Bimodal labelling of S. aureus for detection of bacterial colonization in skin infections by MRI

Introduction: Staphylococcus aureus is a versatile human pathogen potentially leading to a diverse array of infections once the natural skin and mucosal barriers are breached (1). Besides minor skin and wound infections it is also a frequent cause of serious and life-threatening diseases such as pneumonia, endocarditis, arthritis, sepsis, peritonitis and osteomyelitis (2, 3). For the understanding of staphylococcal disease mechanisms several mouse models are in use and in order to visualize the course of an infection in real time in vivo imaging techniques have been developed. Due to high spatial resolution and high tissue contrast, especially MRI represents a versatile method to image bacterial infections non-invasively (4). Using the application of iron-oxide particles, cell labelling and tracking methods have been developed recently, and thus monitoring of localization and migration of macrophages and neutrophiles during an infection was achieved. However, the visualization of bacterial colonization within inflamed tissue is still challenging and sources of chronic infections with persistent bacteria have not been directly visualized by MRI. Labelling protocols for mammalian cells such as tumor cells or stem cells (5) are widely established but labelling of bacteria is particularly difficult due to their small size and protective bacterial cell wall. Here, we developed a protocol for labelling bacterial cells with rhodamine-coated VSOPs, which allowed us to visualize bacterial colonies in tissue and to distinguish infection from inflammation both by MRI and FRI (Fluorescence Reflectance imaging).