About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.
O157:H7 is called the serotype of the strain. The O number correspond to the presence of a given antigen on the main bacterial wall, it is called the somatic antigen. H is the flagellar antigen carried on the flagell of the bacteria. And K antigen on the capsule. Both latter structure are not always found in all strains.
A little bit more on this can be found here:
and with google (keywords: serotyping, E. coli)
For the genome it is a question of size. This 20% difference in size have been found between the 2 E. coli genomes available: MG 1655 (it is just the name of the most widely used strains in the lab, not a serotype ) and a particular clone of O157:H7. The difference is mostly explained by the presence of a number of genes specific for virulence and other adaptation.
But it does not help with the understanding of a definition of bacterial species! Just as gene flow between species... It is much easier with bigger animals
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