This section last updated: 2021-06-05 (11 months ago)
Author: David Baorto, MD, PhD
Date Written: 2011-07-09
Salmonella nomenclature is complex and has caused confusion. At the present time, by molecular methods the genus Salmonella is known to have only two species: Salmonella enterica_and _Salmonella bongori. Salmonella enterica is divided into six subspecies, each of which is further divided into numerous serovars (or serotypes) based on serological testing of somatic (O) and flagellar (H) antigens. Serovars are then designated by a formulaic concatenation of the antigens, known as the “antigenic formula”. The terms “serovar” and “serotype” appear to be used interchangeably, however, there has been a preference expressed for the term “serovar” by the Association of Public Health Laboratories and the World Health Organization (which uses “serovar” in its documentation). As a result, that will be the term used in this document.
A significant number of what are now known to be serovars were originally thought to be distinct species of Salmonella, and those species names became rooted in common clinical usage. As a result, Salmonella nomenclature deviates from that of other bacteria, which, for the most part, do not have common names assigned to serovars. (Even the well-known _E. coli_O157:H7 is known by its antigenic formula.) Salmonella, on the other hand consists of 2579 individual serovars (2007 WHO documentation), about 1400 of which have common names.
Here is where it gets tricky(er). While it would be impractical to eradicate commonly used names for important serovars, formatting is used to indicate that the names are not that of species, generally by NOT italicizing and instead capitalizing the first letter of the serovar name as follows: “Salmonella enterica subsp. enterica serovar Typhimurium”. Since all the commonly named serovars are in the subspecies enterica, that middle designation can be left off, resulting in the name “Salmonella enteric serovar Typhimurium”.
This code has one of seven possible answers to differentiate the two species and six subspecies in the case of enterica. When performed by the CDC, the answer is derived from a series of about 50 biochemical tests and consists of seven possibilities:
Salmonella enterica subspecies enterica (type I)
Salmonella enterica subspecies salamae **(type II)
Salmonella enterica subspecies arizonae (type IIIa)
Salmonella enterica subspecies diarizonae (type IIIb)
Salmonella enterica subspecies houtenae (type IV)
Salmonella enterica subspecies indica (type VI)
This code has a discrete set of over 2500 possible answers representing the antigenic formula concatenated from the O and H antigens and sometimes other antigens found by agglutination testing. This code is intended to report only the antigenic formulae of the serovar, not the common name for the serovar.
The format of the antigenic formula generally is expected to contain the subspecies type (I,II,IIIa, etc.), then followed by somatic (O) antigens, flagellar (H) phase 1 antigens, flagellar (H) phase 2 antigens, and other antigens separated by a colon. So, a Salmonella type IV (Salmonella enteric subspecies houtenae) with O antigen 43 and H (phase 1) antigens z36, z38 would be reported as "IV 43:z36,z38:-".
This code is intended to report a final answer for the serovar found, thus has an answer list similar in size to 56475-7. It will report the common name for the antigenic formula reported by that code (if there is a common name). If there is no common name, it will report the antigenic formula. In about 90% of cases the antigenic formula reported by 56475-7 will cleanly map to a serovar to report by this code. In the remaining 10% of cases, however, serovars can be further distinguished even in the light of identical antigenic formulas. These distinctions can involve further biochemical testing or sometimes further antigen testing.