Abstract
Electroceutical wound dressings, especially those involving current flow with silver based electrodes, show promise for treating biofilm infections. However, their mechanism of action is poorly understood. We have developed an in vitro agar based model using a bioluminescent strain of Pseudomonas aeruginosa to measure loss of activity and killing when direct current was applied. Silver electrodes were overlaid with agar and lawn biofilms grown for 24 h. A 6 V battery with 1 kΩ ballast resistor was used to treat the biofilms for 1 h or 24 h. Loss of bioluminescence and a 4-log reduction in viable cells was achieved over the anode. Scanning electron microscopy showed damaged cells and disrupted biofilm architecture. The antimicrobial activity continued to spread from the anode for at least 2 days, even after turning off the current. Based on possible electrochemical ractions of silver electrodes in chlorine containing medium; pH measurements of the medium post treatment; the time delay between initiation of treatment and observed bactericidal effects; and the presence of chlorotyrosine in the cell lysates, hypochlorous acid is hypothesized to be the chemical agent responsible for the observed (destruction/killing/eradication) of these biofilm forming bacteria. Similar killing was obtained with gels containing only bovine synovial fluid or human serum. These results suggest that our in vitro model could serve as a platform for fundamental studies to explore the effects of electrochemical treatment on biofilms, complementing clinical studies with electroceutical dressings.
Highlights
Biofilms are aggregates of microorganisms with high cell densities embedded in a self-produced extracellular polymeric substance (EPS) that are adherent to each other and/or a surface[1]
The bacterial lawn in this model satisfies the criteria for being considered a biofilm as it is an immobile community of bacteria attached to a living surface and embedded in an extracellular polymeric matrix (EPS) that they have produced
A systematic evaluation of the bactericidal effects of electroceutical wound dressing is presently lacking, and this study using an in vitro agar model, is a significant advancement in understanding the extensive parametric and mechanistic aspects of why electroceutical dressings are effective in eliminating bacterial biofilms
Summary
Biofilms are aggregates of microorganisms with high cell densities embedded in a self-produced extracellular polymeric substance (EPS) that are adherent to each other and/or a surface[1]. There have been several novel approaches to developing electroceutical dressings, that use either electric fields or currents to remediate biofilms while accelerating wound healing[12,13,14]. These alternates to antibiotic treatments are in various stages of commercialization but lack a fundamental understanding of the underlying principles behind their efficacy and limitations. While most of the previous studies have been done in liquid media that are less applicable to wound biofilms, our current study focuses on the use of an in vitro agar based model as it mimics conditions similar to soft tissues in terms of providing a soft surface and a diffusion dominated environment. The influence of an applied current on the activity and killing of PA biofilm bacteria was assessed by bioluminescence, viable cell counts, and scanning electron microscope (SEM) imaging
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