Identifying different types of microorganisms with terahertz spectroscopy

Abstract

Most microbial detection techniques require pretreatment, such as fluorescent labeling and cultivation processes. Here, we propose novel tools for classifying and identifying microorganisms such as molds, yeasts, and bacteria based on their intrinsic dielectric constants in the THz frequency range. We first measured the dielectric constant of films that consisted of a wide range of microbial species, and extracted the values for the individual microbes using the effective medium theory. The dielectric constant of the molds was 1.24-1.85, which was lower than that of bacteria ranging from 2.75-4.11. The yeasts exhibited particularly high dielectric constants reaching 5.63-5.97, which were even higher than that of water. These values were consistent with the results of low-density measurements in an aqueous environment using microfluidic metamaterials. In particular, a blue shift in the metamaterial resonance occurred for molds and bacteria, whereas the molds have higher contrast relative to bacteria in the aqueous environment. By contrast, the deposition of the yeasts induced a red shift because their dielectric constant was higher than that of water. Finally, we measured the dielectric constants of peptidoglycan and polysaccharides such as chitin, α-glucan, and β-glucans (with short and long branches), and confirmed that cell wall composition was the main cause of the observed differences in dielectric constants for different types of microorganisms.

Fig. 1.

(a) Schematic images of cell wall structures for molds, yeasts, and bacteria, including their compositions. (b) Plots of frequency-dependent dielectric constants of representative microbial films of yeast (S. cerevisiae), bacteria (E. coli), and mold (A. niger).

Fig. 2.

(a) Real part of dielectric constants at 1 THz for three different types of microbial films, including molds (6 species), bacteria (6 species), and yeasts (2 species). (b) Plot of complex dielectric constants of individual microorganisms extracted using the effective medium theory.

Fig. 3.

(a) A schematic presentation of THz metamaterials sensing of liquids containing low-density microorganisms. (b) The resonance shifts of THz metamaterials measured for low-density mold (top) and yeast (bottom) solutions with densities of 2.4 × 10⁸ and 8 × 10⁷ units/ml, respectively. (c) Plot of frequency shift as a function of number density for different species.

Fig. 4.

(a) Schematic images of molecular structures for peptidoglycan, chitin, α-glucan, and β-glucans. (b) Real (filled boxes) and imaginary (open boxes) parts of dielectric constants for peptidoglycan and polysaccharides films, measured at 1 THz.