Why is there now only one Salmonella species? Once upon a time, I chanced upon an old microbiology book that detailed the rather colorful world of enterobacteria. _Salmonella_ in particular stood out, as it seemed there were a lot of species: _typhi_ / _typhosa_ , _paratyphi_ , _gallinarum_ , _typhimurium_ , _choleraesuis_ , and quite a bunch of others that I have now forgotten. Flipping through a newer book, it now seems that all of these "species" were collated under _choleraesuis_ (and now more recently _enterica_ ), with all those species being demoted to "strains" (or maybe I should use the current term of art, "serovar"). Unfortunately, the book didn't give much in the way of explaining about this merger. So, why is there now only _S. enterica_? If " _S. typhi_ " is a mere serovar, how come the species name is still used in the literature?

Simply put, old habits die hard; physicians and other medical personnel have grown up with the old species designations so will continue to use them. This is somewhat the reverse of the case with _E. coli_ , where 80-90% of the genome is variable across strains.

Lin-Hui gives a brief history, where strains identified early were given specific names within _Salmonella_ like _typhi_ , _typhimurium_ , _panama_ , etc. In 1966 it was proposed to lump serovars together among three species _S. choleraesuis_ , _S. typhosa_ , and _S. kauffmannii_. Shortly thereafter in 1972 with the advent of DNA techniques, it was shown they really should be all piled together. There is one exception, _Salmonella choleraesuis subsp. bongori_ , separated in 1989.