We aim to characterise virus tropism by identifying permissive cell types that support complete virus replication by using Tip-enhanced Raman Spectroscopy (TERS). TERS integrates atomic force microscopy (AFM) and near-field optical recordings with Raman spectroscopy, enabling direct, label-free characterisation of molecular organisation and thus visualisation of virus entry and exit at nanometer resolution. Virus tropism is defined as the selective ability of a virus to infect specific host species, tissues, or cell types. It is determined by virus factors such as receptor binding and replication requirements, as well as by host determinants, including receptor availability and intracellular conditions. As a result, tropism ultimately defines whether productive replication occurs and thereby shapes virus pathogenesis, disease progression, and transmission. However, inferring tropism from virus properties alone remains difficult and typically requires elaborate infection experiments. In this project, TERS will be employed to resolve virus entry and exit strategies with unprecedented spatial detail, Fig. C02.1. Beyond imaging, TERS provides molecular fingerprints and resolves nanoscale surface structures, thereby revealing chemical and conformational changes that accompany virus-cell interactions. Time-resolved nanoscale studies will further allow us to monitor replication at the level of single infection events and to detect heterogeneous infection patterns that are masked in bulk analyses. By combining structural and temporal information, we aim to precisely define the host cell factors and physiological states that permit or restrict virus replication, thus contributing directly to research goals G3 and G4. Our approach represents a novel, rapid, label-free, and phenotypic method to define virus tropism. In addition to TERS, we will employ nanoscale infrared spectroscopy (nanoIR), a technique that combines AFM with IR absorption to provide chemical composition and conformational information at nanometer resolution. The combination of nanoIR and TERS will yield complementary spectroscopic data to unravel structural changes during virus-cell interactions, potentially down to amino acid resolution. To further dissect the influence of host determinants on virus tropism, we will additionally apply a cellular senescence model.