Antigenic Structure of Pasteurella pestis and the Isolation of a Crystalline Antigen.∗

The present report is a summary of investigations initiated in 1942 to extend the knowledge of the antigenic structure of P. pestis, and to use this information to place plague prophylaxis on a sounder basis. The detailed experimental data will be reported at a later date.

All the studies, unless otherwise stated, were performed with dried plague bacilli of the virulent “Yreka” strain. The bacilli were grown on agar for 3 days at 37°C and suspended in saline. This was precipitated by one to 2 volumes of acetone cooled to—70°C and left overnight. Repeated washings with acetone and drying in vacuo produced a bacterial powder of high antigenicity and toxicity (LD₅₀ for 20 g mice: 20-40 μg).

Extraction of the acetone-dried plague bacilli with neutral salt solutions (0.85%, 2.5% sodium chloride solutions or 0.4% sodium acetate solution) yielded a water-soluble and a water-insoluble antigenic component. The water-soluble fraction is quite toxic for mice and rats (LD₅₀ for 20 g mice, 8-15 μg), and has a high immunogenic value for these species, but a low value for guinea pigs. The water-insoluble portion is nontoxic for mice, and produces a very poor immunogenic response in mice and rats. However, this fraction, when tested as an alum precipitate, has a high immunogenic value, equal to that of whole plague bacilli, for guinea pigs.