Increased Expression of Periplasmic Cu,Zn Superoxide Dismutase Enhances Survival of Escherichia coliInvasive Strains within Nonphagocytic Cells

ABSTRACTWe have studied the influence of periplasmic Cu,Zn superoxide dismutase on the intracellular survival of Escherichia colistrains able to invade epithelial cells by the expression of theinvgene from Yersinia pseudotuberculosisbut unable to multiply intracellularly. Intracellular viability assays, confirmed by electron microscopy observations, showed that invasive strains of E. coliengineered to increase Cu,Zn superoxide dismutase production are much more resistant to intracellular killing than strains containing only the chromosomalsodCcopy. However, we have found only a slight difference in survival within HeLa cells between a sodC-null mutant and its isogenic wild-type strain. Such a small difference in survival correlates with the very low expression of this enzyme in the wild-type strain. We have also observed that acid- and oxidative stress-sensitiveE. coliHB101(pRI203) is more rapidly killed in epithelial cells than E. coliGC4468(pRI203). The high mortality ofE. coliHB101(pRI203), independent of the acidification of the endosome, is abolished by the overexpression of sodC. Our data suggest that oxyradicals are involved in the mechanisms of bacterial killing within epithelial cells and that high-level production of periplasmic Cu,Zn superoxide dismutase provides bacteria with an effective protection against oxidative damage. We propose that Cu,Zn superoxide dismutase could offer an important selective advantage in survival within host cells to bacteria expressing high levels of this enzyme.