Increased expression of periplasmic Cu,Zn superoxide dismutase enhances survival of Escherichia coli invasive strains within nonphagocytic cells

We have studied the influence of periplasmic Cu,Zn superoxide dismutase on the intracellular survival of Escherichia coli strains able to invade epith elial cells by the expression of the inv gene from Yersinia pseudotuberculo sis but unable to multiply intracellularly. Intracellular viability assays, confirmed by electron microscopy observations, showed that invasive strain s of E. coli engineered to increase Cu,Zn superoxide dismutase production a re much more resistant to intracellular killing than strains containing onl y the chromosomal sodC copy. However, me have found only a slight differenc e 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 ob served that acid- and oxidative stress-sensitive E. coli HB101(pRI203) is m ore rapidly killed in epithelial cells than E. coli GC4468(pRI203). The hig h mortality of E. coli HB101(pRI203), independent of the acidification of t he endosome, is abolished by the overexpression of sodC. Our data suggest t hat oxyradicals are involved in the mechanisms of bacterial killing within epithelial cells and that high-level production of periplasmic Cu,Zn supero xide dismutase provides bacteria with an effective protection against oxida tive damage. We propose that Cu,Zn superoxide dismutase could offer an impo rtant selective advantage in survival within host cells to bacteria express ing high levels of this enzyme.