(Invited) Bacterial Identification by Using Photogate-Type Optical Sensor
Authors: Ishii Hiromu , Tanaka Sawako , Ishida Makoto , Sawada Kazuaki , Machida Katsuyuki , Nikaido Yasuhiko , Saito Mitsumasa , Yoshida Shinichi 
Published: February 2020

Sensing bacteria such as in the medical front without their cultivation is attracting a great deal of interest to avoid an outbreak of bacterial infection because definitive diagnosis takes a long time [1-3]. For the quick test in medical front, we have been developing a portable small sensor that can detect pathogenic bacteria, Legionella, by detecting their fluorescence on a chip without their cultivation [4, 5]. There are about 50 species in the genus Legionella [6]. Some of them are known to emit fluorescence when they are irradiated with ultraviolet (UV) light at around 360 nm of wavelength. Applying an optical sensor therefore can detect and identify each species of Legionella.
This work describes our recent researches on bacterial identification using a photogate-type optical sensor. Our used bacteria in this study are Legionella dumoffii (L. dumoffii) and Legionella erythra (L. erythra) which respectively emit blue and red fluorescence when they are irradiated by UV-light as shown in Fig. 1. The schematic cross section of photogate-type optical sensor fabricated in our lab. is illustrated in Fig. 2. Fluorescence emitted from Legionella entered to a depth W in the sensing area through the photogate generates photocurrent, Iout according to the following formula, Iout = qSλ/hc x (1-exp(-αW) ) x Φ0(t), q: elementary charge, S: sensing area, λ: wavelength of fluorescence, h: Planck constant, C: light velocity, α: absorption coefficient of light with wavelength of λ in Si, W: penetration depth of light with wavelength of λ, and Φ0(t): intensity of the fluorescence at time, t [7-9]. Obtaining the characteristic fluorescent wavelength, λ and the UV-photoirradiation time dependence of its photointensity, Φ0(t) enables to identify the species of Legionella.
The characteristic wavelengths were selected from the fluorescent spectra obtained using a combination of fluorescence microscope and spectrometer between 400 nm and 800 nm of wavelengths both for L. dumoffii and L. erythra. The UV-photoirradiation time dependence of the fluorescent intensities at characteristic wavelengths was then obtained both for L. dumoffii and L. erythra. For the L. dumoffii case, the fluorescent intensity at its characteristic wavelength of 470 nm rapidly decreased with the photoirradiation time, meaning a fluorescent material in the cells of L. dumoffii changes to a non-fluorescent material by UV-photoreaction. For the L. erythra case, on the other hand, the fluorescent intensity at its characteristic wavelength of 675 nm gradually increased to saturate with the photoirradiation time, meaning the production of fluorescent material began to occur when L. erythra was irradiated by the UV-light. Thus, the fluorescent behavior of Legionella under UV-photoirradiation differs largely each other.
Finally, the photocurrent generated by the fluorescence emitted from L. dumoffii and L. erythra at their characteristic wavelengths were measured by using the photogate-type optical sensor schematically shown in Fig. 2. The UV-photoirradiation time dependences of the photocurrent were found to be in good agreement with those of fluorescent intensity obtained by the combination of fluorescence microscope and spectrometer, showing that the photogate-type optical sensor is able to be used for bacterial identification instead of a bulky system of spectrometer and fluorescence microscope. This will pave the way for portable bacterial sensors for use in medical front etc.
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Figure 1