Next, the effects of PB-3 were investigated in an in vitro cellular-bacterial infection model. A549 cells were infected with a capsid-deletion mutant of S.pn. (D39Δcps). In this bacterial strain the capsid gene was deleted to ensure that cellular damage predominantly depended on PLY. In cells infected with D39Δcps, after 16 h of incubation approximately 100% of the cells were killed as indicated by the loss of mitochondrial TMRE signal, strong nuclear executioner caspase activation as well as morphological signs of cell damage (Figure , II). In contrast, cells infected with D39Δcps treated four times after regular intervals of 4 h with PB-3 (6 µM) almost entirely survived and did not show signs of apoptosis (Figure , III). Cells infected with a second mutant of S.pn.