Characterization of the Madrid E strain of Rickettsia prowazekii purified by renografin density gradient centrifugation

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Abstract
The avirulent Madrid E strain of Rickettsia prowazekii cultivated in chicken yolk sacs could be purified successfully with a Renografin density gradient method developed previously for Rickettsia typhi. Recovery during purification, viability, and lack of contamination with host cell components were similar for the two species, although yields of R. prowazekii per yolk sac were lower. Purified typhus rickettsiae provided satisfactory antigens in the complement fixation, Ouchterlony double-diffusion, and microagglutination tests. The retention of the typhus soluble group antigen during purification was readily demonstrated by complement fixation tests. However, removal of the soluble group antigen by ether treatment was not always adequate for the demonstration of type-specific particulate antigens. Heat-killed R. prowazekii cells gave higher serum microagglutination titers than untreated or formalized cells, a difference was noted for R. typhi cells. Although the protein profiles of whole cells and extracts of R. typhi and R. prowazekii on sodium dodecyl sulfate-polyacrylamide gels were relatively similar, a small but reproducible, difference in the electrophoretic mobilities of their malate dehydrogenases was detected. Purification of typhus rickettsiae on Renografin gradients has no apparent adverse effects on their metabolic or antigenic properties.