Optical Methods for Label-Free Detection of Bacteria

Abstract

Pathogenic bacteria are the leading causes of food-borne and water-borne infections, and one of the most serious public threats. Traditional bacterial detection techniques, including plate culture, polymerase chain reaction, and enzyme-linked immunosorbent assay are time-consuming, while hindering precise therapy initiation. Thus, rapid detection of bacteria is of vital clinical importance in reducing the misuse of antibiotics. Among the most recently developed methods, the label-free optical approach is one of the most promising methods that is able to address this challenge due to its rapidity, simplicity, and relatively low-cost. This paper reviews optical methods such as surface-enhanced Raman scattering spectroscopy, surface plasmon resonance, and dark-field microscopic imaging techniques for the rapid detection of pathogenic bacteria in a label-free manner. The advantages and disadvantages of these label-free technologies for bacterial detection are summarized in order to promote their application for rapid bacterial detection in source-limited environments and for drug resistance assessments.

Keywords: Raman spectroscopy; bacteria detection; dark-field microscopy; label-free; rapid detection; surface plasmon resonance.

Figure 1

Schematic diagrams of (a) SPR optical system and (b) SPR microscopy.

Figure 2

Schematic diagram of the rapid antimicrobial susceptibility test at single bacteria level using SPR microscopy [51]. Adapted with permission from Ref. [51]. Copyright © 2015 American Chemical Society.

Figure 3

Schematic detection principle of E. coli hydrogenated amorphous silicon a-Si:H surface modified with anti-fimbrial antibodies against the major pilin protein fimA. (a) Surface structures of E. coli expressing fimA selectively captured and positively charged Au-NRs incubated with E. coli for SERS sensing. (b) Anti-fimbriae modified array, optical imaging of spots after interaction with E. coli and SERS spectra after capturing bacteria [97]. Adapted with permission from Ref. [97]. Copyright © 2020 Elsevier B.V.

Figure 4

Schematic and detection principle of GNP/monolith modified substrate for the capture of E. coli. (a) Cross-sectional view of E. coli captured on gold nanoparticles modified substrates. (b) SERS enhancement factor of porous substrate functionalized with 40 nm gold nanoparticles simulated by FDTD. (c) In the simulation, the geometry of the model is reduced to two hemispheres coated with 40 nm spherical gold nanoparticles, separated by 10 nm; the electric field intensity distributions in x-y plane and y-z plane of gold on porous monolithic substrate excited by 633 nm laser are calculated. (d) SERS spectra of 40 nm gold nanoparticles/substrate functionalized with cysteamine [98]. Adapted with permission from Ref. [98]. Copyright © 2015 Elsevier B.V.

Figure 5

Schematic diagram of counting E. coli under dark-field, using antibody functionalization of MNP to form a gold ring structure around E. coli. (a) MNP probe was obtained by culture of E. coli antibody onto MNP. E. coli samples are first mixed with MNP probes to form probe-E. coli complexes. (b)The complex of E. coli and MNP probes was separated by a magnet and then counted under a dark-field microscope. [119]. Adapted with permission from Ref. [119]. Copyright © 2018 The Author(s).

Figure 6

Schematic of detection of E. coli with dark-field microscopy. (a) Samples containing E. coli. (b) an anti-E. coli antibody functionalized gold surface. (c) Dark-field microscopy is used to inspect the surface of the gold sheet after 75 min incubation with the field sample and rinse with phosphate buffer solution, enlarging the image. (d) Statistical image analysis was used to count the bacteria captured by the antibodies [40]. Adapted with permission from Ref. [40]. Copyright © 2019 MDPI.

Figure 7

Bacteria detection principle by a single-particle imaging approach. (a) Schematic diagram of bacteria detection by single-particle imaging. (b) The inhomogeneity of particle morphology is identified by tracking the fluctuations of scattering intensity in free solution. (c) Convection induced by an electric heater was used to screen individual bacteria in a small field of view [121]. Adapted with permission from Ref. [121]. Copyright © 2022 The Author(s).

Figure 8

The principle of tracking the rapid identification of 1 um polystyrene spheres and single cell phenotypic characteristics of E. coli. (a) E. coli rotation-induced scattering intensity fluctuation tracking compared with 1 µm microbeads. (b) SVM classification result of one representative infection negative sample. (c) SVM classification result of one representative infection positive sample. [122]. Adapted with permission from Ref. [122]. Copyright © 2022 American Chemical Society.

Figure 9

Schematic illustration of the experimental arrangement. (a) Covalent binding of phage to SiO₂ on fiber surface. (b) Resonance wavelength change with analyte refractive index transmission spectrum [124]. Adapted with permission from Ref. [124]. Copyright © 2012 Elsevier B.V.