Discovery of a biofilm electrocline using real-time 3D metabolite analysis

Abstract

Bacteria are social organisms that possess multiple pathways for sensing and responding to small molecules produced by other microbes. Most bacteria in nature exist in sessile communities called biofilms, and the ability of biofilm bacteria to sense and respond to small molecule signals and cues produced by neighboring biofilm bacteria is particularly important. To understand microbial interactions between biofilms, it is necessary to perform rapid, real-time spatial quantification of small molecules in microenvironments immediately surrounding biofilms; however, such measurements have been elusive. In this study, scanning electrochemical microscopy was used to quantify small molecules surrounding a biofilm in 3D space. Measuring concentrations of the redox-active signaling molecule pyocyanin (PYO) produced by biofilms of the bacterium Pseudomonas aeruginosa revealed a high concentration of PYO that is actively maintained in the reduced state proximal to the biofilm. This gradient results in a reduced layer of PYO that we have termed the PYO “electrocline,” a gradient of redox potential, which extends several hundred microns from the biofilm surface. We also demonstrate that the PYO electrocline is formed under electron acceptor-limiting conditions, and that growth conditions favoring formation of the PYO electrocline correlate to an increase in soluble iron. Additionally, we have taken a “reactive image” of a biofilm surface, demonstrating the rate of bacterial redox activity across a 2D surface. These studies establish methodology for spatially coordinated concentration and redox status measurements of microbe-produced small molecules and provide exciting insights into the roles these molecules play in microbial competition and nutrient acquisition.

Bacteria possess the ability to sense a wide-range of microbe-produced extracellular small molecules and respond through modulation of specific behaviors. Some extracellular small molecules serve as dedicated signals that have evolved to permit microorganisms to communicate both with self (same species or strain) and nonself cells (1). Such signals allow a bacterial community to coordinate its behavior based on local cell density and mass transfer of the surrounding environment, a process referred to as quorum sensing (QS) (2). Other small molecules serve as chemical cues that provide information to a recipient microbe, but differ from a signal in that they did not evolve explicitly for that purpose (1). Cues are chemically and functionally diverse and include primary metabolites arising from central metabolism as well as secondary metabolites. Although dedicated signals have been the focus of significant research over the last 20 y, it is clear that primary and secondary metabolites also impact the species composition and behaviors of microbial communities in vitro and in vivo (3–5). In some cases, once-presumed dedicated signals have been shown to also act as cues, suggesting that many small molecules may be multifunctional (1, 6–8).

Both bacterial signals and cues are structurally diverse but, in general, are small molecular mass (<1,000 Da) and synthesized from central metabolites. The responses to such molecules are also diverse, including but not limited to: biosynthesis of antimicrobials (9–12), production of light (13, 14), colonization and proliferation within disease sites (4, 15), and resistance to killing by the innate immune system (5). As with many microbiological studies, elucidation of the molecular details of signal/cue synthesis and response has been performed by using bacteria grown in homogenous planktonic (free-swimming) populations. Although such studies have provided detailed insight into synthesis and response of these small molecules, bacteria in nature often do not exist in high-density planktonic communities. Instead, natural bacterial populations often exist in high-density, low-cell number biofilm communities (16). A biofilm is a bacterial community attached to a surface, or to each other, and surrounded by an extracellular polymeric matrix. Biofilms are distinct from well-mixed planktonic cultures in that they are spatially heterogeneous, both in regard to chemical gradients and bacterial phenotypes (17, 18). This heterogeneity has been proposed to have important biological consequences by producing a diverse community better able to resist external stresses, a phenomenon termed the “insurance hypothesis” (19).

