Fourier Transform Infrared and Raman Spectroscopy for Characterization of Listeria monocytogenes Strains

Abstract

The purpose of this study was to characterize the variation in biochemical composition of 89 strains of Listeria monocytogenes with different susceptibilities towards sakacin P, using Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The strains were also analyzed using amplified fragment length polymorphism (AFLP) analysis. Based on their susceptibilities to sakacin P, the 89 strains have previously been divided into two groups. Using the FTIR spectra and AFLP data, the strains were basically differentiated into the same two groups. Analyses of the FTIR and Raman spectra revealed that the strains in the two groups contained differences in the compositions of carbohydrates and fatty acids. The relevance of the variation in the composition of carbohydrates with respect to the variation in the susceptibility towards sakacin P for the L. monocytogenes strains is discussed.The food-borne pathogenic bacterium Listeria monocytogenes represents a major concern with respect to food safety, especially because the bacterium is able to grow in refrigerated ready-to-eat products with an extended shelf life, a consumer product type with increasing popularity (25). Bacteriocins are antibacterial peptides produced by certain bacteria-for example, lactic acid bacteria (LAB)-in order to kill or inhibit the growth of other closely related bacteria (4). LAB have been used for the production and preservation of food for a long time, and since both the bacteria and their bacteriocins have been consumed for centuries, they are generally regarded as safe to eat. Thus, it has been suggested that the growth of L. monocytogenes in food might be limited by the addition of bacteriocins and/or bacteriocin-producing LAB (16). This suggestion has been supported by the results of several studies (8, 9). Katla et al. (10) investigated 200 strains of L. monocytogenes isolated from food or food-processing facilities for their susceptibilities towards four bacteriocins produced by LAB, among them sakacin P, which is a class IIa bacteriocin (15). The results showed that the different strains had different susceptibilities to the various bacteriocins. The reason for this variation is unknown, but if bacteriocins and/or bacteriocinproducing LAB are to be used as protective cultures against L. monocytogenes, it is important to understand the basis for this variation in order to certify the effects of these protective agents. A previous investigation of L. monocytogenes strains that were resistant to class IIa bacteriocins suggested that the resistance was coupled to several changes in the cell surfaces of the bacteria (26).Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy are chemical analytical methods that have also been used to collect information about whole bacterial cells (14). The outputs from these methods are FTIR and Raman spectra that contain signals from the organic functional groups in the sample. Since the bands in FTIR spectra are due to polar functional groups while the bands in Raman spectra ar...

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