M. tuberculosis is localized in the cytosol of primary human macrophages. (A) TEM images representing the observed localization of M. tuberculosis in GM-CSF– and M-CSF–differentiated macrophages at 48 h after infection. “Membrane bound” represents bacteria in a phagosome (with one surrounding host membrane, such as in subpanels A and E, highlighted by white arrowheads), bacteria in phagolysosomes (loose vesicles with one surrounding host membrane, such as in subpanels C and G), or bacteria in autophagosomes (with two or more surrounding host membranes, such as in subpanels D and H); “Cytosol” represents bacteria with no surrounding host membranes (such as in subpanels B and F). Bars, 500 nm. (B) Quantification of GFP-Mtb subcellular localizations in resting (blue-outlined bars) or activated (red-outlined bars) GM-CSF and M-CSF macrophages at 2, 24, and 48 h after infection. Quantification was performed by stereological analysis of TEM images of resin sections of infected macrophages. Data are from one experiment, and the total numbers of cells analyzed in each condition (n) are shown.