Dairy Microbiology, National Dairy Research Institute, Karnal, 132001, India

Probiotic lactobacilli possess outer cell wall structures composed of macromolecular paracrystalline arrays of proteins or glycoproteins known as surface layer proteins (S-layer proteins) for maintaining cellular structural integrity and conferring protection to the cells. Subunits of S-layer proteins are linked to each other and to the underlying cell surface by non-covalent forces. The presence of S-layer has now been described in several species of lactobacilli such as Lactobacillus acidophilus, L. helveticus, L. casei, L. brevis, L. buchneri, L. fermentum, L. bulgaricus, L. plantarum, L. crispatus,L. kefir and L. parakefir which range from 25 to 71 kDa. S-layer proteins consist of two functional domains i.e. the self-assembly domain and cell wall targeting domain. The interaction between Lactobacillus surface layer protein (S-layer protein) and gastro-intestinal epithelial cells activates various signaling pathways for conferring protection to the host by competitively blocking the adhesion of high risk pathogenic bacteria such as enteropathogenic E. coli, Salmonella typhi, Shigella dysenteriae and Listeria monocytogenes etc. Some of the probioticproperties such as adhesion, aggregation or pathogen inhibition have been related with the presence of S-layers. The S-layer protein from L. crispatus was found to prevent the adhesion of potential pathogen like E. coli 0175:H7 upon co-incubation of E. coli O157:H7 or Salmonella typhimurium with S-layer proteins from L. crispatus ZJ001 which prevented the adhesion of the pathogens to HeLa cells through a competitive exclusion mechanism. Besides their application in prevention of GI tract disorders, they have several other applications such as exploring them as nanoparticles due to the self-assembly nature of native and recombinant protein in suspension.