Microbial Growth Inhibition by Alternating Electric Fields

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http://aac.asm.org/cgi/content/full/52/10/3517Weak electric currents generated using conductive electrodes have

been shown to increase the efficacy of antibiotics against bacterial

biofilms, a phenomenon termed " the bioelectric effect. " The purposes of the present study were (i) to find out whether insulated

electrodes that generate electric fields without " ohmic " electric

currents, and thus are not associated with the formation of metal ions and free radicals, can inhibit the growth of planktonic bacteria

and (ii) to define the parameters that are most effective against

bacterial growth. The results obtained indicate that electric

fields generated using insulated electrodes can inhibit the growth of planktonic Staphylococcus aureus and Pseudomonas

aeruginosa and that the effect is amplitude and frequency dependent, with a maximum at 10 MHz. The combined effect of the electric field and chloramphenicol was found to be additive. Several possible mechanisms underlying the observed

effect, as well as its potential clinical uses, are discussed.

The use of physical means as an aid for modern medicine in the continuous

battle against pathogenic microorganisms holds new prospects

that only recently have begun to be widely recognized. Light

sources of various types are being used for photodynamic therapy

in dentistry and dermatology (10, 13, 33). Ultrasound waves are used for human dental plaque removal (16) and, in combination with antibiotics, for the eradication of bacterial biofilms in vitro and in vivo (3, 7, 19, 21). In addition, thermotherapy, originally developed as a tool against cancerous tumors, has been found to be effective against cutaneous leishmaniasis (22).

The major drawback of the methods mentioned above is their limited selectivity;

thus, ultrasonic waves and thermotherapy nonspecifically produce

heat that may cause severe collateral damage. Similarly, the

illumination of the photosensitizers in photodynamic therapy can

harm tissues in the vicinity of the target area. Other downsides of

photodynamic therapy include the need to deliver the photosensitizers

to the treated area and the low tissue penetration of the radiation, limiting the application of this treatment to topical infections (13, 14).

The use of an additional physical means, weak electric currents, to

inhibit bacterial growth was suggested by Rosenberg et al. (24), who observed that electrolysis resulted in the arrest of Escherichia

coli cell division. Further investigation of this phenomenon revealed that transition platinum complexes produced

at the platinum electrodes during electrolysis were responsible

for the bacterial growth inhibition. These derivatives were found not to be specific to bacteria; they were also toxic to

human cells. In fact, this work eventually led to the discovery of

the known chemotherapeutic agent cisplatin (25). In the years to follow, it was demonstrated that low-intensity electric currents, mostly direct current (DC) (1, 5, 20, 31, 32), as well as alternating electric fields of as much as 10 MHz (4, 18), can enhance the efficacy of antibacterial agents against bacterial biofilms. In all of these studies, the electric currents were generated using conductive electrodes, allowing

for the formation of metal ions and free radicals at the electrode surface. Like cisplatin, these products are toxic to human cells, and therefore the use of such electric currents was limited.

Recently it was demonstrated that low-intensity alternating electric

fields at frequencies of 100 to 200 kHz can inhibit the growth of proliferating cancerous cell lines, both in vitro and

in vivo, without affecting normal quiescent cells (11, 12). These

fields, termed " tumor-treating fields " (TTFields), were generated

by means of electrically insulated ceramic electrodes, thus ensuring that during the application of the field there is no

electrolysis and that no biocides or ions are produced at the electrode surface and released into the medium. Clinical investigations,

supported by in vitro studies, have demonstrated the safety of the use of TTFields. Evidence was presented indicating that

the mechanism at the basis of this inhibitory effect was related

to the unidirectional dielectrophoresis forces produced by the nonhomogeneous electric fields generated in the vicinity of

the cleavage plain that gradually develops and separates the

newly formed daughter cells from each other. Since a similar process

occurs in rapidly replicating prokaryotic organisms, it is reasonable to assume that they can be targeted by appropriately tuned

electric fields. The field parameters required for affecting bacteria

are expected to be significantly different from those affecting

mammalian cells due to the significant differences in size (see Kirson et al. [11]).

In view of these considerations, the objectives of the present study

were to test the feasibility of using alternating electric fields

generated by insulated electrodes for the inhibition of planktonic bacteria and to define the effective field parameters for

the inhibition process.