Foci of Listeria monocytogenes persist in the bone marrow

Murine listeriosis is one of the most comprehensive and well-studied models of infection, and Listeria monocytogenes has provided seminal information regarding bacterial pathogenesis. However, many aspects of the mouse model remain poorly understood, including carrier states and chronic colonization which represent important features of the spectrum of host-pathogen interaction. Bone marrow has recently been shown to harbor L. monocytogenes, which spreads from this location to the central nervous system. Bone could, therefore, be an important chronic reservoir, but this infection is difficult to study because it involves only a few bacteria and the extent of infection cannot be assessed until after the animal is sacrificed. We employed in vivo bioluminescence imaging to localize L. monocytogenes bone infections over time in live mice, revealing that the bacteria grow in discrete foci. These lesions can persist in many locations in the legs of mice and are not accompanied by a histological indication such as granuloma or a neutrophil infiltratate. We demonstrate that highly attenuated hly mutants, which have defective intracellular replication, are capable of prolonged focal infection of the bone marrow for periods of up to several weeks. These results support the recently proposed hypothesis that the bone marrow is a unique niche for L. monocytogenes.