Polynucleotide sequence similarity tests were carried out to determine the extent of divergence present in a number of Escherichia coli strains, obtained from diverse human, animal, and laboratory sources, and closely related strains of Shigella, Salmonella, and the Alkalescens-Dispar group. At 60 C, relative reassociation of deoxyribonucleic acid (DNA) from the various strains with E. coli K-12 DNA ranged from 100 to 36%, with the highest level of reassociation found for three strains derived from K-12, and the lowest levels for two "atypical" E. coli strains and S. typhimurium. The change in thermal elution midpoint, which indicates the stability of DNA duplexes, ranged from 0.1 to'14.5 C, with thermal stability closely following the reassociation data. Reassociation experiments performed at 75 C, at which temperature only the more closely related DNA species form stable duplexes, gave similar indications of relatedness. At both temperatures, Alkalescens-Dispar strains showed close relatedness to E. coli, supporting the idea that they should be included in the genus Escherichia. Reciprocal binding experiments with E. coli BB, 02A, and K-12 yielded different reassociation values, suggesting that the genomes of these strains are of different size. The BB genome was calculated to be 9% larger than that of K-12, and that of 02A 9% larger than that of BB. Calculation of genome size for a series of E. coli strains yielded values ranging from 2.29 x 109 to 2.97 x 109 daltons. E. coli strains and closely related organisms were compared by Adansonian analysis for their relatedness to a hypothetical median strain. E. coli 0128a was the most closely related to this median organism. In general, these data compared well with the data from reassociation experiments among E. coli strains. However, anomalous results were obtained in the cases of Shigella flexneri, S. typhimurium, and "atypical" E. coli strains.

A strain of Escherichia coli isolated from an epidemic of “nonspecific” diarrhea has been shown to harbor a plasmid which specifies enterotoxin production.