Mechanisms of plasma-induced microbial inactivation have commonly been studied with physicochemical techniques. In this letter, Escherichia coli K-12 and its Delta recA, Delta rpoS, and Delta soxS mutants are employed to discriminate effects of UV photons, OH radicals, and reactive oxygen species produced in atmospheric discharges. This microbiological approach exploits the fact that these E. coli mutants are defective in their resistance against various external stresses. By interplaying bacterial inactivation kinetics with optical emission spectroscopy, oxygen atoms are identified as a major contributor in plasma inactivation with minor contributions from UV photons, OH radicals, singlet oxygen metastables, and nitric oxide. (c) 2007 American Institute of Physics.