ESCHERICHIA-COLI CLONE SONNEI (SHIGELLA-SONNEI) HAD A CHROMOSOMAL O-ANTIGEN GENE-CLUSTER PRIOR TO GAINING ITS CURRENT PLASMID-BORNE O-ANTIGEN GENES

O antigen is part of the lipopolysaccharide present in the outer membr ane of gram-negative bacteria. The surface-exposed O antigen is subjec t to selection by the host immune system, which may account for the ma intenance of many different O-antigen forms. Characteristically, all g enes specific to O-antigen synthesis are clustered in a region close t o the his and gnd genes on the chromosome of Escherichia coil and rela ted species. Shigella sonnei, essentially a clone of E. coil (E. coil clone Sonnei), is an important human pathogen and is unusual in that i ts O-antigen gene cluster is located on a plasmid, Our results suggest that it once had a normal chromosomal O-antigen gene cluster which ha s been largely deleted. We suggest that the O antigen encoded by the p lasmid-borne genes offered a selective advantage in adapting to a new environment and that the chromosomal O-antigen genes were eventually i nactivated, We also identified, by PCR and sequencing, a potential anc estor of E. coli Sonnei among the 166 known E. coli serotype strains.