Stiidy Of Interaction Among Yersinia Pestis Plasmids And Its Relation To Pathogenicity

Plague is an awful and natural infectious focus disease caused by Yersinia pestis. Inhuman history, three major plague pandemics have ever occurred, which once broughthideous disaster to humans. After the founding of New China, plague was controlledeffectively, but recently epidemiology and incidence of plague in the world exhibitedupward trend, and plague was classified as reemerging infectious disease by WHO in2000. Plague natural infectious focuses range widely, animal plague epidemic regioncontinue to grow, and human plague cases increased steadily in our country. In addition,Y. pestis is a typical biological warfare and bioterrorism agent. Therefore, study of Y.pestis pathogenicity is very important in maintaining the health of the people, economicconstruction and national security.Y. pestis has evolved from Y. pseudotuberculosis serotype O:1b. A typical Y. pestisstrain contains three virulent plasmids (pMT1, pPCP1and pCD1). The plasmid pCD1isinherited from Y. pseudotuberculosis, whereas pMT1and pPCP1was horizontallyacquired from outside. Y. pestis can cause awfully lethal disease, whereas Y.pseudotuberculosis only cause mild enteritis, indicating that newly acquired plasmidsare closely associated with strong pathogenicity of Y. pestis. Now then, is thereinteraction between the inherent plasmid and newly acquired plasmids? If this is thecase, does it effects on pathogenicity? In order to answer above two questions, based onMicrotus Y. pestis strain201, interaction among plasmids and its relation topathogenicity will be studied by virulence analysis, gene knockout and proteinexpression determination etc.Y. pestis strain201contains a cryptic plasmid pCRY except for plasmids pPCP1,pMT1and pCD1. The virulence of fifteen plasmid-deficient mutants from Y. pestis strain 201was evaluated by survival rate analysis in this study. The result indicated that theplasmid pCD1is essential to Y. pestis pathogenicity, the plasmid pMT1exhibited certainvirulence for mice, and the plasmid pPCP1and pCRY alone or together has a lowervirulence than the plasmid pCD1or pMT1. The result of bacterial load in mouse organsshowed that bacteria can be isolated from the mice infected with all theplasmid-deficient mutants containing pCD1, including pCD1+pMT1+pCRY+,pPCP1+pCD1+pMT1+, pPCP1+pCD1+pCRY+, pPCP1+pCD1+, pCD1+pCRY+and pCD1+,except for pCD1+pMT1+. This result indicated that the plasmid pMT1may havedepressant effects on pCD1, which can be counteracted by pPCP1or pCRY. Afterwards,the secretion of Yops was determined by combining SDS-PAGE electrophoresis withmass spectrography analysis. The results showed that comparing with otherplasmid-deficient strains, the expression of YopN and LcrV was inhibited inpCD1+pMT1+strain, demonstrating once more that the plasmid pMT1may havedepressant effects on pCD1, which can be counteracted by pPCP1or pCRY.To study the relation between the interaction among plasmids and pathogenicity,the mutants pPCP1+pCD1+pMT1+pla, pPCP1+pMT1+pla, pCD1+pMT1+ymt andpMT1+ymt were constructed by gene knockout method. The virulence analysis resultsindicated that murine toxin was a major virulent factor, which can be degraded by Plaunder the circumstance of loss of pCD1. Compared with pPCP1+pCD1+pMT1+,bacterial load of pPCP1+pCD1+pMT1+pla in mouse organs is evidently lower,indicating that the ability that the plasmid pPCP1promotes infection in vivo is mainlycontributed to the protein Pla. Currently, we only know that the plasmid pCRY maycounteract the inhibition of the plasmid pMT1, but the plasmids pPCP1, pMT1andpCD1are necessary to maintain full virulence of Y. pestis.