Promoted proliferation of an SOD-deficient mutant of Escherichia coli under oxidative stress induced by photoexcited TiO2

An SOD null mutant of Escherichia coli (IM303) and its wild-type strain (MM294) were cultivated with or without sublethal oxidative stress generated from photoexcited TiO2. Concerning maximum specific growth rate of the cells, mum, measured under various conditions, the mum value of IM303 cells increased notably in the presence of TiO2 illuminated with light (I = 12.5 W m(-2)), being about two times higher than that of the cells grown in the absence of TiO2 and light. The mum value of IM303 cells under the oxidative condition restored to a level comparable to that of wild-type MM294 cells, which coincided with the finding that the content of reactive oxygen species lowered in IM303 cells under the oxidative stress. Colony isolation was conducted to obtain the cells prevailing in the early culture phase of IM303 cells in the presence of TiO2 and light. It was found that the isolates exhibited the outgrowing properties with the increased mum values under both the conditions with and without TiO2 and light. It was also indicated that in the culture of typically selected isolate, the cells started to grow with a relatively short lag in a threonine-minus medium.