Numerous technological advancements have allowed for the examination of chemical gradients within biofilms, primarily through the use of in situ responsive fluorophores or microprobes (20–23). However, a fundamental question still exists regarding the nature of such gradients in bulk solution surrounding a biofilm. This question is important because these gradients are likely critical for determining how spatially separated bacteria communities sense and respond to neighboring communities. The primary challenge to these experiments is the ability to quantify and spatially resolve small molecules immediately surrounding biofilms in real-time. In a previous study (24), our laboratories have begun to address the challenge of spatial metabolite profiling through the use of scanning electrochemical microscopy (SECM) to quantify a primary metabolite produced by a bacterial biofilm in real-time. SECM has the unique ability to set the exact distance from a ultramicroelectrode sensing tip to a biological substrate through a feedback approach curve (25) and, thus, is able to measure the local concentration of redox active small molecules over a biofilm. SECM also has the ability to scan over a substrate in the x-y direction, providing a spatial concentration profile over the surface.

In this study, we used SECM to interrogate production of the QS-controlled, redox-active secondary metabolite pyocyanin (PYO) produced by biofilms of the pathogenic bacterium Pseudomonas aeruginosa. PYO was chosen for study because it is a multifunctional molecule, serving both as a QS signal (26), a potent antimicrobial in vitro and in situ (3, 27), and as a virulence factor important for P. aeruginosa pathogenesis (28, 29). Our data reveal that P. aeruginosa biofilms produce high levels of PYO that is actively maintained in the reduced state proximal to the biofilm. This results in a reduced layer of PYO that we have termed the PYO “electrocline” (i.e., a gradient of redox potential), which extends several hundred microns away from the biofilm surface. We also demonstrate that the PYO electrocline is formed under electron acceptor-limiting conditions and that growth conditions favoring formation of the PYO electrocline correlate to an increase in soluble iron.

Results

Real-Time Quantitative Detection of PYO.

We first sought to measure PYO concentrations directly above P. aeruginosa colony biofilms by using a unique biofilm SECM apparatus (Fig. 1A). For these experiments, the supernatant above the biofilm was removed and replaced with fresh medium at the beginning of the experiment to establish baseline (no PYO) SECM readings (11, 30). For PYO quantification, peak current was obtained from square wave voltammetry (Fig. S1), a unique application of SECM, and converted to PYO concentration using a calibration curve (Fig. S2). PYO was not immediately produced by newly formed P. aeruginosa biofilms; however, after only 5 h, the local (20 μm above the biofilm surface) PYO concentration was ≈2.5 μM (Fig. 1B). The delay in PYO production was also observed in planktonic cells (Fig. S3) and is not surprising because it is QS controlled.

Fig. 1.

(A) Schematic diagram of the SECM apparatus. (B) Real-time quantitative detection of PYO produced by a P. aeruginosa biofilm at 36 °C. (C) Z direction reduced and oxidized PYO profiles at 36 °C. Zero on the x axis represents the surface of biofilm. (D) Proposed schematic model of PYO electrocline formation above a biofilm. To spatially map the reduced and oxidized PYO profile, the tip was biased at −0.3 V (oxidizing potential) and −0.8 V (reducing potential), respectively.

Spatial Mapping Reveals a PYO Electrocline.

PYO can exist in a reduced or oxidized state, governed by the half reaction:

and the redox state of PYO has important implications for its biological activity (31). Indeed, reduced PYO displays significant antimicrobial activity (27) and the ability to reduce insoluble Fe³⁺ to soluble Fe²⁺ (32), whereas the oxidized form can potentially function as an alternative electron acceptor (33, 34). Thus, to understand the biological function of PYO, it would be valuable not only to measure PYO concentration, but also the PYO redox state. SECM is an excellent tool for these experiments because it allows both redox states of PYO to be assessed in real-time by biasing the SECM tip at −0.3 V (oxidizing at tip) and −0.8 V (reducing at tip). Z-direction spatial mapping of the PYO redox ratio above the P. aeruginosa biofilm revealed that the concentration of reduced PYO increased steadily as the tip approached the biofilm surface (Fig. 1C). This increase in reduced PYO correlated with a decrease in oxidized PYO at the biofilm surface (Fig. 1C). We have termed this area, beginning at the surface of the biofilm and extending to ≈400 μm above the biofilm, the PYO electrocline because the redox status of PYO transitions to a more reduced state within this region (Fig. 1D).

