Mechanisms of plasma-induced microbial inactivation have commonly been studied with physicochemical techniques. In this study, Escherichia coli K-12 BW25113 and its ΔrecA, ΔuvrA, ΔrpoS and ΔsoxS mutants are employed to discriminate effects of DNA damage, UV photons, reactive oxygen species and environmental stress produced in an atmospheric-pressure DC plasma jet. With the help of optical emission spectroscopy, this biological method exploits the fact that reactive oxygen species, especially long-lying radical like O3 and O2 (1∆g), are playing a main role in inactivation process.