Exopolysaccharides from Lactic Acid Bacteria and Bifidobacteria

Most bacterial cells are covered by an envelope of similar architecture consisting of a cytoplasmic membrane, a cell wall, and, if present, other external structures such as the outer membrane in Gram-negative bacteria or layers of polysaccharides or proteins, which could be present either in Gram-negative or Gram-positive bacteria. Particularly, the cell wall of a Gram-positive envelope consists of a phospholipid bilayer membrane, in which some proteins are embedded, surrounded by a thick layer of peptidoglycan (murein), which is a structural polysaccharide consisting of alternating β-(1→4)-linked N-acetyld-glucosamine and N-acetyl-muramic acid residues cross-linked by peptide side chains (Figure 7.1). Several secreted proteins associated with peptidoglycan are present, as well as other carbohydrate structures, namely, lipoteichoic acids, teichoic acids, and polysaccharides. The latter components comprise the capsular polysaccharides (CPS), which remain attached to the peptidoglycan forming a capsule like in Gram-negative bacteria, and the slime exopolysaccharides (EPS), which are secreted into the environment (Holts et al. 2009). In addition to the structural function, these outer cellular molecules act as a barrier to protect bacteria against adverse environmental conditions and could also be sensors to communicate with other biotic and abiotic components of the ecosystems they inhabit.