The pathogenic bacterium Listeria monocytogenes is the third most common cause of bacterial meningitis in neonates, and causes abortion and stillbirth. Additionally, sporadic outbreaks of listeriosis continue to claim lives, particularly in the very young and the elderly. Here, Jonathan Hardy and colleagues use in vivo bioluminescent imaging to visualize L. monocytogenes in mice. They found that live and attenuated forms of the bacterium reside in the bone marrow and persist for weeks following acute infection. The results draw attention to the bone marrow as a site of residual infection, both during and after treatment. Additionally, this study demonstrates that growth mechanisms still function in attenuated Listeria strains enabling the bacteria to colonize the bone marrow, which is important because these attenuated strains are being tested for their ability to induce anti-tumor immune responses in cancer patients.

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