Monocytes Are the Predominant Cell Type Associated with in the Gut, but They Do Not Serve as an Intracellular Growth Niche.

After foodborne transmission of the facultative intracellular bacterial pathogen , most of the bacterial burden in the gut is extracellular. However, we previously demonstrated that intracellular replication in an as yet unidentified cell type was essential for dissemination and systemic spread of In this article, we show that the vast majority of cell-associated in the gut were adhered to Ly6C monocytes, a cell type that inefficiently internalized With bone marrow-derived in vitro cultures, high multiplicity of infection or the use of opsonized bacteria enhanced uptake of in CD64 monocytes, but very few bacteria reached the cell cytosol. Surprisingly, monocytes that had upregulated CD64 expression in transition toward becoming macrophages fully supported intracellular growth of In contrast, inflammatory monocytes that had increased CD64 expression in the bone marrow of BALB/c/By/J mice prior to exposure in the gut did not support growth. Thus, contrary to the perception that can infect virtually all cell types, neither naive nor inflammatory Ly6C monocytes served as a productive intracellular growth niche for These results have broad implications for innate immune recognition of in the gut and highlight the need for additional studies on the interaction of extracellular, adherent with the unique subsets of myeloid-derived inflammatory cells that infiltrate sites of infection.