Strain is, in microbiology, a phenotypic variant of a species, or even of a lower taxon, usually propagated clonally, due to the interest in the conservation of its defining qualities. In a more basic way it can be defined as a set of bacterial species that share at least one characteristic.There are scientific societies, collections of crop type, which store a large diversity of microorganisms and to diffuse them at the request of researchers; in these collections, taxonomic attribution of each clone is perfectly assured up to level of strain.