CONICET | Buscador de Institutos y Recursos Humanos

S-layer structures are macromolecular paracrystalline arrays that completely cover the bacterial cell surface [1,2]. They are attached to the underlying cell wall by non-covalent bonds and usually may be dissociated and solubilized into protein monomers by chaotropic agents such as guanidine hydrochloride and dissociating agents such as 5 M-LiCl. S-layers have been described in several species of Lactobacillus, such as L. acidophilus, L. helveticus, L. casei, L. brevis, L. buchneri, L. fermentum, L. bulgaricus, L. plantarum, L. kefir and L. parakefir [3,4]. On the basis that several Lactobacillus species with an S-layer structure play essential roles in many fermentation processes in food industry, silage fermentations, and in probiotics for humans and animals, the structural characterization of lactobacilli S-layers has become increasingly important [5]. In recent years Raman spectroscopy has broadened its applications due to the high content of molecular structure information that it provides. In Food Microbiology, it has been used for microorganism characterization and for the analysis of proteins [6,7]. As in FTIR spectroscopy, the position of a given band in a Raman spectrum depends on the inter and intramolecular interactions, including peptide-bond angles and hydrogen-bonding patterns. Therefore, Raman spectroscopy can also be used for protein structural analysis. In this work, S-layer proteins extracted from three species of heterofermentative lactobacilli isolated from kefir grains (L. kefir, L. parakefir and L. brevis) have been structurally characterized. This approach allowed the determination of secondary structure composition (%) of S-layer proteins according to the relative areas of the component bands obtained from the peak fitting of the amide I and III regions of the raw Raman spectra [8,9].