HtrA Protease of Campylobacter jejuni Is Required for Heat and Oxygen Tolerance and for Optimal Interaction with Human Epithelial Cells

Campylobacter jejuni is a predominant cause of food-borne bacterial gastroenteritis in the developed world. We have investigated the importance of a homologue of the periplasmic HtrA protease in C. jejuni stress tolerance. A C. jejuni htrA mutant was constructed and compared to the parental strain, and we found that growth of the mutant was severely impaired both at 44°C and in the presence of the tRNA analogue puromycin. Under both conditions, the level of misfolded protein is known to increase, and we propose that the heat-sensitive phenotype of the htrA mutant is caused by an accumulation of misfolded protein in the periplasm. Interestingly, we observed that the level of the molecular chaperones DnaK and ClpB was increased in the htrA mutant, suggesting that accumulation of nonnative proteins in the periplasm induces the expression of cytoplasmic chaperones. While lack of HtrA reduces the oxygen tolerance of C. jejuni, the htrA mutant was not sensitive to compounds that increase the formation of oxygen radicals, such as paraquat, cumene hydroperoxide, and H₂O₂. Using tissue cultures of human epithelial cells (INT407), we found that the htrA mutant adhered to and invaded human epithelial cells with a decreased frequency compared to the wild-type strain. This defect may be a consequence of the observed altered morphology of the htrA mutant. Thus, our results suggest that in C. jejuni, HtrA is important for growth during stressful conditions and has an impact on virulence.