# Anti-Bacterial Action of Plasma Multi-Jets in the Context of Chronic Wound Healing
## 3 Results
### Inactivation of Staphylococcus aureus  
Figure 7 presents the multi-spot plasma treatment of _S. aureus_ inoculated membrane set on agar substrates, in various experimental conditions. Figure 6: (**a**) Current delivered on a metallic target with 4 jets: bold black trace, 3 jets thin grey trace, 2 jets: bold grey trace and 1 jet: thin black trace, (**b**) image of KI-enriched agar samples exposed to four multi-jets generated with the low voltage applicator during left: 60 s and right: 240 s.  
As expected, the control gas treatment shows no bactericidal action, as shown in Figure 7c where the bacterial lawn full development is observed in the same manner as for the untreated sample in Figure 7b. This confirms that the helium gas flow has no impact on the inoculated membrane, inducing no drying or spray action, as the 24-h incubation results in the same growth of the bacterial lawn as for the untreated sample. In Figure 7d, it is observed that the impact of the y 4, 12 and 12 secondary jets, distributed from the center to the periphery of the plasma applicator, results in a full inactivation of the bacteria. Because of an imperfect centering of the plasma applicator with respect to the sample, only four secondary jets distributed along the 38 mm diameter outer ring of the multi-spot applicator impacted the bacterial lawn, which was about 40 mm in diameter. This is visible in the top right region of Figure 7d. Indeed, 24 h after the multi jet treatment, the bacterial lawn growth is fully inhibited at the positions of these 32 plasma jet's impact. This first indicates that the plasma action is localized in the region where the plasma jets impact and does not induce any strong effect in between the secondary jet's regions during plasma jet exposure, but after the following 24-h incubation as well. The ratio of the sum of these 4, 12, 12 and 4 plasma spot impact surfaces over the bacterial lawn surface results in about an 8% coverage for a 5-min-long static exposure with the multi-spot applicator. Thus, the decontamination of the full surface for the extended bacterial lawn would require a displacement of the plasma applicator, as will be documented in Section 3.4. One observes that the higher intensity of the central multi-jets, as discussed in Figure 1, results in larger inactivation spots as compared to the peripheral multi-jets effect. As the visible diameters of all the secondary jets is approximately constant, this means that the plasma light emission cross section observed on the sample surface is not the best indicator to predict the inhibition zone diameter. Both plasma generated reactive species and transient electric fields inherent with the multi-jets delivery may have an effect away from the plasma emitting light volume. Figure 7e indicates that 5 min treatment with the sample rotation (rotation speed was of 1 revolution/min) is enough to prevent bacterial lawn development at the plasma jet impact positions. This means that plasma exposures shorter than 5 min are sufficient to inactivate _S. aureus_. One also notes that the inactivation is more efficient along the two inner  
Figure 7: Inactivation of _S. aureus_ with the multi-spot plasma applicator. (**a**) Multi-spot plasma treatment of inoculated membrane on agar substrate, (**b**) H24 image of the untreated sample, (**c**) H24 image of the sample exposed at H0 to 5 min helium flow, (**d**) H24 image of the sample exposed at H0 to 5 min plasma treatment in static mode, (**e**) H24 image of the sample exposed at H0 to 5 min plasma treatment with a 1 rpm rotation, (**f**) H24 image of the sample exposed at H0 to 1-min plasma treatment in static mode. H0 and H24 stand for hour zero as the time origin and 24 h later, respectively. circles where 4 and 12 multi-jets, respectively, are distributed, than for the third and fourth larger diameters circles exposed with the 12 and 24 multi-jets, respectively, ignited out of the channels from the two outer rings of the applicator.