Voltammetric Detection and Profiling of Isoprenoid Quinones Hydrophobically Transferred From Bacterial Cells

Abstract

We have developed a novel bacterial detection technique by desiccating a bacterial suspension deposited on an electrode. It was also found that the use of an indium-tin-oxide (ITO) electrode dramatically improved the resolution of the voltammogram, allowing us to observe two pairs of redox peaks, each assigned to the adsorption of isoprenoid ubiquinone (UQn) and menaquinone (MKn), which were present in the bacterial cell envelopes, giving midpeak potentials of -0.015 and -0.25 V versus Ag|AgCl|saturated KCl| at pH 7.0, respectively. Most of the microorganisms classified in both the Gram-negative and -positive bacteria gave well-defined redox peaks, demonstrating that this procedure made the detection of the quinones possible without solvent extraction. It has been demonstrated that the present technique can be used not only for the detection of bacteria, but also for profiling of the isoprenoid quinones, which play important roles in electron and proton transfer in microorganisms. In this respect, the present technique provides a much more straightforward way than the solvent extraction in that one sample can be prepared in 1 min by heat evaporation of a suspension containing the targeted bacteria, which has been applied on the ITO electrode.

Electrochemical Investigation of Isoprenoid Quinone Productions byShewanella oneidensisMR-1 Detected by Its Destructive Adsorption on an Indium-Tin-Oxide Electrode

“…S2 a), in comparison to the use of other electrode materials, such as Au and glassy carbon. 21 The hydrophobicity forced these quinones into adsorption on the electrode during the desiccation, and, indeed, yielded symmetric CV peaks, which were indicative of the surface-confined redox events, as shown in Fig. 4.…”

“…4. 21 It is noteworthy that the absence of redox signals from other substances, such as redox proteins, allowed us to exclusively obtain information about the respiratory quinones. It should also be noted that the in-situ heating of microbes on an electrode offers a new and straightforward electrochemical assay technique for the investigation of hydrophobic materials in microbial cells.…”

“…(Theoretically, the peak potential of UQn is affected by its adsorption energy, but the variation in the n value from 8 to 10 would not cause a significant change in the potential. 21 ) Due to unavailability of the MMK compound, the R f −0.66 band in the anaerobic culture was analyzed by NMR and successfully assigned to MMK7 (Fig. S3b).…”

“…Purification procedures of the microbe were reported in our previous study. 21 The density of S. oneidensis was determined by its OD 600 value, which was calibrated to the count number on 3 M Petrifilm (1 OD 600 ≡ 1.76 × 10 8 CFU mL −1 ). The culture was made in Nutrient Broth (NB; EIKEN CHEMI-CAL Co. Ltd., Japan) in an airtight plastic tube.…”

“…We have evaluated this thermal-lysis adsorption technique for more than 10 different species of Gram-negative and positive bacteria and confirmed well-defined adsorption cyclic voltammograms (CVs) for most of them. 21 In this article, the production of the isoprenoid quinones in S. oneidensis has been studied by this technique. …”