Laboratory strains of Escherichia coli K-12: things are seldom what they seem

K-12 was originally isolated 100 years ago and since then it has become an invaluable model organism and a cornerstone of molecular biology research. However, despite its pedigree, since its initial isolation K-12 has been repeatedly cultured, passaged and mutagenized, resulting in an organism that carries many genetic changes. To understand more about this important model organism, we have sequenced the genomes of two ancestral K-12 strains, WG1 and EMG2, considered to be the progenitors of many key laboratory strains. Our analysis confirms that these strains still carry genetic elements such as bacteriophage lambda (λ) and the F plasmid, but also indicates that they have undergone extensive laboratory-based evolution. Thus, scrutinizing the genomes of ancestral K-12 strains leads us to examine whether K-12 is a sufficiently robust model organism for 21st century microbiology.