Environmental Aspects of Yersinia pestis Virulence and Selection in the Natural Plague Focus

Pathogenicity factors ensure Y. pestis dissemination in the ambient environment and in carriers’ and vectors’ organisms, protective barrier penetration and the homeostasis-impairing ability. These are specific traits of the rodent-flea ecological niche. Virulence is a key prerequisite for pathogenic bacteria persistence in the host populations, and also an environmental selection factor resulting from interdependence of the main carrier and Y. pestis. Y. pestis populations form with different species of main carriers; their properties are determined by Y. pestis specific conditions and flexibility. With variability and occurrence of mutants with broad range of properties, the limiting factor for Y. pestis in focus is the main carrier’s genetically determined resistance level. Formation of Y. pestis population norm is the main carrier’s prerogative. Circulation of Y. pestis populations, typical for the focus, is provided by stabilising selection based on selective advantage, elimination of deviations from the norm and fleas’ capability of transmitting the agent. The Y. pestis existence in the rodent-flea ecological niche is based on specific levels of constitutive and functional virulence, which are crucial for the agent selection. Flexibility of Y. pestis population virulence is ensured by properties affecting the virulence and those “fine-tuning” populations in the main carrier’s organism.