O-antigen is the most variable cell wall constituent of Gram-negative bacteria. Escherichia coli and Salmonella enterica are closely related species. In this work, we present structural and genetic evidence for the close relationship between O-antigens of E. coli O71 and S. enterica O28. The E. coli O71 O-antigen was found to consist of tetrasaccharide-repeating units containing d-GalpNAc, d-Galp, l-Rhap, and d-Quip3NAc, with multiple O-acetyl lateral groups. It is very similar to the known structure of the S. enterica O28 O-antigen, which has the same backbone units, but with a lateral Glc residue instead of O-acetyl groups. The O-antigen gene clusters of E. coli O71 and S. enterica O28 were sequenced and found to contain the same genes with high-level similarity. All of the genes expected for the synthesis of the common backbone structure of the two O-antigens were identified based on homology. It is proposed that the two gene clusters had originated from the same ancestor, and diverged by acquiring prophage genes to carry out side-chain modifications. This is a new pair of the closely related E. coli and S. enterica O-serogroups. The serogroup-specific genes of E. coli O71 and S. enterica O28 were also identified.

Escherichia coli, Salmonella enterica, O-antigen structure, O-antigen gene cluster, PCR assay

13C NMR data:
Linkage	Residue	C1	C2	C3	C4	C5	C6
3,4,3,3	Ac	175.7	23.6
3,4,3	aDQuip3N	96.4	71.9	55.3	77.8	68.6	19.5
3,4	aLRhap	102.9	68.7	77.4	71.5	71.0	18.2
3	bDGalp	106.4	71.9	74.6	77.4	76.5	62.9
2	Ac	176.0	23.9
	aDGalpN	98.7	49.5	78.6	70.1	72.6	62.6


1H NMR data:
Linkage	Residue	H1	H2	H3	H4	H5	H6
3,4,3,3	Ac	-	1.97
3,4,3	aDQuip3N	5.05	3.57	4.33	3.52	4.20	1.23
3,4	aLRhap	5.21	4.30	3.87	3.62	3.78	1.34
3	bDGalp	4.48	3.55	3.77	4.04	3.74	3.76-3.77
2	Ac	-	2.02
	aDGalpN	5.26	4.37	3.95	4.25	4.11	3.75-3.75


1H/13C HSQC data:
Linkage	Residue	C1/H1	C2/H2	C3/H3	C4/H4	C5/H5	C6/H6
3,4,3,3	Ac		23.6/1.97
3,4,3	aDQuip3N	96.4/5.05	71.9/3.57	55.3/4.33	77.8/3.52	68.6/4.20	19.5/1.23
3,4	aLRhap	102.9/5.21	68.7/4.30	77.4/3.87	71.5/3.62	71.0/3.78	18.2/1.34
3	bDGalp	106.4/4.48	71.9/3.55	74.6/3.77	77.4/4.04	76.5/3.74	62.9/3.76-3.77
2	Ac		23.9/2.02
	aDGalpN	98.7/5.26	49.5/4.37	78.6/3.95	70.1/4.25	72.6/4.11	62.6/3.75-3.75

There is only one chemically distinct structure:

SVGMWSMILES3D molIonize

 

*O[C@@H]1[C@@H](C)O[C@H](O[C@@H]2[C@@H](O)[C@H](C)O[C@@H](O[C@H]3[C@@H](CO)O[C@@H](O[C@@H]4[C@@H](NC(C)=O)[C@@H](*)O[C@H](CO)[C@@H]4O)[C@H](O)[C@H]3O)[C@@H]2O)[C@H](O)[C@H]1NC(C)=O

698.672 g/mol (C₂₈H₄₆R₂N₂O₁₈, R = next and previous repeats, not counted in mol weight)