Avian (ortho)reovirus (ARV), which belongs to Reoviridae family, is a major domestic fowl pathogen and is the causative agent of viral tenosynovitis and chronic respiratory disease in chicken. ARV replicates within cytoplasmic inclusions, so-called viral factories, that form by phase separation and thus belong to a wider class of biological condensates. Here, we evaluate different optical imaging methods that have been developed or adapted to follow formation, fluidity and composition of viral factories and compare them with the complementary structural information obtained by well-established transmission electron microscopy and electron tomography. The molecular and cellular biology aspects for setting up and following virus infection in cells by imaging are described first. We then demonstrate that a wide-field version of fluorescence recovery after photobleaching is an effective tool to measure fluidity of mobile viral factories. A new technique, holotomographic phase microscopy, is then used for imaging of viral factory formation in live cells in three dimensions. Confocal Raman microscopy of infected cells provides "chemical" contrast for label-free segmentation of images and addresses important questions about biomolecular concentrations within viral factories and other biological condensates. Optical imaging is complemented by electron microscopy and tomography which supply higher resolution structural detail, including visualization of individual virions within the three-dimensional cellular context.
