This week will we tackle an unresolved problem in biology: how do we define species? Although assigning definitions to terms may seem trivial, the assumptions we make in microbiology about metabolism based on 16S rRNA identity (species) are highly dependent on this definition. The paper we will discuss finds great genotypic diversity within a single species from a single environment, begging the question: what is a species, after all? Two additional articles exploring this issue, one comparing the genomes of 3 strains of E. coli, and one discussing the history and current state of the species issue, will be posted as supplementary reading.