Brief note
Lack of effect of an externally applied electric field on bacterial adhesion to glass

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Abstract

Deposition to glass of Streptococcus salivarius HB-C12 and Staphylococcus epidermidis 3399 in a parallel plate flow chamber in the absence and presence of an externally applied electric field has been studied experimentally. No effect on bacterial adhesion, including initial deposition rates, numbers of adhering bacteria after 4 h, spatial distributions of adhering bacteria and air bubble induced detachment, was found. A theoretical analysis shows that electric fields applied over a 150 μm thin glass substratum do not have a sufficiently strong effect on its surface potential to influence bacterial adhesion.

Introduction

The formation of a biofilm occurs in several sequential steps, starting with the formation of a conditioning film, followed by bacterial transport and initial adhesion, anchoring and eventually growth [1]. Several authors have claimed to be able to influence biofilm formation by using electric fields. For example, Costerton and co-workers reported that they could enhance the efficacy of antibiotics in killing biofilm bacteria by using only low electric field strengths (1.5–30 V/cm) and current densities (15 μA/cm2 to 2.1 mA/cm2) [2], [3]. Others [4] have demonstrated inhibition of bacterial colonisation of surfaces by using an electric current of only 10 μA and medical devices such as catheters provided with an electric field generator are claimed to inhibit bacterial attachment [5], [17]. In all cases, the exact mechanism by which the electric field influences biofilm formation is not clear.

In this note, we study the effect of an applied electric field on bacterial adhesion to glass. Adhesion of bacteria is determined by physico-chemical interactions [6] and the classical DLVO approach describes bacterial adhesion as an interplay of Lifshitz–Van der Waals and electrostatic interactions, with a possible influence of an external electric field. Experimentally, a distinction must be made between the application of an electric field with and without electrical charge transfer. In most studies, adhesion to an electrode surface is studied, which enables charge transfer. Morisaki et al. [7] determined the applied electric field at which the initial deposition rate of Pseudomonas syringae to indium tin oxide (ITO) coated glass was essentially zero and from this value derived the microbial attachment force to be 5.0×10−11 N per bacterium. Charge transfer during deposition of Leuconostoc mesenteroides and Streptococcus thermophilus to stainless steel was studied by Boulangé-Petermann et al. [8]. Adhesion of bacteria from an aerated 0.15 M NaCl solution was accompanied by a change in rest potential of the stainless steel in contact with the solution from 50 mV with respect to a saturated calomel electrode (SCE) to 170 mV/SCE and caused changes in the current present under potentiostatic conditions. Similar experiments were done with Escherichia coli adhering to carbon-cloth electrodes [9], [10], demonstrating that application of an electrode potential of 0.74 V/SCE resulted in the death of cells attached to the electrode due to oxidation of the intracellular coenzyme A. To our knowledge, no reports have been made on the effects of an applied electric field on bacterial adhesion in the absence of charge transfer, i.e. when the substratum is not used as an electrode. It has therefore hitherto been unclear if effects on bacterial adhesion of electric field application result from the applied electric potential or whether they are caused by charge transfer between the electrode and bacteria, electrophoresis or other electrochemical reactions.

The aim of this paper is to determine experimentally the effect of an externally applied electric field on the deposition of two bacterial strains to glass in a parallel plate flow chamber.

Section snippets

Parallel plate flow chamber and data analysis

Deposition of bacteria to glass in the presence and absence of an externally applied electric field was studied using a parallel plate flow chamber [11] with image analysis. The top and bottom glass plates of the flow cell (5.5 by 3.8 cm) were separated by two Teflon spacers of 0.06 cm thickness. Bacterial deposition was determined to a 150 μm thick bottom glass plate, partially coated with a gold layer (transmittance of 80% for white light and resistivity of 100 Ω per square, coated area of

Results and discussion

Fig. 2 shows the measured initial deposition rates of S. salivarius HB-C12 and S. epidermidis 3399 to glass as a function of the potassium phosphate concentration in the absence of an applied electric potential. Initial deposition rates in the presence of an applied electric field (−4 and +4 kV) were not significantly different (for all cases P>0.1, Student's t-test) as can be seen in Table 1 were the average measured paired differences in the initial deposition rate in the presence and absence

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