Because P. aeruginosa continually produces PYO during the course of the experiment, it is not possible to determine whether the PYO electrocline is due to increased production of reduced PYO or reduction of extracellular PYO. To examine this question, biofilms of P. aeruginosa Δphz1/2, a strain unable to produce phenazines because of deletion of both phz biosynthetic operons (26), were grown in the presence of a biologically relevant amount of exogenous PYO (33). Z-direction spatial mapping of the PYO redox ratio above the P. aeruginosa Δphz1/2 biofilm revealed a PYO electrocline, indicating that biosynthesis of PYO is not required for electrocline formation (Fig. 2A). The height of the electrocline extended beyond 400 μm, and similar to the PYO electrocline formed by WT P. aeruginosa, cyclic voltammograms (Fig. S4) showed that nearly all PYO was in the reduced form at a distance of 100 μm above the biofilm. Based on the observation that P. aeruginosa Δphz1/2 formed an electrocline similar to WT P. aeruginosa in the presence of exogenous PYO, we chose to use this model system for the remaining experiments because utilization of a constant level of PYO provided greater sensitivity and reliability.

Fig. 2.

(A) Z direction reduced and oxidized PYO profiles above a P. aeruginosa Δphz1/2 biofilm at 25 °C. (B) Z direction reduced PYO profiles above a P. aeruginosa Δphz1/2 biofilm in LB-MOPS (no addition) with 100 mM NO₃⁻ (excess nitrate), 100 μM Fe³⁺ (excess Fe³⁺), or with heat killed cells (dead cells). (C) Z direction reduced PYO profiles above P. aeruginosa napA and fbcC mutants. (D) Measurement of Fe²⁺ in bulk solution in LB-MOPS (no cells), P. aeruginosa Δphz1/2 mutant biofilms in LB-MOPS, P. aeruginosa biofilms in LB-MOPS with 100 mM NO₃⁻, P. aeruginosa fbcC⁻ mutant in LB-MOPS, P. aeruginosa napA⁻ mutant in LB-MOPS with 100 mM NO₃- (napA⁻ + NO₃⁻), and a heat-killed P. aeruginosa biofilm (dead cells). In each case, 200 μM oxidized PYO was added to MOPS-LB growth medium. For A–C, the y axis was normalized to the concentration of oxidized PYO (200 μM) added to each biofilm.

It is important to note that for all PYO electroclines, a drop in PYO concentration is observed as the tip approaches closer than 60 μm of the biofilm surface. The reason for this drop is because when the tip is distal from the biofilm (i.e., >2,000 μm), the SECM instrument is operating in generation-collection mode but when the tip is within the electrocline, it switches to feedback mode. As a result, when the tip is proximal to the biofilm surface (i.e., within 60 μm), the response at the tip depends on how fast oxidized PYO at the tip is being reduced back by the biofilm. In these experiments, the rate of PYO reduction by the biofilm is not sufficient to exceed the rate of oxidation of PYO by the tip, hence the current drops at 60 μm or closer to the biofilm.

Determining the Mechanism of PYO Electrocline Development.

Because PYO is proposed to serve as an alternative electron acceptor (33), we hypothesized that electrocline formation was due to the flow of electrons from carbon source oxidation to PYO. If this hypothesis is true, we predict that addition of another extracellular electron acceptor such as NO₃⁻ would reduce flux of electrons to PYO, thereby resulting in collapse of the PYO electrocline. To test this prediction, the impact of NO₃⁻ on the PYO electrocline produced by P. aeruginosa Δphz1/2 in the presence of exogenous PYO was assessed. As expected, in the presence of excess NO₃⁻, the PYO electrocline collapsed (Fig. 2B). These data suggest that the presence of alternative electron acceptors is a critical parameter affecting the size of the PYO electrocline.

