Escherichia coli 0123 O antigen genes and polysaccharide structure are conserved in some Salmonella enterica serogroups

The serotyping of O and H antigens is an important first step in the characterization of Salmonella enterica. However, serotyping has become increasingly technically demanding and expensive to perform. We have therefore sequenced additional S. enterica O antigen gene clusters to provide information for the development of DNA-based serotyping methods. Three S. enterica isolates had O antigen gene clusters with homology to the Escherichia coli 01 23 O antigen region. O antigen clusters from two serogroup 058 S. enterica strains had approximately 85 % identity with the E. coli 0123 O antigen region over their entire length, suggesting that these Salmonella and E. coli O antigen regions evolved from a common ancestor. The O antigen cluster of a Salmonella serogroup 041 isolate had a lower level of identity with E. coli 0123 over only part of its O antigen DNA cluster sequence, suggesting a different and more complex evolution of this gene cluster than those in the 058 strains. A large part of the Salmonella 041 O antigen DNA cluster had very close identity with the O antigen cluster of an 062 strain. This region of DNA homology included the wzx and wzy genes. Therefore, molecular serotyping tests using only the 041 or 062 wzx and wzy genes would not differentiate between the two serogroups. The E. coli 01 23 O-antigenic polysaccharide and its repeating unit were characterized, and the chemical structure for E. coli 0123 was entirely consistent with the O antigen gene cluster sequences of E. coli 0123 and the Salmonella 058 isolates. An understanding of both the genetic and structural composition of Salmonella and E. coli O antigens is necessary for the development of novel molecular methods for serotyping these organisms.