Intravital two-photon microscopy of host-pathogen interactions in a mouse model of Staphylococcus aureus skin abscess formation.

Staphylococcus (S.) aureus is a frequent cause of severe skin infections. The ability to control the infection is largely dependent on the rapid recruitment of neutrophils (PMN). To gain more insight into the dynamics of PMN migration and host-pathogen interactions in vivo, we used intravital two-photon (2-P) microscopy to visualize S. aureus skin infections in the mouse. Reporter S. aureus strains expressing fluorescent proteins were developed, which allowed for detection of the bacteria in vivo. By employing LysM-EGFP mice to visualize PMN, we observed the rapid appearance of PMN in the extravascular space of the dermis and their directed movement towards the focus of infection, which led to the delineation of an abscess within 1 day. Moreover, tracking of transferred labelled bone-marrow neutrophils showed that PMN localization to the site of infection is dependent on the presence of G-protein-coupled receptors on the PMN, whereas Interleukin-1 receptor was required on host cells other than PMN. Furthermore, the S. aureus complement inhibitor Ecb could block PMN accumulation at thesite of infection. Our results establish that 2-P microscopy is a powerful tool to investigate the orchestration of the immune cells, S. aureus location and gene expression in vivo on a single cell level.