The thermostable direct hemolysin (TDH) is considered as a major virulence factor of Vibrio parahaemolyticus. We observed this potential in several human cancer cell lines by using the TDH-producing wild-type (RIMD2210633) as well as tdh-deletion mutant of V. parahaemolyticus and found that the deletion of tdh did not affect cytotoxicity to any of the cell lines tested. DNA fragmentation and annexin V staining showed that both wild-type and tdh-mutant trigger apoptosis in these cells. To understand the molecular basis of cell death in the absence of TDH, gene expression profile of human colon cancer cell line HCT116 infected with tdh-deletion mutant was carried out using human cDNA microarrays consisting of 33,000 known genes. In infected cells, differentially expressed genes including genes for early growth response, growth arrest and DNA damage, and activating transcription factor that affect programmed cell death pathways were detected. Interestingly, mutant strains having a deletion in type 111 secretion system 1 (TTSS1) failed to elicit DNA fragmentation in HCT1 16 cells. Our results strongly suggest that apoptosis requires functional TTSS1 and TTSS1-dependent translocation factor(s) to be associated with the host cell death, and thus pathogenesis of V. parahaemolyticus. (c) 2005 Elsevier Inc. All rights reserved.