Acquisition of the rfb-gnd cluster in evolution of Escherichia coli O55 and O157

The rfb region specifies the structure of lipopolysaccharide side chains th at comprise the diverse gramnegative bacterial somatic (O) antigens. The rf b locus is adjacent to gnd, which is a polymorphic gene encoding 6-phosphog luconate dehydrogenase. To determine if rfb and gnd cotransfer, we sequence d gnd in five O55 and 13 O157 strains of Escherichia coli. E. coli O157:H7 has a gnd allele (allele A) that is only 82% identical to the gnd allele (a llele D) of closely related E. coli O55:H7. In contrast, gnd alleles of E. coli O55 in distant lineages are >99.9% identical to gnd allele D. Though g nd alleles B and C in E. coli O157 that are distantly related to E. coli O1 57:H7 are more similar to allele A than to allele D, there are nucleotide d ifferences at 4 to 6% of their sites. Alleles B and C can be found in E. co li O157 in different lineages, but we have found allele A only in E. coli O 157 belonging to the DEC5 lineage. DNA 3' to the O55 gnd allele in diverse E. coli lineages has sequences homologous to tnpA of the Salmonella enteric a serovar Typhimurium IS200 element, E. coli Rhs elements (including an H-r pt gene), and portions of the O111 and O157 rfb regions. We conclude that r fb and gnd cotransferred into E. coli O55 and O157 in widely separated line ages and that recombination was responsible for recent antigenic shifts in the emergence of pathogenic E. coli O55 and O157.