Studies on the Specific Soluble Proteins of Pasteurella Pestis and P. Pseudotuberculosis

Abstract

Summary and Conclusions A complement fixation test has been employed in the study of the specific protein fractions of the virulent and avirulent plague and pseudotuberculosis strains in the elucidation of the antigenic structure of these organisms. The P⅓ fraction of the soluble proteins of virulent and protective plague strains grown in enriched casein hydrolysate broth specifically fixes the complement, no cross-reactions being observed with the antisera against either the protein fraction or the whole of the avirulent nonprotective plague or pseudotuberculosis organisms. The test confirms the findings of other serological tests, viz. precipitation and agglutination, carried out with these protein fractions or their antisera. The water-extractable fraction of a virulent plague strain is the most specific antigen found, but the tests fails to distinguish between the virulent and the relatively avirulent protective strains and a comparatively larger dose is required for mouse protection. Antigenic deterioration of plague strains can be detected by cross complement fixation tests. Antisera against pseudotuberculosis strains have given divergent results amongst themselves and further work with their protein fractions is indicated. Antiplague rabbit sera are preferable to anti-plague horse sera for the complement fixation test. The immunogenic values of the protein fractions and of whole vaccines prepared from various plague and pseudotuberculosis strains, have been determined. The results confirm the serological findings. Three categories of plague strains can be distinguished, viz., (1) virulent protective, (2) avirulent protective and (3) avirulent non-protective, the pseudotuberculosis strains falling under the last group. The protective power of the Haffkine plague vaccine lies in the protein fractions of its supernatant. The P⅓ fraction (antigen A) of the first or the second group of organisms is fully responsible for the mouse protection given by the vaccine. Its immunogenic values seem to depend on the quality of the medium used. The corresponding P½-⅓ fraction (antigen B), on the other hand, gives practically no protection, even in heavy doses, but further work is needed to confirm this observation. The results of mouse-protection tests with the anti-protein sera support the above findings. Certain future lines of studies on plague have been indicated, including the possibility of replacing with advantage the whole vaccine by the P½ fraction for human immunization.