Rapid detection of food- and waterborne bacteria using surface-enhanced Raman spectroscopy coupled with silver nanosubstrates

Abstract

Development of rapid and sensitive methods to detect pathogens is important to food and water safety. This study aimed to detect and discriminate important food- and waterborne bacteria (i.e., Escherichia coli O157:H7, Staphylococcus epidermidis, Listeria monocytogenes, and Enterococcus faecelis) by surface-enhanced Raman spectroscopy (SERS) coupled with intracellular nanosilver as SERS substrates. An in vivo molecular probing using intracellular nanosilver for the preparation of bacterial samples was established and assessed. Satisfactory SERS performance and characteristic SERS spectra were obtained from different bacterial samples. Distinctive differences were observed in SERS spectral data, specifically in the Raman shift region of 500-1,800 cm(-1), and between bacterial samples at the species and strain levels. The detection limit of SERS coupled with in vivo molecular probing using silver nanosubstrates could reach the level of single cells. Experiments with a mixture of E. coli O157:H7 and S. epidermidis for SERS measurement demonstrate that SERS could be used for classification of mixed bacterial samples. Transmission electron microscopy was used to characterize changes of morphology and cellular composition of bacterial cells after treatment of intracellular nanosilver. The results indicate that SERS coupled with intracellular silver nanosubstrates is a promising method for detection and characterization of food- and waterborne pathogenic and non-pathogenic bacterial samples.

Surface-enhanced Raman spectroscopy introduced into the International Standard Organization (ISO) regulations as an alternative method for detection and identification of pathogens in the food industry

“…1467 cm −1 are observed. These bands are detected also in SERS spectra of many Gram-positive and Gram-negative bacteria species like E. coli or S. epidermidis [40, 41] and are assigned as follows: 649 cm −1 (guanine and tyrosine); 723 cm −1 (C–N stretching mode of the adenine part of flavin adenine dinucleotide, FAD); 958 cm −1 (C=C deformation or C–N stretching); 1030 cm −1 (C–C stretching); 1095 cm −1 (O–P–O stretching in DNA); 1220 cm −1 (amide III); 1467 cm −1 (CH 2 deformation) [42]. The differences between these two spectra, especially in the ratio of intensities of some bands, e.g., 649, 723, and 1030 cm −1 , originate from bacteria responding to environmental changes (XLD or Chrom Sal) by changing their metabolic profiles and composition of the cell walls [43].…”

IR and Raman Spectroscopy for Pathogen Detection

“…They could even discriminate three strains of E. coli from each other by simple PCA classification [130]. Some groups have investigated mixed bacterial samples and were able to tell them apart from pure cultures by PCA [130,143] or Euclidian distance [128,129]. Surprisingly, the mixed samples did not appear in the area between the pure samples but were shifted toward one constituent of the mixture [129,143] or even showed larger difference from both original components [128,130].…”

“…It should be noted that only very few of these groups report the impact of different batches of the applied SERS substrate on their SERS spectra [113,130,143,144], and most used either novel SERS substrates [130,144] or formed the colloidal NPs directly inside the cells [143].…”

“…Over several years, the group of Zeiri and Efrima has developed a reproducible method to selectively form NPs either on the surface of bacteria resulting in a silver coating, or inside the cells [127]. With this technique or variations thereof, several groups were able to measure the SERS spectra of single bacteria [113,143].…”

Surface-enhanced Raman scattering detection of bacteria on microarrays at single cell levels using silver nanoparticles

“…Rapid and accurate methods for the analysis of bacteria in water and food are in high demand for health care and food hygiene [1,2]. Many methods have been developed for routine detection of bacteria, including plating and culturing [3,4], flow cytometry and FISH (fluorescence in-situ hybridization) [5], MALDI/TOF MS [6], enzyme-linked immunosorbent assay (ELISA) [7], and polymerase chain reaction (PCR) [8].…”