Coxiella burnetii exhibits morphological change and delays phagolysosomal fusion after internalization by J774A.1 cells.

Coxiella burnetii, the etiological agent of Q fever, is an obligate intracellular bacterium proliferating within the harsh environment of the phagolysosome. Mechanisms controlling trafficking to, and survival of pathogens within, the phagolysosome are unknown. Two distinct morphological variants have been implicated as playing a role in C. burnetii survival. The dormant small-cell variant (SCV) is resistant to extracellular stresses and the more metabolically active large-cell variant (LCV) is sensitive to environmental stresses. To document changes in the ratio of SCVs to LCVs in response to environment, a protein specific to SCV, ScvA, was quantitated. During the first 2 h after internalization of C. burnetii by J774A.1 cells, the level of ScvA decreased, indicating a change from a population containing primarily SCVs to one containing primarily LCVs. In vitro experiments showed that 2 h of incubation at pH 5.5 caused a significant decrease in ScvA in contrast to incubation at pH 4.5. Measuring in vitro internalization of [(35)S]methionine-[(35)S]cysteine in response to pH, we found the uptake to be optimal at pH 5.5. To explore the possibility that after uptake C. burnetii was able to delay phagolysosomal fusion, we used thorium dioxide and acid phosphatase to label phagolysosomes during infection of J774A.1 cells. We determined that viable C. burnetii was able to delay phagolysosomal fusion. This is the first time that a delay in phagolysosomal fusion has been shown to be a part of the infection process of this pathogenic microorganism.