Complementation of a DnaK-deficient Escherichia coli strain with the dnaK/dnaJ operon of Brucella ovis reduces the rate of initial intracellular killing within the monocytic cell line U937.

Facultatively intracellular bacteria express heat shock proteins after phagocytosis by macrophages. Using non-pathogenic Escherichia coli strains and the human monocytic cell line U937, we showed that deletion of the dnaK gene significantly increased the rate of initial intracellular killing of bacteria. Trans-complementation of the deletion mutant with the dnaK/dnaJ operon of Brucella ovis restored the pattern of intracellular elimination of the control strain expressing dnaK. These differences were not observed using antibody-opsonized bacteria and activated cells. In vitro, strains expressing dnaK resisted hydrogen peroxide better than the deletion mutant; in contrast, the mutant complemented by dnaK/dnaJ of B. ovis tolerated low pH and low H2O2 better than the wild-type strain and the deletion mutant. Our results suggested the participation of DnaK in protection of intracellular bacteria against antimicrobial macrophage factors.