M30 epitope immunohistochemistry was used to detect caspase-cleaved cytokeratin-18 which is detectable in early stages of apoptosis. Images are fluorescent confocal staining at ×600 magnification. The negative control was unchallenged HGECs with only media added (A). The positive control was HGECs treated with camptothecin 4 μg/ml for 4 hours (B). HGECs challenged with live P. gingivalis 33277 at MOI:100 for 24 hours show marked staining (C), while HGECs challenged with heat-killed bacteria under the same conditions show no detectable apoptosis (D). Challenging HGECs with an MOI:100 for 4 hours or MOI:10 for 4 and 24 hours showed no positive staining (no apoptosis) (data not shown).