Heterogeneity of genome sizes among natural isolates of E. coli

Comparisons of the genetic maps of Escherichia coli K-12 and Salmonella typhimurium LT2 suggest that the size and organization of bacterial chromosomes are highly conserved. Employing pulsed-field gel electrophore-sis, we have estimated the extent of variation in genome size among 14 natural isolates of E. coli. The BlnI and NotI restriction fragment patterns were highly variable among isolates, and genome sizes ranged from 4,660 to 5,300 kb, which is several hundred kilobases larger than the variation detected between enteric species. Genome size differences increase with the evolutionary genetic distance between lineages of E. coli, and there are differences in genome size among the major subgroups of E. coli. In general, the genomes of natural isolates are larger than those of laboratory strains, largely because of the fact that laboratory strains were derived from the subgroup of E. coli with the smallest genomes. The most comprehensive information concerning the evolu-tion of bacterial chromosomes has been based on alignments of the genetic maps of Escherichia coli K-12 and Salmonella typhimurium LT2 (5, 24, 42, 44). Although these enteric species diverged an estimated 120 to 160 million years ago (36), the overall similarity in their linkage maps and the relative paucit