Digital fluorescence imaging of fusion of influenza virus with erythrocytes

Fusion of influenza virus with human erythrocytes at pH 5.2 was followed by fluorescence microscopy using a cooled slow-scan CCD camera. The high sensitivity of the CCD permits repetitive digital imaging of the same cells with minimal photobleaching. The experimental conditions were such that only a small number of virus particles were adsorbed per cell. Quantitative analysis of the data indicated that for most cells only a single fusion event took place. This was, however, sufficient to cause haemolysis within 30 min at 20–22°C for about 60% of cells. There was a highly variable time lag between fusion and haemolysis. The lateral diffusion coefficient of virus particles on the cell surface when bound at pH 7.4 was < 2 × 10^?13 cm²·s^?1. The technique should be of value for more detailed studies of the dynamics of viral and other membrane fusion events.