To further confirm apoptosis was triggered after infection, we measured caspase activation and substrate cleavage by western blot analyses and detected that the cleavage of poly (ADP-ribose) polymerase (PARP), one of the main cleavage targets of caspase-3 [26], which increased during the late stage of infection in the two cell lines (Fig 2A and 2B). Here, CASP3 is linked to infection.