The Escherichia coli serA-linked capsule locus and its flanking sequences are polymorphic, genetic evidence for the existence of more than two groups of capsule gene clusters.

Two families of Escherichia coli capsules, termed groups I and II, have been defined previously on the basis of a number of biochemical and genetic criteria. Recently, a third group of capsules, termed I/II has been suggested on the basis of chemical structure and mode of expression. In this paper, we show that group I capsule-producing strains lack the serA-linked group II capsule genes. In addition, group I/II capsule-producing strains lack the group II capsule genes despite the former genes also mapping near to serA. Therefore, the genetic data presented in this paper support the existence of three groups of capsule gene clusters, two of which are linked to serA. Sequences flanking the K4 capsule genes were found in the chromosome of all E. coli strains examined and were sometimes present in multiple copies at different loci, indicating that this chromosomal region is highly polymorphic.