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J Bacteriol, June 1998, p. 2983-2986, Vol. 180, No. 11
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Escherichia coli Clone Sonnei (Shigella sonnei) Had a Chromosomal O-Antigen Gene Cluster Prior to Gaining Its Current Plasmid-Borne O-Antigen Genes

Vincent Lai, Lei Wang, and Peter R. Reeves*

Department of Microbiology, The University of Sydney, Sydney, New South Wales 2006, Australia

Received 6 November 1997/Accepted 20 March 1998

O antigen is part of the lipopolysaccharide present in the outer membrane of gram-negative bacteria. The surface-exposed O antigen is subject to selection by the host immune system, which may account for the maintenance of many different O-antigen forms. Characteristically, all genes specific to O-antigen synthesis are clustered in a region close to the his and gnd genes on the chromosome of Escherichia coli and related species. Shigella sonnei, essentially a clone of E. coli (E. coli clone Sonnei), is an important human pathogen and is unusual in that its O-antigen gene cluster is located on a plasmid. Our results suggest that it once had a normal chromosomal O-antigen gene cluster which has been largely deleted. We suggest that the O antigen encoded by the plasmid-borne genes offered a selective advantage in adapting to a new environment and that the chromosomal O-antigen genes were eventually inactivated. We also identified, by PCR and sequencing, a potential ancestor of E. coli Sonnei among the 166 known E. coli serotype strains.

* Corresponding author. Mailing address: Department of Microbiology, The University of Sydney, Sydney, N.S.W. 2006, Australia. Phone: (612) 351 2536. Fax: (612) 351 4571. E-mail: reeves{at}angis.su.oz.au.

J Bacteriol, June 1998, p. 2983-2986, Vol. 180, No. 11
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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