# Susceptibility of _Staphylococcus epidermidis_ to Argon Cold Plasma Jet by Oxygen Admixture
## 3 Results and Discussion
### Anti-Bacterial Activity of Non-Thermal APPJ  
The application of a cold plasma device as an antibacterial and safety agent for the stimulation of wound healing was recently tested by [PERSON] et al. [27] using human dermal samples. Their study demonstrated the in vitro and in vivo safety and efficacy in _Staphylococcus aureus_ reduction by using a flexible dielectric barrier discharge plasma device. The effect of non-thermal APPJ using pure argon and argon mixed with oxygen molecules on the reduction of colony-forming units (CFUs) of _S. epidermidis_ count is shown in Figure 7. There is a steady decrease in the count of _S. epidermidis_ under all treatments with the maximum reduction achieved for the non-thermal APPJ when argon is mixed with 0.2% oxygen. The reduction for later cases was dropped from 78 \(\times\) 10\({}^{6}\) CFU/mL (initial count) to 1.2 \(\times\) 10\({}^{6}\) CFU/mL as compared to 2.9 \(\times\) 10\({}^{6}\) for pure argon plasma delivered for  
Figure 6: Effect of O\({}_{2}\) admixture variation on atomic O and OH radical intensities at 2 mm below the jet nozzle at fixed 14 kV and 29 kHz. one minute. Increasing the addition of oxygen to the argon lowered the toxicity of the pure argon plasma where the CFUs of _S. epidermidis_ recorded 3.3, 3.5, 3.9, and 4.3 \(\times\) 10\({}^{6}\) CFU/mL after one minute for the plasma-derived from argon mixed with 0.6%, 0.8%, 1.0%, and 1.2% oxygen, respectively. Generally, the extension of exposure time, under different treatments, from 1 up to 16 min leads to a gradual increase in the reduction of CFU of _S. epidermidis_ reaching 0.9 \(\times\) 10\({}^{6}\) CFU/mL for pure argon plasma after 16 min. However, there is a steady decrease in the count of _S. epidermidis_ from 78 \(\times\) 10\({}^{6}\) to 3.5 \(\times\) 10\({}^{6}\) CFU/mL on exposure to plasma for 1 min using pure argon as operating gas. The maximum reduction in CFU was observed at 0.2% O\({}_{2}\). No bacterial growth was observed at the later concentration applied for 8 min and the same case was detected at 0.4% O\({}_{2}\) applied to 16 min. [PERSON] et al. [28] tried cold atmospheric pressure argon plasma against 78 genetically different _S. aureus_ strains for clinical and epidemiological implications. Their study established the high complexity of microbial defense against antimicrobial treatment and strain-dependent susceptibility of _S. aureus_ to plasma treatment. Several authors indicated that the degree of microbial inactivation probably depends on the type of micro-organisms, the inactivation medium, number of cells, operating gas mixture, gas flow, and physiological state of cells [29; 30]. The difference in the effectiveness of each treatment or operating gas mixture is probably related to the emitted reactive species produced by the atmospheric plasma. The anti-bacterial activity of the cold plasma is probably due to the generation of oxygen and OH-radicals, which are almost produced in an equal amount at argon mixed with 0.2% oxygen, as shown in the spectroscopic measurements. The intense bombardment of the generated radicals with the bacterial cell walls induces severe irreversible cell wall damage, leading to its destruction [31]. In previous works [32; 33; 34; 35], the antimicrobial effectiveness of charged particles, free radicals, and UV-emitting species was described. Moreover, the cold atmospheric plasma may induce destruction in cellular constructions and cell permeation [36] as well as destruction and cracking of the cell walls or cell membrane of micro-organisms [37]. In addition, [PERSON] et al. [38] showed that the energized electrons and ions could collide with organic molecules on the cell and disrupt the chemical bonds and induce perforations to the cell membrane. Moreover, the reactive oxidizing agents and reactive nitrogen species (e.g., ROS and RNS) produced in APPJ could react with and damage the cell or the contents.