Coinfection of fibroblasts with Coxiella burnetti and Toxoplasma gondii: to each their own.

Intracellular pathogens have evolved distinct strategies to subvert host cell defenses. At diametrically opposed ends of the spectrum with regard to the host endosomal/lysosomal defenses are the obligate intracellular protozoan Toxoplasma gondii and the bacterium Coxiella burnetti. While the intracellular replication of T. gondii requires complete avoidance of the host endocytic cascade, C. burnetti actively subverts it. This results in these organisms establishing and growing in very different vacuolar compartments. In this study we examined the potential interaction between these distinct compartments following coinfection of mammalian fibroblasts. When present within the same cell, these organisms exhibit minimal interaction with each other. Colocalization of T. gondii and C. burnetti within the same vacuole occurs at a low frequency in doubly infected cells. In such instances only one of the organisms appears to be replication competent, emphasizing the different requirements for survival and/or intracellular growth. The potential basis for both the lack of interaction between these distinct pathogen-containing compartments, and the mechanisms to address their low frequency of colocalization are discussed in the context of our understanding of the biology of the organisms and membrane traffic in eukaryotic cells.