Outer membrane tube formation by <i>Francisella novicida</i> involves extensive envelope modifications and is linked with type VI secretion and alterations to the host phagosomal membrane

<i>Francisella tularensis</i> is an intracellular bacterial pathogen that causes the zoonotic disease tularemia. Following uptake by host cells, the bacteria rapidly escape the phagosome and replicate intracellularly. In previous studies, we found that <i>Francisella</i> produces tubular extensions of its cell surface in response to specific cues and during macrophage infection. In the present study, we used cryogenic electron tomography to examine tube formation by the model <i>Francisella</i> sp., <i>F. novicida</i>. This analysis revealed that tube formation involves extensive bacterial envelope alterations and a dynamic cytoplasmic organelle. Furthermore, tubes produced by bacteria within infected macrophages were associated with the breakdown of the phagosomal membrane. In addition, we found that the <i>Francisella</i> type VI secretion system, which is essential for phagosomal escape, co-localized with the bacterial tubes. These findings reveal the cellular transformations that occur during membrane tubulation by <i>Francisella</i> and suggest a role for the tubes in phagosomal escape.