## 1 Introduction  
Hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) is extensively used as a biocide, particularly in applications where its decomposition into non-toxic by-products is important such as food and medical instruments [1]. The peroxide bond is unstable, so H\({}_{2}\)O\({}_{2}\) in low concentration is easily decomposed to water and oxygen [2]. The bactericidal effect becomes stronger with the increase in H\({}_{2}\)O\({}_{2}\) concentration, but it can irritate the skin or eyes and damage some materials [3]. A solution of 3% or less H\({}_{2}\)O\({}_{2}\) is widely used in wound disinfection and dental care [4, 5]. However, it is a mild disinfectant, so the inactivation potential of H\({}_{2}\)O\({}_{2}\) was insufficient in some cases to ensure food or medical safety [6, 7].  
In order to increase the sterilizing power of this H\({}_{2}\)O\({}_{2}\), a combination with other mild technologies has been attempted [8, 9]. The combination with the cold atmospheric pressure plasma (CAP) is one such attempt. CAP is a specific type of non-thermal atmospheric pressure plasmas at effluent temperatures acceptable for heat sensitive specimens. It is a partly ionized gas, which is made up of atoms, molecules, free electrons, ions, UV irradiation, and reactive species such as reactive oxygen/nitrogen species (RONS) including ozone. CAP is divided into dielectric barrier discharge (DBD) and Jet types by electrode structures. In DBD, plasma is localized at the vicinity of the electrodes, and the excited gas mass is diffused or delivered to the surface. In Jet, the plasma plume emitting light from excited molecules is directly exposed to the sample. Combination of H\({}_{2}\)O\({}_{2}\) with DBD has been tried, and the results showed enhanced disinfection efficiency on dried bacteria and biofilms [10, 11]. In both cases, authors assumed the generation of hydroxyl radical (OH-) radicals from H\({}_{2}\)O\({}_{2}\). Combination with Jet type CAP has not yet been studied, but it is expected that the OH- radical generation rate would be higher.  
Jet type CAP has been attracting attention as a suitable tool for disinfection of exposed body parts such as surface treatment. The Leibniz Institute for Plasma Science and Technology (INP) group has been intensively researching the bactericidal power and the effect on treatment recovery in the treatment of chronic wounds, and it has been shown to be effective in the treatment of skin cancer [12; 13]. However, in the case of a plasma jet, it takes a long time to treat a large area because the application area is narrow. Therefore, a method for efficiently sterilizing in a short time while directly contacting the plum is required. The combination of H\({}_{2}\)O\({}_{2}\) and jet type is expected to increase efficiency in terms of time and sterilization power by adding a plasma effect to the effect of the existing disinfectant.  
In this study, we used less than 3% H\({}_{2}\)O\({}_{2}\) and CAP simultaneously. CAP has a thin and long plume, and the combination of H\({}_{2}\)O\({}_{2}\) and CAP was applied to _E. faecalis_ homogenate in aqueous solutions and gels. _E. faecalis_ has been reported as a main cause for the persistent oral infections [14; 15]. The bactericidal efficiency was evaluated with the electron microscopy and the colony forming unit (CFU) assay.  
The aim of this study was to investigate the efficiency of simultaneous treatment of Jet type CAP and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) for bacterial disinfection. The underlying mechanisms were studied by measuring extracellular chemical changes in the water solution and by measuring biological changes such as the trans-membrane potential, the intracellular oxidative stress, and the membrane permeability.