FIG. 2.

Photorhabdus asymbiotica induces apoptosis of host cells. (A) Australian strains of P. asymbiotica induce apoptosis of THP-1 cells upon 6 h of infection. THP-1 cells were infected with the American and Australian Photorhabdus clinical strains at an MOI of 100:1 (E. coli XL1-Blue was used as a negative control, and staurosporine [STAU] was used as a positive control). TUNEL-positive cells were determined by fluorescence microscopy. At 6 h postinfection, all Australian strains induced apoptosis of THP-1 cells. Almost no TUNEL-positive THP-1 cells were found at 6 h postinfection when infection was done with the American strains. Results are representative of three independent experiments. (B) Apoptosis induced by P. asymbiotica strain AU92 is specific for macrophage-like cells. THP-1 cells and insect hemocytes were incubated for 6 h with P. asymbiotica at an MOI of 100:1. After DAPI staining, some THP-1 cells presented nuclear fragmentation, and these cells were TUNEL positive. Insect granulocytes (Gr) were TUNEL positive, in contrast to plasmatocytes (Pl). (C) Ultrastructure of THP-1 cells following infection with P. asymbiotica (by TEM). Control cells (left) and THP-1 cells (right) are shown after 6 h of incubation with AU92 (MOI, 100:1). Note the presence of typical apoptotic bodies (arrow).