**Parameters A**ff**ecting the Antimicrobial Properties of Cold Atmospheric Plasma Jet**

**Bih-Show Lou 1,2, Chih-Ho Lai 3,4,5,6, Teng-Ping Chu 7, Jang-Hsing Hsieh 7,8, Chun-Ming Chen 7, Yu-Ming Su 7, Chun-Wei Hou 1, Pang-Yun Chou 9 and Jyh-Wei Lee 7,8,9,10,\***


Received: 19 October 2019; Accepted: 7 November 2019; Published: 9 November 2019

**Abstract:** Using the Taguchi method to narrow experimental parameters, the antimicrobial efficiency of a cold atmospheric plasma jet (CAPJ) treatment was investigated. An L9 array with four parameters of CAPJ treatments, including the application voltage, CAPJ-sample distance, argon (Ar) gas flow rate, and CAPJ treatment time, were applied to examine the antimicrobial activity against *Escherichia coli* (*E. coli*). CAPJ treatment time was found to be the most influential parameter in its antimicrobial ability by evaluation of signal to noise ratios and analysis of variance. 100% bactericidal activity was achieved under the optimal bactericidal activity parameters including the application voltage of 8.5 kV, CAPJ-sample distance of 10 mm, Ar gas flow rate of 500 sccm, and CAPJ treatment time of 300 s, which confirms the efficacy of the Taguchi method in this design. In terms of the mechanism of CAPJ's antimicrobial ability, the intensity of hydroxyl radical produced by CAPJ positively correlated to its antimicrobial efficiency. The CAPJ antimicrobial efficiency was further evaluated by both DNA double-strand breaks analysis and scanning electron microscopy examination of CAPJ treated bacteria. CAPJ destroyed the cell wall of *E. coli* and further damaged its DNA structure, thus leading to successful killing of bacteria. This study suggests that optimal conditions of CPAJ can provide effective antimicrobial activity and may be grounds for a novel approach for eradicating bacterial infections.

**Keywords:** Taguchi method; antimicrobial efficiency; cold atmospheric-pressure plasma jet (CAPJ); *Escherichia coli*; DNA double-strand breaks; scanning electron microscopy
