The interaction of Coxiella burnetii phase I and phase II in Earle's cells

Abstract

Coxiella burnetii (Cb) in phase I (Ph I) multiplies very poorly in L-cells as shown by light and electron microscopy and infectivity titrations; Cb antigens, however, are detectable by immunofluorescence in most cells. Phase II (Ph II) organisms, in contrast, multiply well and produce typical particles in well-defined cytoplasmic vacuoles. Density gradient centrifugation has been applied to test the heterogeneity of the animal-maintained Nine Mile strain in Ph I and the corresponding egg-grown strain in Ph II. The evidence indicates that both strains are mixtures of Ph I and Ph II organisms, although they reacted as Ph I or Ph II antigens in the complement fixation test. The interaction of Ph I and Ph II organisms was studied in mixed infections in L-cells; suppression, or a less marked enhancement, of the growth of Coxiella burnetii was observed, depending on the conditions. It is suggested that Ph I and Ph II rickettsiae represent genetically different but closely related organisms, Ph I being the "parent" strain and Ph II the "mutant" strain; "phase variation" of Coxiella burnetii results from a selection of the mutant in the population under the prevailing environmental conditions.