Search

Bacterial cells possessing a certain zeta potential are immobilized by electrochemical deposition within conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy). These conducting polymers serve as a biocompatible matrix for trapping bacteria on an indium-tin-oxide (ITO)-coated glass substrate. The biological functions of bacteria were not affected by the chemical structure and electrical conductivity of the matrix. The viability of the bacteria on the ITO glass was monitored by dark-field microscopy. The cell density of Escherichia coli increased logarithmically during incubation in nutrient broth medium, leading to definitive formation of a biofilm on PPy. The facultative E. coli anaerobe sustains metabolism under aerobic and anaerobic conditions, but proliferates more extensively in the presence of oxygen. The conducting PPy film also facilitates electrochemical evaluation of the respiratory activity of bacterial cells and establishes that facultative anaerobic and aerobic bacteria exhibit similar respiratory activities under aerobic conditions.

In this study, electrochemical detection of viable bacterial cells was performed using a tetrazolium salt, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), which was converted to an insoluble and redox active formazan compound in viable microbial cells. The insolubility of this formazan was effectively exploited as a surface-confined redox event. An indium-tin-oxide electrode was applied to a microbial suspension that had been incubated with MTT and was heated to dry for the extraction and adsorption of formazan. Drying led to the appearance of a distinctive voltammetric oxidation peak at +0.1 V vs Ag|AgCl, the magnitude of which was successfully correlated to the number of viable microbes in the suspension. Thus, the electrochemical detection of formazan was effectively coupled with the thermal lysis of microbes. It is also noteworthy that this lysis-adsorption technique was highly selective to the hydrophobic formazan molecule due to the removal of hydrophilic cell components during equilibration in a phosphate buffer before voltammetric measurement. This technique was capable of detecting microbes above 2.8 × 10 1 CFU mL -1 and required only a 1 h incubation. The results of this study indicate that the sensitivity of the present technique is up to 10 000-fold higher than that of MTT colorimetry. The higher sensitivity was mainly ascribed to the concentration of the microbially produced formazan on the electrode by thorough desiccation of the bacterial suspension.

Real-time evaluation of bacterial viability is important for various purposes such as hygiene management, development of antibacterial agents, and effective utilization of bacterial resources. Here, we demonstrate a simple procedure for evaluating bacterial viability using gold nanoparticles (Au NPs). The color of bacterial suspensions containing Au NPs strongly depended on the bacterial viability. We found that the dispersion state of Au NPs affected the color of the suspension, based on the interaction of Au NPs with substances secreted by the bacteria. This color change was easily recognized with the naked eye, and viability was accurately determined by measuring the absorbance at a specific wavelength. This method was applicable to various bacterial species, regardless of whether they were Gram-positive or Gram-negative.

Nanometer-sized composite particles, which consisted of gold nanoparticles encapsulated by an N-isopropylacrylamide copolymer, were successfully synthesized using a one-step process. Shape complementary cavities of the O157-antigen were formed on the composite utilizing temperature-dependent affinity changes of the copolymer. The composite bound to enterohemorrhagic Escherichia coli (E. coli) O157 at 298 K and enhanced light-scattering intensity of the cell due to the optical properties of the gold nanoparticles. Moreover, the composite showed excellent selectivity (>15) against other types of E. coli such as O26 and O Rough. Recognition of the O157-antigen ceased upon heating to 313 K but was restored upon cooling to 298 K. During repeated temperature cycling around the phase transition temperature of the copolymer (305 K), the composite reproducibly showed recognition behavior at 298 K. The binding ability of the composite could be switched reversibly. Therefore, it was concluded that the molecular structure of the O157-antigen was memorized by the composite, rather than being molded into it. This technique is applicable not only for the detection of a target bacterium but also for an identification of new bacterial threats by the simple formation of the specific antigen-imprinted composite.