V. Lai et al., ESCHERICHIA-COLI CLONE SONNEI (SHIGELLA-SONNEI) HAD A CHROMOSOMAL O-ANTIGEN GENE-CLUSTER PRIOR TO GAINING ITS CURRENT PLASMID-BORNE O-ANTIGEN GENES, Journal of bacteriology, 180(11), 1998, pp. 2983-2986
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.