Strikingly, many discrepancies between the isolates were observed, as revealed by multilocus sequence typing (MLST), the presence of virulence-associated genes, core genome MLST, and single nucleotide polymorphism/indel analyses. As laboratory culture conditions could be responsible for major changes in bacterial strains, we conducted a genome analysis of isolates of this emblematic strain from different culture collections (England, France, the United States, Germany). Not long after, the strain began an odyssey and landed in many laboratories across the world. More than a century ago, Theodor Escherich isolated the bacterium that was to become Escherichia coli, one of the most studied organisms.

Lenski, Nature 536:165–170, 2016, ) that shares numerous molecular features. This strain history can be viewed as unintentional experimental evolution in culture collections all over the word since 1885, mimicking the long-term experimental evolution of E. coli of Lenski et al. Other mutations in rpoS and efflux pump- and porin-encoding genes highlight the acclimatization of the strain through self-preservation and nutritional competence regulation.

This mutator phenotype was probably acquired during the storage of the strain, promoting adaptation to a specific environment. By studying one of the oldest strains available, the ancestral Escherich strain, we were able to identify its mutator status leading to tremendous genetic diversity among the isolates from various collections and allowing us to reconstruct the phylogeographic history of the strain.

Several genes can be impacted, each of them being associated with a typical mutational spectrum. Mutator phenotypes have been described in laboratory-evolved bacteria, as well as in natural isolates.

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