Abstract
Background: Plasma-activated solutions can be obtained by exposing deionized water, saline, and citrate solution to microhollow cathode discharge. Those plasma-activated solutions showed different temporal post-discharge effects, and those plasma-activated solutions demonstrated varied antibacterial activity. Antibacterial activity of three types of plasma-activated solutions was investigated on Escherichia coli (E. coli, gram-negative) and Staphylococcus aureus (S. aureus, gram-positive). Plasmaactivated water (PAW) and plasma-activated saline (PAS) were much more potent than plasma-activated citrate solution (PAC) as inactivating on both bacteria. Although plasma-activated solutions did not have noticeable effects on biofilm structures, they diffused freely in biofilms and inactivated bacteria in biofilm effectively and efficiently, resulting in bacterial cell membrane damage.
Methods: A non-thermal and microhollow cathode discharge (MHCD) device was used to generate plasma-activated solutions. E. coli and S. aureus were used as models for antibacterial activity and antibiofilm assay. In the antibacterial activity assay, the optical density (OD570) of bacterial growth was measured by using microplate reader. In the anti-biofilm assay, CV staining and MTT assay, were used to assess biofilm susceptibility to plasma-activated solutions, and LIVE/DEAD staining kit was used to stain and observe live and dead bacterial distributions in the biofilms and visualized by Zeiss confocal fluorescence microscopy. Results: Plasma treatments can cause the acidification of treated solutions (water and saline), but buffer citrate solution provided sufficient buffer capacity. Although bacteria could tolerate acidic solutions prepared from acids, they were unable to survive in plasma treated solutions after an exposure of 5 minutes or more. More importantly, plasma-activated solutions were also inactive to biofilms and all bacteria embedded in both one-day old biofilms (E. coil and S. aureus) were killed after 3 hours of contact time. However, PAW and PAS are deprived of biofilm etching function. Despite completely inactivated bacteria, PAW and PAS treated biofilms maintained the same biomass as the controls after a long incubation time. This result supports the conclusion that different reactive species in plasma are responsible for the antibacterial and etching activity of plasma. Conclusion: Since plasma-activated solutions damaged bacteria by causing cell membrane lysis, they showed low cell selectivity and were active to both gram-positive and gram-negative bacteria and biofilms. Therefore, plasma-activated solutions can be alternates to replace chemicals, antibiotics and antiseptics for industrial, biomedical bio-fouling applications.Keywords: Antibiotic resistance, biofilms, plasma-activated solutions, antibacterial, plasma discharge, sterilization.
Graphical Abstract
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