To definitively assess whether the collapse of the PYO electrocline was due to diversion of electron flux away from PYO to NO₃⁻, the impact of NO₃⁻ on the PYO electrocline was assessed by using biofilms of a P. aeruginosa napA mutant. NapA is the periplasmic NO₃⁻ reductase important for reduction of NO₃⁻ under aerobic conditions (35); thus, the P. aeruginosa napA mutant is unable to divert electrons to NO₃⁻ under aerobic conditions. As expected, NO₃⁻ had no impact on the PYO electrocline in the napA mutant (Fig. 2C). In fact, the ratio of reduced/oxidized PYO within the electrocline almost doubled for the napA mutant biofilm grown in the presence or absence of NO₃⁻ (Fig. 2C). This increase is likely due to reduction of trace amounts of NO₃⁻ present in our complex growth medium (35) by the WT P. aeruginosa biofilm. Collectively, these results indicate that the magnitude of the PYO electrocline is impacted by the presence of alternative electron acceptors. As a control for these experiments, the PYO electrocline produced by a P. aeruginosa fbcC mutant was also examined. FbcC is a subunit of cytochrome bc1, which has been reported to be the site at which PYO accepts electrons from aerobically growing P. aeruginosa (36). As expected for the P. aeruginosa fbcC mutant biofilm, the electrocline was almost completely abolished, confirming that this respiratory cytochrome is important for PYO electrocline formation (Fig. 2C).

Electrocline Formation Correlates with Fe²⁺ Enrichment.

In addition to alternative electron acceptors, we predicted that exogenous molecules that actively facilitate reoxidation of PYO will also diminish the PYO electrocline. To test this prediction, the PYO electrocline was examined after addition of excess Fe³⁺ (100 μM, ≈10× the concentration of the LB-MOPS growth medium). Fe³⁺ has been shown to oxidize PYO under aerobic conditions (32, 34). As expected, the PYO electrocline height decreased by 100 μm in the presence of excess Fe³⁺; however, the ratio of reduced/oxidized PYO within the electrocline remained the same. Based on these data, we hypothesized that a PYO electrocline may enrich the local environment around a biofilm with bioavailable Fe²⁺ even in the presence of oxygen. Using a Ferrozine-based Fe(II) assay (37), we measured Fe²⁺ from media exposed to colony biofilms for 1 h. Interestingly, Fe²⁺ levels increase under PYO electrocline forming conditions (Fig. 2D), whereas conditions that collapse the electrocline (NO₃⁻ addition or inactivation of cytochrome bc1) abrogate this increase in Fe²⁺ levels. Together, these data support a model in which metabolically active cells generate a reduced PYO electrocline, which subsequently serves as an electron donor to increase the ratio of Fe²⁺/Fe³⁺.

Electrochemical Imaging of a P. aeruginosa Biofilm.

In addition to detailed spatial characterization of the PYO electrocline, the ability of P. aeruginosa biofilms to rapidly reduce oxidized PYO generated at the ultramicroelectrode tip indicated that SECM could also be used to generate a redox activity map of the biofilm surface. For these experiments, an SECM image of a 1-mm diameter P. aeruginosa Δphz1/2 biofilm was performed at a constant height above the biofilm in the presence of exogenous PYO (Fig. 3). The tip was held at −0.3 V (PYO oxidizing potential) while scanning over the biofilm at a distance of 20–30 μm from the biofilm surface maintaining a constant speed of 8 μm/0.02 s. The false color contrast of the electrochemical image shows reactivity mapping of the biofilm as it reduces PYO that was oxidized by the SECM tip just above the biofilm (Fig. 3). The oxidized PYO diffused in the gap (tip-biofilm) where it was bacterially reduced resulting in higher current (red color) recorded by the tip (Fig. 3). Thus, the image can be seen as mapping of electron transfer from the biofilm to PYO, thereby providing a real-time examination of the PYO reducing ability of the biofilm. Biofilm redox mapping is an intriguing application for SECM and provides proof of principle that the redox activity of a biofilm, a strong indicator of metabolic activity, can be determined spatially in real-time.

