Some "lactose negative" strains of E.coli (the late lactose fermenters) can mimic Shigella spp., especially in cases that they produce only little amount of gas from glucose (Strain G).
Escherichia coli encompasses an enormous population of bacteria that exhibit a very high degree of both genetic and phenotypic diversity. Genome sequencing of a large number of isolates of E. coli and related bacteria shows that a taxonomic reclassification would be desirable. However, this has not been done, largely due to its medical importance and Escherichia coli remains one of the most diverse bacterial species: only 20% of the genome is common to all strains.
In fact, from the evolutionary point of view, the members of genus Shigella (S.dysenteriae, S.flexneri, S.boydii, S.sonnei) should be classified as E. coli strains, a phenomenon termed taxa in disguise.
Cultivation 24 hours, 37°C in an aerobic atmosphere.