Intraspecies and temperature-dependent variations in susceptibility of Yersinia pestis to the bactericidal action of serum and to polymyxin B

Abstract

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 degrees C; fleas, 25 degrees C; or winter hibernation, 6 degrees 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 subspecies causing human plague), high levels of resistance to PMB and NHS were detected at 25 degrees 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 degrees C and 37 degrees 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 degrees C, all strains were highly sensitive to NHS. Variations in the PMB resistance of different bacterial cultures related to both the content of cationic components (4-amino-4-deoxyarabinose in lipid A and glycine in the core) and a proper combination of terminal monosaccharides in the LPS. The NHS resistance correlated with an elevated content of N-acetylglucosamine in the LPS. Structural variation in the LPS of Y. pestis correlates with the organism's ability to resist innate immunity in both fleas and mammals.

FIG. 1.

PMB binding by Y. pestis LPS preparations isolated from organisms grown at 25°C or 37°C. The standard deviation at all points was less than 10% of the value.

FIG. 2.

Bactericidal activities of NHS (A) and IHS (B) versus those of HI-NHS and HI-IHS, respectively, toward Y. pestis strains grown at different temperatures. Lane 1, KM218 (6°C); lane 2, KM218 (25°C); lane 3, KM218 (37°C); lane 4, KIMD1 (6°C); lane 5, KIMD1 (25°C); lane 6, KIMD1 (37°C); lane 7, KM260 (11) (6°C); lane 8, KM260 (11) (25°C); lane 9, KM260 (11) (37°C); lane 10, 1146 (25°C); lane 11, 1146 (37°C); lane 12, EV11M (25°C); and lane 13, EV11M (37°C). Each bar depicts the mean of three independent determinations. Error bars represent 95% confidence intervals; a nonoverlapping 95% confidence interval error bar indicates a difference at a P value of <0.05.