Review on SERS of Bacteria

Abstract

Surface enhanced Raman spectroscopy (SERS) has been widely used for chemical detection. Moreover, the inherent richness of the spectral data has made SERS attractive for use in detecting biological materials, including bacteria. This review discusses methods that have been used to obtain SERS spectra of bacteria. The kinds of SERS substrates employed to obtain SERS spectra are discussed as well as how bacteria interact with silver and gold nanoparticles. The roll of capping agents on Ag/Au NPs in obtaining SERS spectra is examined as well as the interpretation of the spectral data.

Keywords: bacteria; cell envelope; nanoparticles; principal component analysis (PCA); secretions; surface-enhanced Raman spectroscopy (SERS).

Figure 1

A cartoon view of the membranes of: (a) Gram-negative bacterium; and (b) Gram-positive bacterium. The membranes of Gram-negative bacteria are composed of two layers: the outer membrane rich in lipopolysaccharide (LPS) and the inner membrane rich in anionic phosphatidylglycerols (PG). Gram-positive bacteria have a cell wall consisting of lipoteichoic acid and peptidoglycan and a cytoplasmic membrane. Reproduced with permission from Elsevier [4].

Figure 2

SEM photomicrographs of E. coli: (a) control; and (b) 10; (c) 20; and (d) 30 μg·mL⁻¹ gum arabic capped Ag NPs (arrows indicate depressions and extensive damage of bacterial cells). Reproduced with permission from John Wiley and Sons [23].

Figure 3

HR-TEM images of E. coli cells: (a) untreated; (b,c) treated with gum arabic Ag NPs; and (d) enlarged view of the membrane of (c). Red arrow indicates Ag NPs attachment on membrane and black and white arrows show partially damaged membranes at various sites. Reproduced with permission from John Wiley and Sons [23].

Figure 4

Schematic diagram of bactericidal activity of Ag NPs on Gram-positive and Gram-negative bacteria. Reproduced with permission from Springer [24]. Ag NPs adhesion to microbial cells, penetration inside the cells, ROS and free radical generation, and modulation of microbial signal transduction pathways have been recognized as the most prominent modes of antimicrobial action [13].

Figure 5

(a) TEM image of E. coli with a wall colloid [30]. Reproduced with permission from the American Chemical Society. (b) SERS spectra obtained for E. coli and B. megaterium coated with silver [31]. Spectra were obtained using 514.5 nm excitation. Reproduced with permission from Elsevier.

Figure 6

(a) TEM image of E. coli 1116 showing the colloid deposits that formed on the cell wall (bacteria(H₂O)@AgNP) [38]. Red arrows indicate Ag composites that have large, elongated structures. (b) SERS spectra obtained for E. coli 5695 and S. epidermidis in contact with clusters of Ag NPs [38]. Spectra were obtained for 15 different batches of samples using 633 nm excitation. Reproduced with permission from the American Chemical Society.

Figure 7

(a) TEM image of E. coli infused with Ag colloid [34]. (b) SERS spectra of E. coli and A. calcoaceticus infused with silver colloid [34]. Spectra were obtained using 514.5 nm excitation. Reproduced with permission from John Wiley and Sons.

Figure 8

(a) AFM image of nanostructured PPX-Cl film deposited with 60 nm Au film [50]. (b) SERS spectra collected for 25 random cells on 1 mm² area showing a highly reproducible fingerprint for E. coli [50]. Spectra were obtained using 785 nm excitation. Reproduced with permission from John Wiley and Sons.

Figure 9

(a) SERS spectra of 20 μM aqueous solutions of the indicated purine components of bacterial SERS spectra. The spectra have been offset for viewing and are normalized to the maximum peak intensity of the 20 μM adenine solution [57]. (b) Comparison of the SERS spectra of cells (blue) for four bacterial strains and their corresponding enriched supernatant (red) [57]. Spectra of cells and supernatant are offset for better viewing. Spectra were obtained using 785 nm excitation. Reproduced with permission from Springer.

Figure 10

(a) TEM image of E. coli 1116 mixed with Ag NPs and 0.01 M NaCl (bacteria-AgNP) [38]. (b) SERS spectra obtained for E. coli 1116 obtained by two different methods: external colloid formation, bacteria(H₂O)@AgNP, and mixing bacteria with Ag colloid, bacteria-AgNP [38]. Spectra were obtained using 633 nm excitation. Each sample was measured three times. Reproduced with permission from the American Chemical Society.

Figure 11

SERS spectra obtained using citrate generated Ag NPs [76]. Ag NPs were mixed with bacterial suspensions and filtered onto a 0.02 μm pore size ceramic filter. Spectra are: (a) citrate; (b) Pseudomonas aeruginosa P2; and (c) different ratios of Ag NPs to E. coli (top to bottom: 3:1, 2:1, 1:1, and 0.5:1 Ag NPs:E. coli). Spectra were obtained using 785 nm excitation. Reproduced with permission from the Elsevier.

Figure 12

SERS spectra of E. coli coated with either an external Ag or Au colloid, FAD adsorbed on either Ag or Au, and RF adsorbed on Ag where spectra were obtained using: (a) 514 nm; and (b) 633 nm laser excitation [34]. Reproduced with permission from John Wiley and Sons.

Figure 13

SERS spectra of E. coli obtained at different growth phases [55]: (a) plot of optical density at 600 nm (OD₆₀₀) as a function of growth time, where cells were harvested at OD₆₀₀ of 0.4, 1.5, and 2.0 (open circles); (b) SEM images of harvested bacteria; and (c) SERS spectra of harvested bacteria, where spectra were obtained by placing the bacteria on an Ag/AAO substrate and using 632.8 nm laser excitation. Reproduced with permission from the PLOS.

Figure 14

Preliminary SERS results obtained for E. coli subjected to different temperatures: (a) 4 °C for 245 min; (b) 37 °C (temperature the cells were cultured); and (c) 43 °C for 64 min. E. coli samples were mixed with citrate-generated Ag NPs and filtered onto a ceramic substrate [76]. SERS spectra were obtained using 785 nm laser excitation.

Figure 15

Deconvoluted SERS spectrum of E. coli (full spectrum shown in Figure 14b) obtained by mixing citrate generated Ag NPs with the bacterial suspension and filtering onto a 0.02 μm pore size ceramic filter [76]. Spectrum was obtained using 785 nm excitation. Data points are shown as black, filled circles. The red line is the summation of all the Lorentzian peaks. Peak assignments are shown.

Figure 16

The comparison of the calculated PCA analysis for three bacteria: S. pneumoniae (green diamonds), H. influenzae (blue squares), and N. meningitidis (red circles) [51]. Reproduced with permission from the Royal Society of Chemistry.

Figure 17

PCA plots of bacteria at: (a) 1 h; and (b) 24 h of incubation [71]. Reproduced with permission from Springer.