Characterization and biological activity of bacterial glycoconjugates in cold adaptation.
The cryosphere, covering about one fifth of the surface of the Earth, comprises several components: snow, river and lake ice, sea ice, ice sheets, ice shelves, glaciers and ice caps, and frozen ground which exist, both on land and beneath the oceans (Vaughan DG, et al. 2013) All these habitats, combining the low temperature and the low liquid water activity, are challenging for all the forms of life (Casanueva et al. 2010) These extreme environments are inhabited by microorganisms of all three domains of life; in particular, cold adapted microorganisms belong to Archea and Bacteria domains. To survive in these harsh life conditions, these microorganisms have developed many adaptation strategies, including the over expression of cold shock and heat shock proteins, the presence of unsaturated and branched fatty acids that maintain membrane fluidity (Chattopadhyay et al. 2006) the different phosphorylation of membrane proteins and lipopolysaccharides (Ummarino et al. 2003; Corsaro et al. 2004; Carillo et al. 2013; Casillo et al. 2015) and the production of cold active enzymes (Huston et al. 2004) antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) and cryoprotectants (Deming et al. 2009) Among cryoprotectants, carbohydrate based extracellular polymeric substances (EPS) have a pivotal role in cold adaptation, as they form an organic network within the ice, modifying the structure of brine channels and contributing in the enrichment and retention of microrganisms in ice (Krembs et al. 2002; Krembs et al. 2011; Ewert et al. 2013) Macromolecules belonging to the external layer are fundamental in adaptation mechanisms, as for example the lipopolysaccharides (LPSs) which constitute the 75% of the outer membrane. LPSs have a structural role since increase the strength of bacterial cell envelope and mediate the contacts with the external environment. The general structure of an LPS is characterized by three distinct portions: the lipid A, composed of the typical glucosamine disaccharide backbone with different pattern of acylation on the two sugar residues, the core oligosaccharide, distinguishable in a inner core and a outer core, and the O chain polysaccharide built up of oligosaccharide repeating units. This latter moiety can be absent, and in that case LPSs are named lipooligosaccharides (LOSs)