In the Enterobacteriaceae, the core oligosaccharide provides the junction between the highly conserved lipid A and the remarkably diverse polysaccharide O antigen. The basic structure of the inner (lipid A-proximal) core is well conserved, perhaps reflecting constraints imposed by its involvement in the structural integrity of the outer membrane. However, non-stoichiometric modifications do create some structural variants. The outer core may show more variation. Efforts to develop immunoprophylactic strategies based on the core oligosaccharide require a detailed understanding of core immunochemistry, the accessibility of specific epitopes in the LPS, and the distribution of specific structures within natural populations. The availability of sequences for the waa (core biosynthesis) loci and functional data for the gene products provide a molecular basis for the known structural diversity in Escherichia coli and Salmonella core oligosaccharide. Surveys of waa-locus organization have established the distribution of these core types in natural populations and have identified genetic variants that provide candidates for additional novel structures.
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