The Role of Glutathione Mediated Oxidative Stress Response Mechanisms in Nitrifying Bacteria

Nitrification in activated sludge is especially susceptible to oxidative chemical toxins. Several oxidative stress response mechanisms exist in bacteria, and one highly conserved biomolecule involved with antioxidant activities is glutathione. In many Gram-negative heterotrophic bacteria, glutathione mediates the glutathione-gated potassium efflux (GGKE) response, which activates secondary stress responses that protect important intracellular components. A search of the genome of the Gram-negative ammonia oxidizing autotroph Nitrosomonas europaea revealed that glutathione synthase was present, while several key enzymes involved with glutathione reduction and GGKE are missing; however, other mechanisms that facilitate potassium efflux for oxidative stress protection may exist. Experiments were performed using the ammonia oxidizing bacterium N. europaea and the nitrite oxidizing bacterium Nitrospira moscoviensis. Pseudomonas aeruginosa PAO1 was used as a positive control. Concentrations of total and oxidized glutathione were measured in N. europaea after exposure to the oxidative chemical sodium hypochlorite. This data showed that glutathione was present but not oxidized to equivalent levels in N. europaea and P. aeruginosa. Additional batch experiments were performed and soluble potassium levels were monitored to observe increases associated with oxidant induced potassium efflux mechanisms. Results using N. europaea and Ni. moscoviensis suggest that neither bacteria effluxed potassium in response to the oxidants N-ethylmaleimide or sodium hypochlorite, indicating that no oxidative stress-induced potassium efflux mechanism exists in these nitrifiers. Coupled with known genomic information, the results of this study provide greater insight into why nitrification is so susceptible to process upset in biological wastewater treatment.