Fig. 3.

A constant height SECM reactive image: of a 1-mm P. aeruginosa biofilm. Colors represent the current measured by SECM and correlates to the rate of PYO reduction by the biofilm from low (blue) to high current (red). A 2D scan was acquired by moving the SECM tip in the x and y axis at a fixed height (20–30 μm) for the tip-substrate (biofilm) distance. The SECM tip was held at −0.3 V to oxidize PYO. Solution used: 200 μM oxidized PYO in MOPS-LB. Dotted line indicates approximate size and position of the biofilm.

Discussion

Bacteria in nature often reside in small, high-density sessile biofilms, and these small populations exhibit high-density signal-dependent phenotypes (38). Thus, the ability to determine small molecule concentrations in 3D space surrounding a biofilm in real-time will not only provide important insight into signal/cue levels proximal to bacterial clusters or biofilms, but will also provide new avenues for probing how biofilm spatial structure impacts small molecule perception and response by neighboring microbial communities.

Our laboratories previously adapted SECM to acquire quantitative data detailing metabolite concentration heterogeneity surrounding a biofilm (24). These initial studies led us to expand the utility of SECM not only to quantify the concentration of the signal/cue PYO, but also to noninvasively determine the redox status of biofilm-produced small molecules in 3D space. In this study, we demonstrate the utility of SECM to determine the concentration (Fig. 1B) and redox state of PYO (Fig. 1C), in real time, surrounding a biofilm—properties unable to be precisely quantified by using other technologies. We also developed an application of square wave voltammetry (SWV) to detect PYO-dependent current as low as ≈0.7 pA (equivalent to ≈0.6 μM PYO) (Fig. S1). Using spatial mapping in the z axis above the biofilm surface, we discovered the presence of a PYO electrocline proximal to the biofilm and demonstrate that this electrocline extends over 400 μm from the biofilm surface (Figs. 1C and 2A).

To explain electrocline formation, we propose intracellular (Fig. 4A) and extracellular models (Fig. 4B), which occur in unison. Fig. 4A shows electron flow from carbon source oxidation to extracellular oxygen via NAD⁺/NADH, the ubiquinone pool, and cytochrome bc1. When excess NO₃⁻ is present in the extracellular environment, electron flow is diverted from ubiquinone to reduce NO₃⁻ to NO₂⁻ through the NapA nitrate reductase (Fig. 4A). When PYO is present, electron flow is diverted from cytochrome bc1 to PYO instead of oxygen (Fig. 4A). Hence, we hypothesized that electrocline formation would be diminished by obstructing the electron flow either through cytochrome bc1 or by diverting electrons from the ubiquinone pool to the NapA nitrate reductase (Fig. 2 B and C). In both cases, diversion of electron flow to PYO led to smaller PYO electroclines (Fig. 2 B and C). In addition, an increased electrocline was observed for a napA mutant even in the presence of NO₃⁻, likely due to additional electron flow to cytochrome bc1 as a result of electron flow divergence from inactivated NapA.

Fig. 4.

(A) A schematic diagram of the proposed intracellular model of electron flow from carbon source oxidation to extracellular oxygen, PYO or NO₃⁻. Dotted arrows indicate abbreviated or unknown electron transport steps; solid arrows indicate direct electron transfer. (B) A schematic diagram of extracellular electron flow. Kf, heterogeneous electron transfer from biofilm to solution; k1, rate of reaction between reduced PYO and molecular oxygen; k2, rate of reaction between reduced PYO and Fe³⁺; k3, rate of reaction between Fe²⁺ and oxygen. (C) A schematic representation of the modulation of a PYO electrocline in response to different extracellular conditions.

