A procedure for monitoring the presence of the virulence plasmid (pYV) in Yersinia pestis under culture conditions

The pathogenicity of Yersinia pestis depends on the presence of a virulence plasmid (pYV). The unstable nature of pYV in Y. pestis leads to the eventual outgrowth of pYV-less cells due to its higher growth rate. Thus, it was necessary to develop procedures to monitor the presence of the plasmid during cultivation, storage, and laboratory manipulations. A procedure was developed to monitor the presence of pYV in Y. pestis by using low calcium response and Congo red binding techniques. The selection of pYV in the isolated clones was confirmed by polymerase chain reaction and by expression of pYV-associated phenotypes. Thus, using this procedure, low calcium response-Congo red-positive clones can be isolated for use in the development of growth models of virulent Y. pestis in food.