Abstract
This paper covers eight Salmonella serogroups, that are defined by O antigens with related structures and gene clusters. They include the serovars that are now most frequently isolated. Serogroups A, B1, B2, C2-C3, D1, D2, D3 and E have O antigens that are distinguished by having galactose as first sugar, and not N-acetyl glucosamine or N-acetyl galactosamine as in the other 38 serogroups, and indeed in most Enterobacteriaceae. The gene clusters for these galactose-initiated appear to have entered S. enterica since its divergence from E. coli, but sequence comparisons show that much of the diversification occurred long before this. We conclude that the gene clusters must have entered S. enterica in a series of parallel events. The individual gene clusters are discussed, followed by analysis of the divergence for those genes shared by two or more gene clusters, and a putative phylogenic tree for the gene clusters is presented. This set of O antigens provides a rare case where it is possible to examine in detail the relationships of a significant number of O antigens. In contrast the more common pattern of O-antigen diversity within a species is for there to be only a few cases of strains having related gene clusters, suggesting that diversity arose through gain of individual O-antigen gene clusters by lateral gene transfer, and under these circumstances the evolution of the diversity is not accessible. This paper on the galactose-initiated set of gene clusters gives new insights into the origins of O-antigen diversity generally.
Highlights
Bacterial surface polysaccharides are among the most variable structures in Biology
In this paper we analyse in detail a set of 8 Salmonella enterica O antigens, being those of serogroups A, B1, B2, C2-C3, D1, D2, D3 and E, which are distinctive in several ways that provide major insights into the evolution of Oantigen diversity
We looked for other homologues by using S. enterica WbaP in a BLASTp search against the non-redundant protein sequences database, using the BLOSUM62 matrix, a gap creation penalty of 11, and gap extension penalty of 1, and identified homologues in Citrobacter koseri, Pectobacterium atrosepticum, P. wasabiae, and several Erwinia and Pantoea species, for which the analysis of adjacent sequence data suggest that these homologues are in polysaccharide gene clusters
Summary
Bacterial surface polysaccharides are among the most variable structures in Biology. They occur in a variety of capacities, including as O antigens of lipopolysaccharide (LPS), capsules, teichoic acids, and protein-associated structures. In S. enterica it is variation in this locus that is used to define serogroups, but there are often other genes located elsewhere on the chromosome, sometimes within prophage genomes, that can be involved in O-antigen synthesis These additional genes are generally not essential for Oantigen incorporation into LPS, and carry out functions such as adding a side-branch glucose residue or O-acetyl groups that may not be present on all repeat units. Le Minor, and Popoff, and later Grimont and Weill, all at the WHO Collaborating Centre on Salmonella in Paris, adopted the principle that a serogroup should be defined by the variation at the major O-antigen locus, with each group having a specific gene cluster at the galF/gnd locus.
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