STRAIN GENE VARIATION. ““As the numbers of sequenced E. coli strains continues to grow, the number of discovered core genes remains about the same, while the number of flex genes grows linearly with the number of strains sequenced. Land et al., asserted that comparison of more than 2000 Escherichia coli genomes finds an E. coli core genome of about 3100 gene families and a total of about 89,000 gene families.”
https://standardsingenomics.biomedcentral.com/articles/10.1186/s40793-016-0180-8