Modulation of the PYO electrocline height also depends on the concentration and redox state of iron (Fig. 4B). According to this model, the amount of reduced/oxidized PYO solely depends on kf or the rate of electron transfer from the biofilm to the outside solution, whereas the height of an electrocline above a biofilm depends on the kinetic balance of k1, k2, and k3 (see Fig. 4B for details). To maintain a PYO electrocline, the kinetics of electron transfer from the biofilm to PYO (kf) must be greater than the kinetics of electron transfer from PYO to oxygen (k1) and Fe³⁺ (k2 and k3). Reduced PYO reacts more readily with molecular oxygen than Fe³⁺ (34); thus, oxygen concentration proximal (≈400 μm) to the biofilm decreases because of respiration and PYO-dependent oxygen reduction. Decrease in oxygen concentration has been demonstrated by PYO cyclic voltammogram data at 100 and 1,500 μm above the biofilm (Fig. S4). Despite the presence of oxygen, PYO can also reduce Fe³⁺to Fe²⁺ at pH 7 (32, 34). Therefore, we hypothesized that the addition of Fe³⁺ to the solution might play a role in modulating electrocline height by affecting k2. Fig. 2B supports this hypothesis showing that a decrease in height of the PYO electrocline is observed upon increasing Fe³⁺ concentration. Because electrocline formation correlates with an increase in soluble Fe²⁺ (Fig. 2D), we propose that an additional role for PYO is to scavenge iron near the biofilm surface by reduction of Fe³⁺ via a PYO electrocline. Modulation of electrocline height based on Fe³⁺ concentration could serve as a mechanism to expand an Fe-reducing electrocline when Fe³⁺concentrations are low. This ability of PYO to enrich Fe²⁺ in areas immediately adjacent to the biofilm may provide P. aeruginosa with a means of obtaining soluble Fe²⁺ instead of relying on energy intensive siderophore-based Fe³⁺ transport mechanisms. Considering that iron is limiting in numerous environments including the human body, the role of the PYO electrocline in iron acquisition may have implications in disease. Based on recent studies, these observations may also extend to other P. aeruginosa phenazines (39).

The measurement of metabolite concentrations in 3D space above a bacterial biofilm has yielded the discovery of a redox electrocline generated by electron transport to a soluble small molecule. In addition to characterizing electrocline formation, we have also obtained a “reactive image” of a bacterial biofilm (Fig. 3), demonstrating the rate of bacterial redox activity across a 2D surface. These findings demonstrate a paradigm for metabolite measurement and present the possibility that electroclines and potentially other chemoclines produced by biofilms might serve important biological roles such as nutrient acquisition.

Materials and Methods

Materials.

All chemicals were used as purchased without any further purification. PYO and potassium ferrocyanide were purchased from Sigma Aldrich. Ten-micrometer Pt wire was purchased from Good fellow. Deionized water (18 MΩ) was used to prepare all solutions.

Bacteria Culture and Biofilm Sample Preparation.

P. aeruginosa PA14 and isogenic mutants were obtained from the PA14 nonredundant transposon library (40). P. aeruginosa Δphz1/2 was generated (26) (See Table S1 for details). Growth medium for all SECM experiments was LB-MOPS at pH 7.2. LB-MOPS consists of a 1/1 (vol/vol) mixture of Luria Bertani Broth (5 g/L yeast extract, 10 g/L tryptone, and 10 g/L NaCl) and MOPS buffer (41). In select experiments, cell supernatants were added to biofilms. To generate supernatants, the P. aeruginosa Δphz1/2 was inoculated into 3 mL of LB-MOPS medium and grown overnight. The culture was then diluted in fresh LB-MOPS to an optical density at 600 nm (OD₆₀₀) of 1, and centrifuged in a 15-mL tube at 7,000 × g for 10 min at 25 °C. Supernatants were decanted and stored at 4 °C until used.

