Biotechnology • Nov. 12, 2022, 2:47 p.m.

How bacteria use electric fields to reach surfaces

How bacteria use electric fields to reach surfaces
  • In particular, the way the microbial cells approach a solid surface in the presence of an electric field remains unclear. It seems tacitly accepted that microbial cells reach the surface of polarised electrodes by random motion and that the electrode impacts biofilm development only during the phase of cell growth on the surface.

Now, It is becoming increasingly obvious that ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">microbial biofilms have a natural tendency to exchange electrons with their support when they grow on a conductive surface. For us, it is around two decades, and microbial electroactive biofilms have been the source of the so-called microbial electrochemical technologies with a huge number of possible application fields such as the production of electrical energy or hydrogen effluent treatments and metal recovery for example. Electroactive biofilms have also been essential roles in ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">microbiologically influenced corrosion. Besides all it is the field of conventional ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">electrochemical processes, more and more cases of inter-species microbial ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">electron transfers are being discovered to be mediated by conductive solids.

Extensive it is fundamental research has led to great advances in our understanding of extracellular electron transfers inside biofilms and between biofilms and electrodes. Nevertheless, we still know very little about the early formation of electroactive biofilms. In particular, the way the ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">microbial cells approach a solid surface in the presence of an electric field remains unclear. It seems tacitly accepted that microbial cells reach the surface of polarised electrodes by random motion and that the electrode impacts ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">biofilm development only during the phase of cell growth on the surface. The possible impact of the electric field on the approach phase, before the cells reach the ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">electrode surface, is rarely evoked.

A few studies have pr osed that ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">bacterial cells can migrate in the electric field as colloids that are uniformly charged or behave as dipoles. In these cases, we have seen bacterial cells have been assumed to migrate passively, without the involvement of a specific sensing process. In contrast, a few reports have postulated that bacteria might detect local electric fields through chemotaxis, by sensing the ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">concentration gradient of redox compounds. Some reports have described the inhibition of ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">cell motility by an applied current, without evoking possible mechanisms. Others have observed that the swimming speed of ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">Shewanella species increases in the vicinity of a polarised electrode. This effect is different from a passive migration process because the swimming speed was enhanced in all directions of motion rather than in the direction of the electric field only. Nevertheless, the effect was conditioned by the cells’ ability to exchange electrons with the electrode and therefore required initial contact with the electrode to be triggered. Apart from these few leads, the approach phase of bacterial cells towards a polarised surface is still very poorly documented.

In contrast, ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">eukaryotic cells have been widely demonstrated to develop sophisticated strategies to sense an electric field and use it to orient their motion. This property, called electrotaxis, has been shown to play key roles in essential ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">physiological processes such as tissue development wound healing, and organ formation. The multiplicity of evidence of electrotaxis in eukaryotic cells prompted us to look for a possible bacterial electrotaxis-related strategy in the formation of electroactive biofilms.

  • The main purpose of this study was to move forward in identifying and deciphering possible microbial electrotaxis by using a complex multi-species consortium.
  • A new type of electrochemical set-up was designed to discriminate the impact of the electric field from that of the polarised electrode. Imposing an electric field through a solution requires electrochemical reactions to occur on the surface of the electrodes.
  • It is consequently difficult to discriminate the impact of the electric field from that of the polarised electrode surface in a conventional electrochemical reactor because the electrodes that create the electric field act also as the electron source or sink for the electroactive species that reach them. The new experimental device and procedure described here made it possible to separate the two effects.
  • It is thus evidenced here that an electric field enhances the formation of electroactive biofilms on solid surfaces. The electric field affects the ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">biofilm formation through indirect action due to the ionic gradient created at the interface.
  • This analysis leads us to consider the capacity of microbial cells to detect interfacial ionic gradients as a major motor of surface colonization.
  • The results described here indicate that controlling local ionic gradients may be an efficient way to enhance or mitigate the formation of biofilms on solid surfaces. Obvious applications can be foreseen in all domains of microbial electrochemical technologies.
  • Beyond that, and in a more speculative way, considering the ubiquitous presence of endogenous electric fields in living organisms and the huge number of interfaces that compose them, the results described here may also open up new research paths in the domains of biomedicine and physiology.

Materials and methods

Inoculum and media

The inoculum solution was garden compost leachate prepared by filtering a mix of 1.5 ​L of garden compost (Lombricompost, Or Brun) and 2.25 ​L of tap water containing 60 ​mM KCl through a large-mesh cloth. The synthetic medium was a phosphate buffer solution supplemented or not with 60 ​mM of KCl. The phosphate buffer solution was obtained by mixing a solution of Na2HPO4 (9.47 ​g ​L-1) and a solution of KH2PO4 (9.08 ​g ​L-1) with a ratio of respectively 61-39% (v/v) to a final pH of 7.0. When indicated in the protocol, the synthetic medium was supplemented with 20 ​mM of ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">sodium acetate and with 10 ​mL ​L-1 of macronutrients solution, 1 ​mL ​L-1 of micronutrients solution, and 1 ​mL ​L-1 of vitamin solution.

 Electrochemical set-up:

The electrochemical reactors were made by assembling three modified Schott glass bottles (diameter 101 ​mm, height 152 ​mm, Duran). The central compartment was separated from the two end compartments with a system constituted of a flat ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">gasket cut from a rubber foil with an outer diameter of 5 ​cm and an inner diameter of 2 ​cm, a membrane with 0.2-μm cut-off threshold (outer diameter 5 ​cm, Pall SAS France), a carbon cloth electrode (outer diameter 3 ​cm, PaxiTech SAS France) electrically connected with a platinum wire, and a second rubber gasket identical to the first one. Such a separation system was arranged between the central compartment and each of the two end compartments, with the carbon cloth electrode facing the central compartment. According to the inner diameter of the flat rubber gasket (2 ​cm), the surface area of the carbon cloth electrodes exposed to the solution was 3.1 ​cm2. Sealing was achieved by pressing the system with an external frame equipped with four bolts.

 Two metallic electrodes were electrically connected with a 2-mm-diameter screwed titanium wire. The electric field was established between these two metallic electrodes by applying a voltage of 8 ​V during 3 ​h (or 2 ​h when indicated) between the DSA used as the anode and the SS electrode used as the cathode, which was separated by around 27 ​cm.

When necessary, a saturated calomel reference electrode (SCE, Radiometer, +0.241 ​V vs SHE) and a platinum grid used as the counter electrode were introduced in the central compartment and each of the two carbon cloth electrodes was individually polarised as a working electrode at -0.20 ​V/SCE with a multi-channel ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">potentiostat (N-stat set-up, ScienceDirect's AI-generated Topic Pages" class="topic-link" style="margin: 0px; padding: 0px; text-decoration-line: underline; text-decoration-thickness: 1px; text-decoration-color: rgb(46, 46, 46); color: rgb(46, 46, 46); word-break: break-word; text-underline-offset: 1px;">Biologic SA, France).

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Journal Reference: Science direct