Exposure of phytopathogenic Xanthomonas spp. to lethal concentrations of multiple oxidants affects bacterial survival in a complex manner

During plant-microbe interactions and in the environment, Xanthomonas campe stris pv, phaseoli is likely to be exposed to high concentrations of multip le oxidants. Here, we show that simultaneous exposures of the bacteria to m ultiple oxidants affects cell survival in a complex manner. A superoxide ge nerator (menadione) enhanced the lethal effect of an organic peroxide (tert -butyl hydroperoxide) by 1,000-fold; conversely, treatment of cells with me nadione plus H2O2 resulted in 100-fold protection compared to that for cell s treated with the individual oxidants, Treatment of X. campestris with a c ombination of H2O2 and tert-butyl hydroperoxide elicited no additive or pro tective effect. High levels of catalase alone are sufficient to protect cel ls against the lethal effect of menadione plus H2O2 and tert-butyl hydroper oxide plus H2O2. These data suggest that H2O2 is the lethal agent responsib le for killing the bacteria as a result of these treatments. However, incre ased expression of individual genes for peroxide (alkyl hydroperoxide reduc tase, catalase)- and superoxide (superoxide dismutase)-scavenging enzymes o r concerted induction of oxidative stress-protective genes by menadione gav e no protection against killing by a combination of menadione plus tert-but yl hydroperoxide. However, X. campestris cells in the stationary phase and a spontaneous H2O2-resistant mutant (X, campestris pv, phaseoli HR) were mo re resistant to killing by menadione plus tert-butyl hydroperoxide. These f indings give new insight into oxidant killing of Xanthomonas spp, that coul d be generally applied to other bacteria.