# Susceptibility of _Staphylococcus epidermidis_ to Argon Cold Plasma Jet by Oxygen Admixture
## 1 Introduction  
_S. epidermidis_ ranks first among the causative agents of nosocomial infections. Staphylococcus are common bacterial colonizers of the skin and mucous membranes of humans and other mammals. _S. epidermidis_ represents the most common source of infections on medical devices [1]. Therefore, prevention and considering sterilization techniques become urgent requirements to avoid the spread of the diseases. One of the efficient sterilization techniques is cold atmospheric pressure plasma sterilization [2, 3]. Several techniques have been used to overcome the restrictions to generate non-thermal plasma at atmospheric pressure such as using Dielectric-barrier discharges (DBD) [4], pulsed power source [5], segmented cathode [6], and the micro-hollow cathode system [7]. Moreover, some investigators used gliding arc discharge and some others use several types of plasma jets [8]. The method by which the non-thermal atmospheric pressure plasma (APP) is generated will depend on the plasma properties required such as volume of the generated plasma, gas temperature, electron density and temperature, and homogeneity of the plasma. It is believed that a combination of the different techniques can be used to fulfill the requirement for generating non-thermal plasma with a high rate of reactive species generation and large-scale plasma. The generation of cold atmospheric pressure plasma opened many applications in several fields including sterilization [9], medical treatment [10], and other biomedical applications [11]. Cold plasma is a new safe non-thermal sterilization technology that uses energetic, reactive gases to inactivate microbes. Atmospheric pressure plasma jets (APPJs) have been generated using low-frequency (LF) power sources in the range of kHz. Since it has a long jet, the APPJs are employed for sterilization for asymmetrical surfaces. Helium (He) plasma jet is the most stable and easiest APPJ to be generated due to its lowest breakdown voltage and lower gas temperature [4]. From the economic point of view, the use of helium is expensive; therefore, an intensive investigation has been done to find other cheaper gases such as air, nitrogen, and argon. Since air and nitrogen have a higher breakdown voltage compared to argon APPJ, argon was selected to be used instead of helium particularly for many industrial and medical purposes. Remarkably, using argon increases generated plasma gas temperature and the possibility for the glow to arc transition. Moreover, the rate of power consumption is relatively low, with high disinfection efficiency and low-cost efficacy [12]. APPJ is a kind of plasma generated at atmospheric pressure, and the plasma is formed as a phenomenon going through a jet. This type of plasma can be generated by alternating current (AC), pulsed, radio frequency (RF) as well as microwave power supplies. Currently, the characteristic properties of APPJ including its usage as a sterilization method have attracted many investigators [13; 14; 15]. Firstly, APPJ is an accessible plasma that does not need any vacuum equipment or any complex materials to be generated. Secondly, it is a flexible system that comes in a shape similar to a pin connected to the power supply and a gas cylinder. This shape gives users the possibility to bring the system into the target. Thirdly, it can be used with heat-sensitive materials including biological samples as they can not tolerate high temperature [16]. Fourthly, it is a low-cost system that just needs a very simple means to operate [17]. Finally, different operating conditions are flexible and can be changed, such as operating gases and their flow rate, the current, applied voltage, and frequency that affect the plasma characteristics and give a wide range of applications [18; 19]. Non-thermal atmospheric pressure plasma jet (APPJ) can be grouped depending on the operating gas into three categories of APPJ: the noble gas plasma jet, the N\({}_{2}\) plasma jet, and the air plasma jet.