Electrolytic Generation of Oxygen Partially Explains Electrical Enhancement of Tobramycin Efficacy against Pseudomonas aeruginosa Biofilm

Abstract

The role of electrolysis products, including protons, hydroxyl ions, reactive oxygen intermediates, oxygen, hydrogen, and heat, in mediating electrical enhancement of killing of Pseudomonas aeruginosa biofilms by tobramycin (the bioelectric effect) was investigated. The log reduction in biofilm viable cell numbers compared to the numbers for the untreated positive control effected by antibiotic increased from 2.88 in the absence of electric current to 5.58 in the presence of electric current. No enhancement of antibiotic efficacy was observed when the buffer composition was changed to simulate the reduced pH that prevails during electrolysis. Neither did stabilization of the pH during electrical treatment by increasing the buffer strength eliminate the bioelectric effect. The temperature increase measured in our experiments, less than 0.2°C, was far too small to account for the greatly enhanced antibiotic efficacy. The addition of sodium thiosulfate, an agent capable of rapidly neutralizing reactive oxygen intermediates, did not abolish electrical enhancement of killing. The bioelectric effect persisted when all of the ionic constituents of the medium except the two phosphate buffer components were omitted. This renders the possibility of electrochemical generation of an inhibitory ion, such as nitrite from nitrate, an unlikely explanation for electrical enhancement. The one plausible explanation for the bioelectric effect revealed by this study was the increased delivery of oxygen to the biofilm due to electrolysis. When gaseous oxygen was bubbled into the treatment chamber during exposure to tobramycin (without electric current), a 1.8-log enhancement of killing resulted. The enhancement of antibiotic killing by oxygen was not due simply to physical disturbances caused by sparging the gas because similar delivery of gaseous hydrogen caused no enhancement whatsoever.

Photodestruction of human dental plaque bacteria: Enhancement of the photodynamic effect by photomechanical waves in an oral biofilm model

“…Ultrasound [51][52][53] and electric fields [54][55][56] have been used to enhance the efficacy of antibiotics in killing biofilm bacteria in sterilization processes. Ultrasonic irradiation enhanced the killing of P. aeruginosa in biofilms by gentamicin by nearly two orders of magnitude [51,52].…”

“…O'Leary et al [53] have shown that statistically significant bactericidal effects of ultrasonic irradiation upon pure cultures of the periodontal pathogens P. gingivalis and A. actinomycetemcomitans were due to the incidental temperature increase [53]. On the other hand, small direct current electric fields achieved a 6-to 8-log increase in killing after 24 hours of exposure to the direct current using biofilms of P. aeruginosa [54][55][56]. An electric current enhanced the efficacy of gentamicin against the oral microorganism Streptococcus gordonii in an in vitro biofilm model [57].…”

Electrochemical biofilm control: mechanism of action

“…Some of the mechanisms proposed in the literature are (1) electrochemical generation of oxidants on the surface (Costerton et al 1994), (2) contraction or expansion of biofilm structure (Stoodley et al 1997), (3) electrolytic generation of oxygen during water electrolysis (Stewart et al 1999), and (4) electrostatic and electrophoretic repulsive forces (Hong et al 2008). However, none of these mechanisms is widely accepted because of conflicting results in the literature and the impossibility of comparing the different electrochemical systems.…”

Microbial Colonization of Medical Devices and Novel Preventive Strategies

“…With application of direct current electric fields between 1.5 and 20 Vcm -2 (current densities of about 15 10 -6 to 2.1 10 -3 Acm -2 ), the concentrations of antibiotics needed to be effective against biofilm bacteria fell from approximately 5000 times to 4 times greater than those necessary for planktonic bacteria in the absence of electricity [82]. Electrolytic generation of oxygen may be partly responsible for this bioelectric effect, at least with the aminoglycoside antibiotic tobramycin against P. aeruginosa biofilm [83]. Enhanced antibiotic activity has also been shown using pulsed electromagnetic fields (PEMF).…”