PHASE VARIATION OF THE NINE MILE AND OTHER STRAINS OF RICKETTSIA BURNETI

Abstract

The classical Nine Mile strain of Rickettsia burneti, which had been maintained in guinea pigs since first isolation, was adapted to growth in yolk sacs. Despite abundant rickettsiae, antigens prepared from each of the first seven yolk sac passages failed to react in complement fixation tests with homologous Q fever antisera. The antisera did react, however, with antigens prepared from the eighth and subsequent passages. This change in behavior during egg adaptation, which resembles that found previously with freshly isolated strains, is termed "phase variation". Rickettsiae in an early stage of egg adaptation which fail to fix complement with antisera are called Phase 1; after further egg adaptation and the development of ability to react with antiserum, they are termed Phase 2. Strains in Phase 2 rapidly reverted to Phase 1 after inoculation into guinea pigs, mice, or hamsters, and naturally occurring strains from a sheep's placenta, cows' milk, and ticks were also found to be in Phase 1. It is possible that all strains used for preparing stock complement fixing antigens are laboratory variants in Phase 2 obtained through multiple yolk sac passages.