Intraspecies and Temperature-Dependent Variations in Susceptibility of Yersinia pestis to the Bactericidal Action of Serum

Lipopolysaccharide (LPS) structure impacts the bactericidal action of cationic peptides, such as polymyxin B (PMB), and sensitivity to killing by normal human serum (NHS). Cultivation of different subspecies strains of Yersinia pestis isolated from unrelated geographic origins at various temperatures (mammals, 37°C; fleas, 25°C; or winter hibernation, 6°C) affects LPS composition and structure. We tested the susceptibilities of various strains of Y. pestis grown at these different temperatures to PMB and serum bactericidal killing. Both properties varied significantly in response to temperature changes. In Y. pestis subsp. pestis (the main subspe-cies causing human plague), high levels of resistance to PMB and NHS were detected at 25°C. However, at the same temperature, Y. pestis subsp. caucasica was highly sensitive to PMB. At both of the extreme temperatures, all strains were highly susceptible to PMB. At 25°C and 37°C, Y. pestis subsp. caucasica strain 1146 was highly susceptible to the bactericidal activity of 80 % NHS. All Y. pestis strains studied were able to grow in heat-inactivated human serum or in 80 % normal mouse serum. At 6°C, all strains were highly sensitive to NHS. Variations in the PMB resistance of different bacterial cultures related to both the content of cationi