Intracellular pathogens often exhibit a complex life cycle and can – protected inside of the host cell – cause persistent infections which are difficult to treat. Currently, the characterization of pathogenesis mechanisms requires labor-intensive methods, most of which involve destruction of the host cell. In this publication, we present a Raman-based, label-free, non-invasive and non-destructive method for localization, visualization and even quantification of intracellular bacteria in three dimensions within intact host cells. A Coxiella burnetii infection model was used to demonstrate the potential of the method. C. burnetii are obligate intracellular bacteria that cause the zoonotic disease Q fever in humans. Quantitative analysis at different time points after infection allowed the researchers to follow the infection cycle with the transition from the – metabolically active large cell variant to the metabolically inactive, but in fectious small cell variant around day B six post infection. During the course of infection, a gradual change in lipid composition was observed. 

The publication contributes to the achievement of the key objectives in the Biophotonics program area by presenting an innovative photonic method that can achieve highest specificity, sensitivity, and resolution for basic and applied research applications in the life sciences and medicine.