Endotoxin containing 2.1 % nitrogen, 1.6% phosphorus, 22.5 % neutral hexose, 15% hexosamine, 25% esterified and amide-linked fatty acids, and 1.4% protein was isolated from Pasteurella pestis strain Alexander by slight modification of a method adapted by Tauber and Russell. The lipopolysaccharide exhibited classical endotoxic biological properties including: (i) toxicity in mice, guinea pigs, and rabbits; (ii) antigenicity in rabbits; (iii) capacity to evoke a biphasic pyrogenic response in rabbits; (iv) capacity to induce tolerance in mice to the lethal effect of endotoxin; (v) capacity to stimulate rapidly acquired resistance in mice to bacterial infection, and (vi) the capacity to produce the localized and generalized Shwartzman phenomena in rabbits. Findings obtained during the study concerning the occurrence, isolation, toxicity, and other biological properties of P. pestis endotoxin provide new evidence that endotoxin could contribute to death in plague. Early plague investigators proposed that Pasteurella pestis produced an endotoxin that contributed to death in plague (14). Early efforts to isolate the endotoxin by classical methods were unsuccessful (8, 9); however, in more recent attempts the phenol extraction method of Westphal et al. (26) has been used, and considerable success has been achieved (7, 24, 25). It is now known collectively from the studies of Davies (7), Walker et al. (25), Walker (23, 24), and Larrabee et al. (11) that P. pestis produces a lipopolysaccharide that kills mice, guinea pigs, rabbits, and monkeys with symptoms and pathological changes characteristic of endotoxin shock at death. These recent findings lend direct support to the endotoxic death concept in plague proposed by earlier investigators. The present investigation, which involved isolating P. pestis endotoxin and testing the toxic and biological properties of the endotoxin, was undertaken to provide additional knowledge concerning P. pestis endotoxin and to determine whether endotoxin could contribute significantly to the pathogenesis of plague. MATERIALS AND METHODS Stock culture. Fully virulent (guinea pig-passed) P. pestis strain Alexander (LD=o < 20 cells for mice and guinea pigs) possessing fraction I antigen, VW antigens, pigmentation, and pesticinogeny was used as a stock culture. Cultivation of organisms. Blood Agar Base slants (Difco, pH 6.8) were inoculated with stock P. pestis
