Enzyme Polymorphism and Genetic Population Structure in Escherichia coli and Shigella

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Abstract
Electrophoretically demonstrable variation in 12 enzymes was studied in > 1600 E. coli isolates from human and animal sources and in 123 strains of the 4 spp. of Shigella. All 12 enzymes were polymorphic; the number of allozymes (mobility variants), which were equated with alleles, averaged 9.3/locus in E. coli and 2.9/locus in Shigella. Some 77% of the allozymes recorded in Shigella were shared with E. coli. A total of 302 unique genotypic combinations of alleles over the 12 loci (electrophoretic types, ET) was distinguished, of which 279 represented E. coli and 23 were Shigella. Among ET, mean allelic diversity per locus was 0.52 for E. coli and 0.29 for Shigella. It was estimated that there are, on the average, .apprx. 0.3 detectable codon differences per locus between pairs of strains of E. Coli and Shigella, which is roughly equivalent to 1.2 amino acid differences per enzyme. Evidence that the enzyme loci studied are a random sample of the genome was provided by a significant positive correlation between estimates of genetic divergence between pairs of strains obtained by DNA reassociation tests and estimates of genetic distance between the same strains based on electrophoresis. A principal components analysis of allozyme profiles revealed that the 302 ET fall into 3 overlapping clusters, reflecting strong non-random associations of alleles, largely at 4 loci. Each of the 4 ET of E. coli that were most frequently recovered from natural populations had an allozyme profile very similar to, or identical with, the the hypothetical model ET of 1 of the groups. ET of Shigella fell into 2 of the groups. No biological significance could be attributed to the genetic structure revealed by multilocus electrophoretic techniques. The electrophoretic data are fully compatible with other molecular and more conventional evidence of a close affinity between E. coli and Shigella, and they raise questions regarding the present assignments of certain strains to species. In support of evidence from DNA reassociation tests and serotyping, the present study suggests that S. sonnei is homogeneous in chromosomal genotype.