In addition to delivery of antibiotics to the site of infection we also are designing ways of increasing antibiotic efficacy against biofilms using the “bioelectric effect.” The bioelectric effect is the synergistic killing effect observed on biofilm cells that are exposed to antibiotics in the presence of an electric direct current (DC) or alternating current (AC) field. The use of electric currents and electromagnetic fields to modulate biologic processes has become an increasingly popular subject of scientific enquiry in recent years. Although many such studies are still in their infancy in orthopaedics, at least one avenue of investigation has shown substantial progress: the use of electromagnetic stimulation to promote healing of problematic bony fractures. Aaron et al1 discuss the results from several trials that indicate electromagnetic fields can be used to accelerate bone formation and healing. Nelson et al21 discuss pulsed electromagnetic fields, capacitive coupled fields (electric fields rather than magnetic fields are used to induce currents), and low-intensity ultrasound as methods that are used to stimulate bone healing and bone formation. Cell studies are reported by Guerkov et al.12. They suggest a cascade of regulatory events is stimulated by the pulsed electromagnetic fields in the human hypertrophic and atrophic nonunion tissues. Ryaby27 presents a review article on the clinical use of electric and electromagnetic fields that are being used in the clinic to assist fracture healing, and a review paper by Otter et al (22) covers a number of the applications of electromagnetic fields to the field of bone healing.
With this background of practical success, efforts are now also underway to determine whether deployment of electric currents and fields against biofilms can become an effective therapy against infection. Early reports in in vitro systems have been encouraging, most particularly in observing the success of the bioelectric effect in potentiating the efficacy of concomitant antibiotic action against biofilm bacteria (Table 2). Costerton et al7 reported that application of a low-intensity direct current (max. 2.1 mA/cm2) to a flow cell in which Pseudomonas aeruginosa biofilm had been established was relatively ineffective in reducing bacterial numbers alone, but increased the killing efficacy of tobramycin by over four orders of magnitude when the two modalities were applied concurrently. The concentration of antibiotic necessary to treat biofilm bacteria was effectively reduced to within the same order of magnitude as that required to achieve MBC values in planktonic bacteria, rather than the thousandfold or more increased concentrations typically required to kill biofilms. Importantly, this increased efficacy brings antibiotic doses down to physiologically and clinically acceptable levels.
These results were confirmed by McLeod et al,18 who again found that direct electric current potentiated the action of tobramycin against pseudomonal biofilms by an almost thousand-fold reduction in bacterial viability in an exposure chamber system (Table 2, Fig. 4). McLeod and co-workers, however, kept current constant for the full duration of treatment and were thus able to compile a dose-response curve for current efficacy, with the lowest optimal current value appearing at 1 mA. Similar encouraging results have been noted for the bioelectric effect against Klebsiella pneumoniae treated with tobramycin, S. epidermidis treated with tobramycin, Streptococcus gordonii treated with gentamicin, and Candida albicans treated with cycloheximide (15,33).
Caubet et al6 have evaluated the use of DC and a 10 megahertz (10 × 10^6 hertz) time-varying AC in an exposure chamber similar to that of McLeod et al.18 These authors used Escherichia coli biofilms treated with either gentamicin or with oxytetracycline and studied the effect of adding either a DC or the ten megahertz AC to the antibiotic in the support medium. The current for the DC and AC work was delivered through the support medium flowing in the exposure chamber through electrodes extending into the medium at either end of the exposure chamber (ie, the AC was not induced in the chamber). The DC plus antibiotic treatment produced a four to five log10 reduction in colony forming units (CFU), achieving very similar results to those of McLeod et al,18 whereas a slightly more modest reduction in CFU of 2.5 to 3.5 logs was achieved with the AC plus the antibiotic. This study supports the hypothesis that enhanced efficacy can be achieved for an antibiotic with the addition of an AC, although the magnitude of the improvement may be less than that obtained with direct current.
Pickering et al24 expand the field even further by examining the effect of an induced current on antibiotic efficacy against biofilm bacteria. In this work, a pulsed electromagnetic field (PEMF) was applied to S. epidermidis biofilm bacteria which induced a current within the biofilm. This induced current reduced the minimum inhibitory concentration of gentamicin by at least 50%, but did not show any significant effect with vancomycin.
Although there is an increasing body of evidence in support of a bioelectric effect against biofilms, the mechanism by which these electrical phenomena exert their actions is unknown, and indeed may well vary with the type of electrical current applied. In the 10 MHz AC system of Caubet et al,6 for example, the authors point out that “there is no transport of ions between the electrodes, no creation of new ions, and no electrolysis” as would occur in a DC system, but a bioelectric effect still appears. Stewart et al29 investigated the mechanisms by which a DC bioelectric effect may operate. Their work discounted the suggestions that reduced pH, increased temperature, or generation of inhibitory ions or reactive oxygen intermediates were the relevant means by which a bioelectric effect manifests. They did find that electrolytic generation of oxygen (potentially increasing the local oxygen concentration in the biofilm microenvironment) appeared to partially explain the augmentation of antibiotic efficiency. Based on this study, we hypothesize that the bioelectric effect results from increased metabolic and replicative activity associated with increased O2 tensions within the biofilm bacteria which make them more susceptible to antibiotic-induced killing. It is well established that antibiotics are much more effective against rapidly metabolizing and dividing bacteria than they are to metabolically quiescent bacteria, and one of the limiting nutrients within the core of the biofilm is O2. We have shown that the provision of O2 deep within the biofilm results in greatly increased metabolism.10
The majority of work evaluating the bioelectric effect has to date been carried out in in vitro systems and has focused mainly on aminoglycoside antibiotics. Only Pickering et al24 failed to find any significant bioelectric effect with vancomycin but they did not evaluate it in a directly applied DC system. In clinical terms for orthopaedics, the bioelectric effect could become important if it can be shown in in vivo conditions and with antibiotics routinely used against the staphylococci, especially vancomycin. We therefore are establishing protocols in which direct or induced currents can be delivered within the joint capsule of an infected knee prosthesis in small animal and large animal models. If the bioelectric effect can be achieved in vivo, the likelihood of developing a human-use device to supply adjuvant electrical therapy to patients with infected implants will be increased substantially.