Sub-THz Vibrational Spectroscopy of Bacterial Cells and Molecular Components

Abstract

In this work, sub-terahertz (THz) spectroscopy is applied to characterize lyophilized and in vitro cultured bacterial cells of non-pathogenic species of Escherichia coli (E. coli) and Bacillus subtilis (BG), spores of BG and DNA fro m E. coli. One of the goals of this research is to demonstrate that Fourier Transform (FT) spectroscopy in the frequency region of 10-25 cm -1 is sensitive enough to reveal characteristic spectral features fro m bio-cells and spores in different environment, to verify the d ifferences between species, and to show the response of spores to vacuum and response of cultured cells to heat. The experimental technique was significantly improved for sensitivity and reliab ility. Observed spectra taken with a spectral resolution of 0.25 cm -1 using FT spectrometer with a detector operating at 1.7 K are rich in well resolved features having spectral widths of ~0.5-1 cm -1 . The reproducibility of experimental results was verified and confirmed. Measured spectra from E. coli DNA and from the entire cell have many similarit ies, thus demonstrating that the cellu lar co mponents might contribute to the v ibrational spectrum of the cell. The results of this work confirm that observed spectroscopic features are caused by fundamental physical mechanism of interaction between THz rad iation and biological macro-mo lecules. Particu larly, the analysis of results indicates that the spectroscopic signatures of microorganis ms originate fro m the co mbination of low frequency vibrational modes or group of modes at close frequencies (vibrational bands) within mo lecular co mponents of bacterial cells/spores, with the significant contribution from the DNA. The significance of this study is justified by necessity for a fast and effective, label free and reagent free optical technology to protect against environmental and other biological threats, as well as for general medical research. The obtained results show that THz v ibrational spectroscopy promises to add quantitative genetic information to the characteristic signatures of biological objects, increasing the detection accuracy and selectivity when appropriate spectral resolution is used.

Biomedical Applications of Terahertz Spectroscopy and Imaging

“…For instance, spores of Bacillus species [35], Escherichia coli, and Bacillus subtilis [36] have different absorption spectra. Additionally, the absorption differences between thermally treated and untreated cells of the same species can be used to assess the living state of bacteria [36].…”

Label-free protein detection using terahertz time-domain spectroscopy

“…Firstly, the content of free water [35,36], which strongly absorbs THz radiation; in solutions, with variation of concentration, the amount of free water that volatilizes naturally and artificially cannot be controlled. Secondly, during the drying process, the spatial conformations are subject to change due to the hot-air blow and the hydration water loss [37], because of the sizes of the pores, which are about three orders magnitude larger than the size of a protein molecule, in nitrocellulose membranes. Thirdly, a part of the measured proteins may have degraded into smaller fragments which are unstable.…”

“…Biological molecular vibrations and rotations involve intra/inter molecular weak hydrogen bonds, van der Waals forces, conformational changes, non-bonded hydrophobic and hydrophilic interactions [17,18]. Moreover, THz radiation excites the aforementioned vibrations and rotations that can be revealed by refractive index spectrum and absorption (or transmission) spectrum of biological molecules by their individual symmetries and structures in the THz frequency range [19,20]. As an emerging field to examine biomolecular structure and to characterize absorption properties of biological materials, THz spectroscopy technology makes itself an appropriate and effective candidate to identify structures and physical properties of macromolecules [21].…”

Sub-terahertz Vibrational Spectroscopy with High Resolution for Biological Molecules and Cells Identification

“…Corresponding values for the dissipation factor, γ and the spectral line width, W, which are reciprocal to τ are between 6 cm -1 and 0.01 cm -1 . Values of W above 1 cm -1 would result in structure less sub-THz spectra, since resolution of vibrational resonances would not be possible when there is a high density of low intensity vibrational modes [10].…”

“…Until recently, a Bruker Fourier transform (FT) transmission spectroscopy instrument (IFS66v) provided the most detailed sub-THz vibrational spectral signatures for biological molecules [5,10,25,26], but had only a moderate spectral resolution of 0.25 cm A number of instruments that are based on time domain or photomixing technologies have recently been introduced for producing and measuring THz radiation, however most of them do not have the sensitivity, or spectral and spatial resolution required for biological molecules and cells characterization [4,5].…”