The Significance of Enterobacterial Mutants for the Chemical Investigation of their Cell‐Wall Polysaccharides

The outer cell surface of Enterobacteriaceae, i.e. the cell wall, consist of a rigid structure (murein) on which additional proteins, lipids, and polysaccharides are deposited. In the bacterial wild types (S forms) the polysaccharide is species‐specific and carries the serologically determinant groups of the respective O antigen. These specific polysaccharides often consist of a large number of monosaccharides; up to eight different monosaccharides may be involved. Bound to lipid A, the cell‐wall lipopolysaccharides represent the endotoxins of the bacteria. During the past few years the structures of the enterobacterial cell‐wall polysaccharides have been elucidated by chemical, immunochemical, biochemical, and genetic investigations, particularly in the Salmonella. Here the polysaccharides consists of a basal polysaccharide common to all species, to which (in the S form) are bound longer species‐specific side chains, consisting of repeating oligosachharide units. Spontaneous S → R mutation leads to R forms which are deficient mutants of the wild types in regard to the biosynthesis of the complete cell‐wall polysaccharide. In contrast to the multiplicity of the serological specificities of the somatic antigens of the S forms, only a few serological types were found among the R forms (R I, R II, etc.). These R polysaccharides correspond to intermediates in the biosynthesis of the wile‐type polysaccharides. The S → R mutation leads to the loss (or block) of an enzymes (transferase, synthetase) participating in the synthesis of the S polysaccharides. Recently many deficient mutants have been isolated and analysed, and in this way numerous stages in the biosynthesis of the cell‐wall polysaccharides, for example, of Salmonella minnesota, have been made accessible to direct analysis. According to these investigations, the cell‐wall polysaccharides of Salmonella consist of a basic polyheptose phosphate core which is bound to lipid A via ketodeoxyoctonic acid. To the basic core are linked pentasaccharide units of(R II structure). All other R forms are structurally deficient mutants of R II. In the complete polysaccharides of Salmonella S forms (wild types), the repeating oligosaccharide units of the specific side, chain are bound to the terminal N‐acetylglucosamine of the R II structure.