Experimental evaluation of interaction between Yersinia pestis and soil infusoria and possibility of prolonged preservation of bacteria in the protozoan oocysts

The character and outcome of interactions between Y. pestis (vaccine strain and soil infusoria Tetrahymena pyriformis (axenic culture) were under experimental study. The parallel use of the bacteriological method and PCR test systems made it possible to follow the dynamics of Y. pestis cells (strain EV) with different plasmid profiles in their interaction with infusoria, as well as their passage into the protozoa cysts. The study revealed the complete utilization of Y. pestis cells lacking virulence factors by infusoria. The presence of plasmids of virulence influenced only the duration of complete bacterial phagocytosis. A drop in the temperature of cultivation to 2 degrees C induced the mass and rapid encystment of infusoria. In the PCR analysis specific DNA fragments of Y. pestis cells, preserved in the latent (uncultivable) state, were detected in the cysts of protozoa within the period of up to 14 months, while the results of bacteriological studies were negative. The data thus obtained are discussed with regard to the possible mechanisms of the existence and prolonged reservation of Y. pestis in the soils of natural foci with participation of protozoa.