Structural characterization of the O-antigen polysaccharide from E. coli strain SC-UY1 supports a fully functional galF-gnd locus in the O20 serogroup.

E. coli strain SC-UY1 was isolated from a case of fulminant neonatal calf diarrhea in Uruguay. The detailed chemical structure of its O-antigen polysaccharide (O-Ag) was elucidated using NMR spectroscopy. The polymer consists of linear pentasaccharide repeating units with the following structure: →2)-α-l-Fucp-(1→3)-α-l-Rhap-(1→4)-α-d-GlcpA-(1→3)-α-l-Fucp2Ac-(1→3)-α-d-GlcpNAc-(1→, representing a novel polysaccharide structure. Sequencing of the chromosomal region located between the housekeeping genes galF and gnd, and its comparison with those of other E. coli strains revealed that this isolate belongs to the E. coli O20 O-genotype. However, the O-Ag structure elucidated herein differs from those previously reported for strains of this serogroup, which have been proposed to originate from a genomic region distinct from the canonical galF-gnd locus, implying that this gene cluster may not be functional. In contrast, the polysaccharide structure elucidated herein is fully consistent with the predicted functions of the genes encoded in the aforementioned chromosomal region, including the identity of the monosaccharide residues. These results demonstrate that the galF-gnd region is fully functional in strain SC-UY1, and that structural variability among the O-antigens of E. coli O20 isolates may underlie previously reported inconsistencies in the serological performance of the O20-specific antisera.