Avian salmonellosis is caused by a group of bacteria of
the genus salmonella. Approximately 2,300 different strains
of salmonellae have been identified, and these are placed
into groupings called “serovars” on the basis of their antigens
or substances that induce immune response by the host,
such as the production of specific antibody to the antigen.
Current taxonomic nomenclature considers the 2,300 different
serovars to be variants of two species,
http://www.nwhc.usgs.gov/publications/field_manual/chapter_9.pdf