Structure of the Lipopolysaccharide Core Region

Most Gram-negative bacteria (exceptions are Treponema pallidum, Borrelia burgdorferi and B. hispanica, Sphingomonas capsulata and S. paucimobili, Thermus thermophilus, and Meiothermus taiwanensis [1–7]) contain lipopolysaccharide (LPS) in their outer membrane [8]. In general, the LPS may be present either in the smooth form (S, possessing the polysaccharide region) or in the rough form (R, lacking the polysaccharide, also called lipooligosaccharide, LOS). Both forms contain lipid A [9] and a core oligosaccharide (OS) that comprises up to 15 sugar residues [8–17]. In the S-form LPS, the core OS region is substituted by a polysaccharide, which most often is an O-specific polysaccharide (O-antigen, for structures see Chap. 3), and in other cases is the enterobacterial common antigen (only in Enterobacteriaceae [18]) or a capsular polysaccharide [19]. For a long time a dogma existed claiming that mutants of LPS-containing Gram-negative bacteria are not viable without a minimal core OS structure. However, viable mutants synthesizing only lipid A or a precursor thereof have been recently isolated [20, 21].