For biofilm formation, P. aeruginosa was grown planktonically overnight and resuspended to an OD₆₀₀ of 1. Five microliters of this suspension was added to the surface of a 25-mm polycarbonate membrane (Whatman no. 110606). Before inoculation, membranes were placed on a Petri dish containing LB agar. Membranes were then overlayed with a “stencil” made of Sylgard 184 PDMS (Dow Corning). PDMS stencils were fabricated by casting ≈1.5 mL of PDMS into a 35-mm Petri dish with a 5-mm OD tube positioned flush with the center bottom of the well. Once polymerized, the stencil was removed from the dish and the tubing was displaced, leaving a 5-mm opening for biofilms to form. After inoculation, biofilms were allowed to form overnight at 37 °C. Biofilms were then moved to room temperature where membranes were gently separated from the PDMS overlay. Films were transferred to the SECM apparatus and overlayed with ≈600 μL of LB-MOPS.

Electrochemical Measurements.

All of the electrochemical measurements were acquired by using SECM (CHI 920C; CH Instruments). A 10-μm Pt electrode was used as the SECM tip. Details about SECM tip fabrication could be found elsewhere (25). Tungsten wire (0.5 mm) and Hg/Hg₂SO₄ (Radiometer) was used as counter and reference electrode, respectively. All of the potentials henceforth mentioned here were referred vs. Hg/Hg₂SO₄ (+0.64 V vs. NHE).

Real-Time Quantitative Detection of PYO Using SECM.

For details on the SECM experimental setup, see Fig. S5 and Fig. S6. The biofilm was gently washed twice with 500 μL of LB-MOPS. Tip-substrate (biofilm) distance was then fixed at 20 μm with the aid of a negative feedback approach curve by using 1 mM ferrocyanide as a redox mediator. Cells were washed three times with LB-MOPS to remove any ferrocyanide in the solution. Finally, 600 μL of supernatant from P. aeruginosa Δphz1/2 was added to the chamber. Supernatant was used to mimic the initial supernatant above the biofilm that had to be removed after the feedback approach curve. Background current using SWV was then measured above the biofilm by an SECM tip. Once the background current was stabilized, the temperature of the water bath surrounding the Petri dish holding each sample was increased to 36 °C by using a copper heating plate connected to a Variac power supply. The temperature was constantly monitored by a thermocouple located outside the Petri dish. The time was set to zero when the temperature reached 36 °C. LB-MOPS (200 μL) was added to the chamber after 1.5 h to compensate for water loss due to evaporation. SWV was recorded at every 10 min until 5 h in the potential range of −0.35 to −0.85 V.

Once the background was recorded, PYO was added to the LB-MOPS covering the biofilm to a biologically relevant final concentration of 200 μM (33). After 30 min at 25 °C, the SECM tip was biased at −0.3 V (oxidizing reduced PYO at tip) and then moved in the z direction toward the biofilm from 2,000 μm away at the speed of 5 μm/s. Similarly, the tip was biased at −0.8 V (reducing oxidized PYO at tip) to map the z direction oxidized PYO profile above the biofilm.

Electrochemical Imaging of a Biofilm Using SECM.

A 1-mm P. aeruginosa biofilm was used for SECM electrochemical imaging. The biofilm was grown and assembled on the SECM stage as described. The SECM stage tilt was fixed by performing a negative feedback approach curve using ferrocyanide as a redox mediator. PYO was then added to 500 μL of LB-MOPS to prepare a 200 μM PYO solution. A feedback mode SECM image of a wild-type biofilm was acquired at 25 °C by biasing the tip at −0.3 V. The tip-substrate (biofilm) distance was kept constant at 20 μm (approximately). Scanning speed was maintained at 8 μm/0.02 s.

Determination of Fe(II) in the Presence of PYO.

Fe²⁺ measurements of biofilm supernatants were performed as described by Viollier et al. (37).