so virulent is offered by evolution wherein Y pestis has accumulated additional genetic material such as pathogenicity islands and plasmids which increases its range of hosts and give it more mechanisms to overcome host immune response.Now the Y pestis in addition to having gained virulence factors it also has changed its target site as Y pestis is normally an enteric pathogen but by evolution it has now become an systemic pathogen.Now both Y pestis and Y pseudotuberculosis posses a large plasmid pCD1 which codes for a type 3 protein secretory system but Y pestis also possesses' 2 virulence plasmids in addition of the large plasmid .These two virulence genes are pPst and pFra.Now the pPst plasmid carries information about plasminogen activator and the pFra gene codes for capsular antigen F1 and murine toxin.

Now Y pestis is also show to cary genes that are required for flea transmission like hms locus and pla .The hms locus is required in flea transmission and a similar hms locus is found in Y pseudotuberculosis but it has hms ^_ thus showing that Y pestis has had mutation in hms locus from Y pseudotuberculosis.Now there are suscpicions that Y Pestis during its evolution is losing genes that are no longer required for it.Like O antigen which plays a important role in protection from the complement system is lacking in Y pestis because of mutations .Y pestis posesses multiple homoserine lactone mediated quorum sensing systems which is also present in its evolutionary counterpart Y pseudotuberculosis.But the quorum sensing monitors the expression of Flagella which is irrelevant for Y pestis as it is non motile.

Thus to summarize Y Pestis is said to be evolved from Y pseudotuberculosis and during the course of its evolution it has accumulated genes that make it capable of increasing its range of host infection and also making it change its habitat from enteric to systemic.

Hope you find the answer beneficial .