Mixed infections of Coxiella burnetii in restrictive host cells: an immunofluorescent study

Abstract

The animal-maintained "phase I" (Ph I) and the egg-grown "phase II" (Ph II) strain of Coxiella burnetii (Cb) were each made up of a natural mixture of particles with different densities (23). When examined by immunofluorescence (IF), their populations behaved as if they were monophasic, both before and after separation by cesium chloride. The Ph I strain, made up of particles of both densities, showed Ph I reactivity and particles of the Ph II strain showed Ph II reactivity. When L-cells (which represent a restrictive host-system for Ph I Cb) were inoculated with large doses of either the Ph I strain of Cb or with particles of a relative density of 1.32 (Ph I), only a few L-cells contained Cb which stained with biphasic serum. The Ph II strain of Cb and particles with a density of 1.27 (Ph II) multiplied well in L-cells and stained readily with Ph II antiserum. Particles which stained with biphasic serum, however, were seen in a few L-cells which had a multiple infection with Ph II Cb. A large inoculum of artificial mixtures of Ph I and Ph II strains of Cb produced many particles in L-cells which stained predominantly with biphasic serum. Interference of growth of Cb occurred using a small inoculum of the mixtures of Ph I and Ph II.Mixtures of active particles of Ph I Cb (host-dependent) and of heat-inactivated Ph II Cb, produced enhancement of growth of Cb. The reactivated Cb were stained predominantly with Ph II antiserum. The possibility that phenotypic mixing of Cb is related to its mode of replication is discussed.