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Biosensors of single-gene knockout mutants and physical methods using mesh and quartz glass are employed to discriminate plasma agents and assess their lethal effects generated in a Direct-Current atmospheric-pressure oxygen plasma jet. Radicals generated in plasma are determined by optical emission spectroscopy, along with the O3 density measurement by UV absorption spectroscopy. Besides, thermal effect is investigated by an infrared camera. The biosensors include three kinds of Escherichia coli (E. coli) K-12 substrains with their mutants, totalling 8 kinds of bacteria. Results show that oxidative stress plays a main role in the inactivation process. Rather than superoxide O2-, neutral reactive oxygen species such as O3 and O2(a1Δg) are identified as dominant sources for oxidative stress. In addition, DNA damage caused by oxidation is found to be an important destruction mechanism.