Staphylococcus aureus is an opportunistic human pathogen and a leading cause of severe and life-threatening systemic infections, which occur when the bacteria disseminate from an initial site of infection to distant organs via the bloodstream. S. aureus bacteremia is associated with high mortality rates. Understanding the mechanisms of host defense against S. aureus systemic infections is therefore crucial for developing effective strategies for prevention and treatment of these infections. Kupffer cells (KC), specialized liver-resident macrophages, are key players in the removal of bacteria in the bloodstream. However, S. aureus has developed strategies to resist elimination by KC that contribute to its ability to persist and colonize additional infection sites such as the kidneys.

Here we sought to decipher the complex interplay between S. aureus virulence factors and the anti-microbial mechanisms of KC in vitro and in vitro. KC were isolated from the livers of C57BL/6 mice and were infected with S. aureus in vitro, followed by monitoring of bacterial intracellular killing and phagocytosis rates. In the in vivo experiments, mice were infected intravenously with GFP-expressing S. aureus, and the number of viable bacteria in the liver was determined 4 and 24 hours after infection. By flow cytometry we determined that monocytes and neutrophils were also recruited to the liver 4 h after infection and thus may contribute to bacterial killing.

Our results demonstrate that in vitro-infected KC efficiently phagocytosed and eradicated S. aureus 2 h after infection. In vivo, we observed a substantial reduction of S. aureus in the liver 4 h after inoculation. However, a bacterial subpopulation survived and disseminated to other organs at later times of infection. We will investigate by Tn-seq, if this subpopulation of S. aureus requires specific virulence determinants to escape KC. Furthermore, only a subpopulation of KC harbored S. aureus, suggesting heterogeneity in the bactericidal capacity of the phagocytes. Single cell RNA sequencing will be used to investigate if this KC heterogeneity influences their antibacterial capacity.