S-layers are paracrystalline bidimensional arrays of proteins or glycoproteins that overlay the cell surface of several genus and species of bacteria and archaea. As the outermost layer of several genus and species of microorganisms, S-layer proteins (SLP) are in direct contact with bacterial environment and thus may be involved in many of their surface properties, including adherence to various substrates, mucins and eukaryotic cells, aggregation and coaggregation with yeasts and other bacteria. In addition, SLP have been reported to be responsible for the bacterial protection against detrimental environmental conditions and to play an important role in surface recognition or as carriers of virulence factors. In this mini-review, we bring together the latest evidences about functional and mechanical properties of bacterial SLP from two different perspectives: (A) their role on bacterial adherence to different substrates and surfaces, and (B) their role as mechanical barriers in bacterial harmful environments.

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FTIR spectroscopy was used to structurally characterize the interaction of S-layer proteins extracted from two strains of Lactobacillus kefir (the aggregating CIDCA 8348 and the non-aggregating JCM 5818) with metal ions (Cd +2 , Zn +2 , Pb +2 and Ni +2). The infrared spectra indicate that the metal/protein interaction occurs mainly through the carboxylate groups of the side chains of Asp and Glut residues, with some contribution of the NH groups belonging to the peptide backbone. The frequency separation between the mCOO À anti-symmetric and symmetric stretching vibrations in the spectra of the S-layers in presence of the metal ions was found to be ca. 190 cm À1 for S-layer CIDCA 8348 and ca. 170 cm À1 for JCM 5818, denoting an unidentate coordination in both cases. Changes in the secondary structures of the S-layers induced by the interaction with the metal ions were also noticed: a general trend to increase the amount of b-sheet structures and to reduce the amount of a-helices was observed. These changes allow the proteins to adjust their structure to the presence of the metal ions at minimum energy expense, and accordingly, these adjustments were found to be more important for the bigger ions.