Submitted to: Journal of Applied Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 16, 2007
Publication Date: August 10, 2007
Citation: Liu, Y., Chen, Y.R., Nou, X., Chao, K. 2007. Potential of surface-enhanced raman spectroscopy for the rapid identification of escherichia coli and listeria monocytogenes cultures on silver colloidal nanoparticles. Journal of Applied Spectroscopy. 6(8)824-831. Interpretive Summary: Rapid, accurate, and preferably routine methods for the identification of foodborne bacteria are increasingly important, not only because of the threat of bio-/agro-terrorism but also due to public health concerns and potential economic loss that are presented by unexpected outbreaks of foodborne illness. Traditional techniques are time-consuming and are not sufficiently rapid to assure the safety of ready-to-eat food products, while fluorescence-labeled antibody / nucleic acid probes based on biosensors and amplification-based methods are multi-step procedures requiring significant volumes of chemical reagents, labor-intensive, and prone to operator-to-operator variation. Rapid microbial detection requires minimal sample preparation, permits routine analysis of a large number of samples with low reagent costs, and must be easy to operate. To limit the probability of bacterial cross-contamination, reducing bacterial exposure to the environment is also desirable. Silver colloidal nanoparticle-based surface-enhanced Raman scattering (SERS) technique is an alternative approach that offers highly structural information and lacks interference from water. In this study, we first determined the characteristic and unique bands from SERS spectra of E. coli and L. monocytogenes cultures, and then developed simple algorithms for rapid identification of bacteria with a 100% success rate. The findings demonstrate the great potential of SERS spectroscopy for routine and rapid detection and screening of bacteria, because of the following advantages: (1) incubated bacterial cultures were directly analyzed and procedures of centrifuging/washing/drying were not involved, (2) bacteria were always retained in suspensions and thus cross-contamination was avoided, (3) much lower volumes of chemical reagents were used compared to the requirements of current traditional methods, (4) the technique required only a short time (about 15-20 min) to complete a test, (5) both the single step of mixing bacterial / silver nanoparticle suspensions and 25 mL minimum culture volumes allowed the analysis of samples on a large scale, and (6) silver nanoparticle colloids were easy and inexpensive to fabricate and demonstrated good binding effect over storage periods of at least four-weeks. This result provides agricultural /medical engineers and researchers a new approach in applying both nanotechnology and Raman spectroscopy for rapid, specific, and routine bacterial identification in processing of raw materials and ready-to-eat food products, in manufacturing process control, and in monitoring of cleaning and hygiene practices.
Technical Abstract: SERS spectra of various batches of bacteria adsorbed on silver colloidal nanoparticles were collected to explore the potential of 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 (K3PO4) 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 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 100% success. A single spectrum acquisition took 5~6 min, and a minimum of 25 mL of silver colloid were directly mixed with 25 mL 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 respective exclusive SERS peaks.