HtrA is a heat-stress protein that functions both as a chaperone and as a serine protease. HtrA has been shown in several organisms to be involved in responses to stressful environmental conditions and involvement of HtrA in virulence has been reported in pathogenic species. A Porphyromonas gingivalis htrA mutant demonstrated no significant difference to the W83 parent strain when subjected to high temperature and pH values from 3 to 11. However, the htrA mutant showed increased sensitivity to H(2)O(2). Cell invasion assays indicated that the total interaction (adherence) with KB cells, human coronary artery endothelial cells and gingival epithelial cells (GEC) was the same for both the wild-type and the htrA mutant. However, the htrA mutant showed increased invasion in KB cells and GEC. Microarray experiments indicated that a total of 253 genes were differentially regulated in the htrA mutant, including a group of stress-related genes, which might be responsible for the observed decreased resistance to H(2)O(2). In animal experiments, a competition assay showed that the htrA mutant did not survive as well as the wild-type. In another in vivo assay, fewer mice infected with the htrA mutant died than mice infected with W83, suggesting that the htrA gene is virulence-related. These data indicate that the htrA gene in P. gingivalis does not relate to stress conditions such as high temperature and pH, but rather to H(2)O(2) stress. The htrA gene also appears to be important for virulence and survival in in vivo animal models.