Potential of Surface-Enhanced Raman Spectroscopy for the Rapid Identification of Escherichia Coli and Listeria Monocytogenes Cultures on Silver Colloidal Nanoparticles

Abstract

Surface-enhanced Raman (SERS) spectra of various batches of bacteria adsorbed on silver colloidal nanoparticles were collected to explore the potential of the SERS technique for rapid and routine identification of E. coli and L. monocytogenes cultures. Relative standard deviation (RSD) of SERS spectra from silver colloidal suspensions and ratios of SERS peaks from small molecules (K(3)PO(4)) were used to evaluate the reproducibility, stability, and binding effectiveness of citrate-reduced silver colloids over batch and storage processes. The results suggested consistent reproducibility of silver colloids over batch process and also stability and consistent binding effectiveness over an eight-week storage period. A variety of mixtures of E. coli/L. monocytogenes cultures with different colloidal batches revealed that, despite large variations in relative intensities and positions of SERS active bands, characteristic and unique bands at 712 and 390 cm(-1) were consistently observed and were the strongest in E. coli and L. monocytogenes cultures, respectively. Two specific bands were used to develop simple algorithms in the evaluation of binding effectiveness of silver colloids over storage and further to identify E. coli and L. monocytogenes cultures with a 100% success. A single spectrum acquisition took 5 approximately 6 min, and a minimum of 25 microL silver colloid was directly mixed with 25 microL volume of incubated bacterial culture. The short acquisition time and small volume of incubated bacterial culture make silver colloidal nanoparticle based SERS spectroscopy ideal for potential use in the routine and rapid screening of E. coli and L. monocytogenes cultures on large scales. This is the first report of the development of simple and universal algorithms for bacterial identification from the respective exclusive SERS peaks.

Differentiation of bacterial strains by means of surface enhanced FT-Raman spectroscopy

“…For instance, Raman spectra of bacteria prepared in distilled water (Fig. 5) differ from the previously published spectra of the same bacteria prepared in the growth media [6]. Despite that, the results obtained in this study are in good agreement with that measured by Luo and Lin [35].…”

“…There are many ambiguities in literature concerning assignment of vibrational spectral bands of bacteria. For instance, the spectral band at 646 cm -1 of bacterial pathogens is assigned to tyrosine skeletal vibration in [8], while in [6] it is assigned to the COO -band of amino acids or C-S stretch mode of cysteine. It is well established that in case of tyrosine water solution the most intense Raman spectral bands are located at 826, 846, 1210, and 1615 cm -1 [9].…”

“…The first attempt to apply SERS spectroscopy with NIR excitation for discrimination of bacterial pathogens is presented in [6]. The studies were performed with a conventional room temperature DTGS detector.…”

Understanding SERS of bacteria

“…[21] The main peak at 712 cm −1 was assigned to a shifted 730 cm −1 peak, due to the influence of differences in growth conditions. The excitation wavelength used in this study is quite long (1064 nm), being far from resonance with the surface plasmons of the silver colloids.…”

IR and Raman Spectroscopy for Pathogen Detection

“…By applying different SERS substrates, numerous groups have shown that SERS spectra of different bacterial species show different spectral features, which allows the qualitative differentiation of the species by their SERS spectra [113,117,120,124,126,[128][129][130][131][132][133][134][135][136][137][138][139][140]. The range of the investigated bacteria comprises Gram-positive as well as Gram-negative bacteria.…”