Spirilla

[canalon]

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:
http://ecoli.bham.ac.uk/path/sero.html
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

Patrick