Infection of Vero cells with Coxiella burnetii phase II: relative intracellular bacterial load and distribution estimated by confocal laser scanning microscopy and morphometry

Coxiella burnetii, the agent of Q fever in man and of coxiellosis in other species, is an intracellular pathogen not yet grown axenically. Confocal la ser fluorescence microscopy and morphometry were used to measure relative C . burnetii phase II loads and their intracellular distribution in aldehyde fixed and DAPI stained Vero cell monolayers. The fluorescence of single hor izontal optical sections provided useful information on relative loads of b acteria in cells and vacuoles. The relative density of the bacteria in the vacuoles was inferred from ratios of fluorescence to vacuolar section areas . Relative bacterial loads, bacterial densities and section areas of large vacuoles increased exponentially between days 2 and 4 of the infection of g amma -irradiated host cells, stabilized between days 4 and 6, and decreased thereafter. Estimated minimum doubling times were higher for the overall c omplement of the intracellular organisms (about 12 h) than for bacteria tha t were confined to larger vacuoles (about 10 h), (C) 2001 Elsevier Science B.V. All rights reserved.