S-layers in lactobacilli: structural characteristics and putative role in surface and probiotic properties of whole bacteria

P. MOBILI, E. GERBINO, E. TYMCZYSZYN, A. GOMEZ ZAVAGLIA

Current Research, Technology and Education Topics in Applied Microbiology and Microbial Biotechnology

Formatex

Lugar: Badajoz; Año: 2010; p. 1224 - 1234

The S-layer is a crystalline bidimensional array of proteic or glycoproteic subunits that represents the outermost layer of several species of archea and bacteria. The subunits are linked to each other and to the underlying cell surface by non-covalent forces, generally assembled in lattices with oblique, square or hexagonal symmetry. Several specific functions have been assigned to the S-layer from different microorganisms (protective coats, molecular sieves, molecule and ion traps, cell adhesion and surface recognition, virulence factors). These potential functions have converted the S-layers in attractive targets for biophysical studies and structural research, particularly in regard to their possible nanotechnological applications. The presence of S-layer proteins has been reported in many species of probiotic lactobacilli. In fact, some probiotic properties of whole bacteria, such as adhesion, aggregation or pathogens inhibition have been related with the occurrence of particular types of S-layers. Different approaches have been used to study the S-layers in lactobacilli, as well as their role in surface and/or probiotic properties. An indirect approach consists in trying to infer S-layer functions through the comparison of specific bacterial properties (i.e.:aggregation, haemagglutination, adhesion to prokaryotic or eukaryotic cells and to extracellular matrix components), before and after the extraction of the protein [1-8]. A direct approach consists in studying the capacity of isolated S-layer proteins to adhere or associate to other bacteria, eukariotic cells (i.e.: yeasts or enterocytes), extracellular matrix components or inert surfaces as well as to inhibit the growth and/or the activity of pathogens. From both approaches, it turns out that S-layer functions in lactobacilli are not only species but also strain specific, and this fact makes difficult the prediction of a given property in S-layer carrying lactobacilli. This correlation can be established if a structural analysis of the S-layers is performed. In this regard, different researchers have studied the nucleotidic sequence of several lactobacilli S-layer protein genes, the primary and secondary structure of proteins (based on predictions from primary structure or from spectroscopic analysis) and posttranslational modifications such as glycosylation. In addition, different structural characteristics of the S-layer proteins have been correlated with the surface and/or probiotic properties of whole microorganisms [8,9]. In this mini-review, an overview of the S-layer proteins from lactobacilli is carried out. The different techniques used for their structural and functional characterization are discussed.