Direct electric current treatment under physiologic saline conditions kills Staphylococcus epidermidis biofilms via electrolytic generation of hypochlorous acid

PLoS One. 2013;8(2):e55118. doi: 10.1371/journal.pone.0055118. Epub 2013 Feb 4.


The purpose of this study was to investigate the mechanism by which a direct electrical current reduced the viability of Staphylococcus epidermidis biofilms in conjunction with ciprofloxacin at physiologic saline conditions meant to approximate those in an infected artificial joint. Biofilms grown in CDC biofilm reactors were exposed to current for 24 hours in 1/10(th) strength tryptic soy broth containing 9 g/L total NaCl. Dose-dependent log reductions up to 6.7 log(10) CFU/cm(2) were observed with the application of direct current at all four levels (0.7 to 1.8 mA/cm(2)) both in the presence and absence of ciprofloxacin. There were no significant differences in log reductions for wells with ciprofloxacin compared to those without at the same current levels. When current exposures were repeated without biofilm or organics in the medium, significant generation of free chlorine was measured. Free chlorine doses equivalent to the 24 hour endpoint concentration for each current level were shown to mimic killing achieved by current application. Current exposure (1.8 mA/cm(2)) in medium lacking chloride and amended with sulfate, nitrate, or phosphate as alternative electrolytes produced diminished kills of 3, 2, and 0 log reduction, respectively. Direct current also killed Pseudomonas aeruginosa biofilms when NaCl was present. Together these results indicate that electrolysis reactions generating hypochlorous acid from chloride are likely a main contributor to the efficacy of direct current application. A physiologically relevant NaCl concentration is thus a critical parameter in experimental design if direct current is to be investigated for in vivo medical applications.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anti-Bacterial Agents / pharmacology
  • Biofilms / drug effects*
  • Biofilms / growth & development
  • Caseins
  • Chlorine / chemistry
  • Chlorine / toxicity*
  • Ciprofloxacin / pharmacology
  • Colony Count, Microbial
  • Culture Media
  • Electricity
  • Electrolysis*
  • Hypochlorous Acid / chemistry
  • Hypochlorous Acid / toxicity*
  • Microbial Sensitivity Tests
  • Nitrates / chemistry
  • Phosphates / chemistry
  • Protein Hydrolysates
  • Pseudomonas aeruginosa / drug effects*
  • Pseudomonas aeruginosa / growth & development
  • Sodium Chloride / chemistry
  • Staphylococcus epidermidis / drug effects*
  • Staphylococcus epidermidis / growth & development
  • Sulfates / chemistry


  • Anti-Bacterial Agents
  • Caseins
  • Culture Media
  • Nitrates
  • Phosphates
  • Protein Hydrolysates
  • Sulfates
  • trypticase-soy broth
  • Sodium Chloride
  • Chlorine
  • Ciprofloxacin
  • Hypochlorous Acid

Grants and funding

This work was supported by Montana State University College of Engineering, Peter Ewing Capital Management LLC, The Allegheny-Singer Research Institute, and The Montana Board of Research and Commercialization Technology Grant Agreement No. 08-03. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.