<i>S. elongatus</i> can grow inside the macrophage cytoplasm.

<p>A.) Time lapse microscopy of macrophages infected with +inv+llo <i>S. elongatus</i> kept in the dark shows the gradual decrease in red autofluorescence over the course of 12 hours. In contrast, when kept in the light, B.) empty vector <i>S. elongatus</i> autofluorescence is observed to gradually decrease over the course of several days (top row), while a significant increase in red <i>S. elongatus</i> autofluorescence was observed in macrophages infected with inv llo <i>S. elongatus</i> for two days post-infection (bottom row). This fluorescence was observed to decrease after the third day of infection. C.) This change in fluorescence over time can be quantified as a change in background subtracted mean fluorescence in ImageJ and averaged over triplicate experiments. Empty vector (blue line) and +inv+llo <i>S. elongatus</i> (red line) show marked differences in growth when infected at similar densities of 1–2 bacterial cells per macrophage. D.) +inv+llo <i>S. elongatus</i> displayed infection density dependent growth rates in macrophages. Each line shows change in mean fluorescence in cells infected at a single starting density, ranging in multiples of two from fewer than one cell per macrophage to approximately 4 bacteria per macrophage. E.) Macrophage cell counts were variable across replicates and over the course of the experiment but displayed no significant difference between macrophages infected with empty vector <i>S. elongatus</i> at low (green line) or high density (blue line), or +inv+llo <i>S. elongatus</i> at low (red line) or high (yellow line) density. F.) When infected at low density of fewer than one bacteria per macrophage, <i>S. elongatus</i> division was observed during 18 hour time-lapse fluorescent microscopy in approximately 1% of macrophages observed, in particular those cells that contained more than one bacterial cell due to stochastic fluctuations.