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A new plasma dental probe (PDP), powered with 4-6-kV ~100-ns electric pulses at repetition rates of up to 2 kHz, generates a room-temperature > 2-cm-long ~2-mm-diameter plasma plume at ambient atmospheric pressure. The shape of the plasma plume depends on the gas flow rate and the pulse voltage. Growth of Bacillus atrophaeus on nutrient agar plates is completely inhibited by exposure to both He and He/(1%)O2 plasmas for 60 s. A 300-s treatment with the He/(1%)O2 plasma disinfected a human tooth root canal colonized with saliva-derived biofilms, demonstrating the potential of the PDP as a simple, safe, and effective tool or adjunct for the disinfection of root canals during endodontic therapy. He/(1%)O2 plasma is more effective than He alone for B. atrophaeus growth inhibition, and the emission spectrum of the He-O2 plasma supports the identification of atomic oxygen as the likely primary reactive chemical species contributing to bacterial inactivation.