Reversion of pneumolysin induced executioner caspase activation redirects cells to survival.

Apoptosis is an indispensable mechanism for eliminating infected cells and activation of executioner caspases is considered as a point of no return. Streptococcus pneumoniae, the most common bacterial pathogen causing community-acquired pneumonia, induces apoptosis via its pore forming toxin pneumolysin, leading to rapid influxes of mitochondrial calcium [Ca 2+]m as well as fragmentation, loss of motility and membrane potential, which is accompanied by caspase-3/7 activation. Using machine-learning and quantitative live-cell microscopy, we identified a significant number of alveolar epithelial cells surviving such executioner caspase activation after pneumolysin attack. Precise single cell analysis revealed the [Ca 2+]m amplitude and efflux rate as decisive parameters for survival and death, which was verified by pharmacological inhibition of [Ca 2+]m efflux shifting the surviving cells towards the dying fraction. Taken together, we identified the regulation of [Ca 2+]m as critical for controlling the cellular fate under pneumolysin attack, which might be useful for therapeutic intervention during pneumococcal infection.