FIENA was used to detect caspase-3 activation, a key molecule in initiation of apoptosis. HGECs were challenged with live or heat-killed P. gingivalis 33277 at MOI:10 and MOI:100 for 4 and 24 hours. Negative control was unchallenged HGECs. Positive control was HGECs challenged with camptothecin 4 μg/ml. Values represent the means ± SD of at least two experiments. Statistical comparisons are to the unchallenged negative control cells (* P < 0.05, ** P < 0.01).