**3. Classification and nomenclature**

To characterise and communicate about this bacterial genus, scientists have used comprehensive *Salmonella* nomenclature. Historically, *Salmonella* strains were classified based on their epidemiology, host range, clinical symptoms, biochemical reactions and surface antigenic patterns. Previously, the name *Salmonella* was derived from the geographical location where the first strain is isolated; for example, *S.* Heidelberg, *S.* Derby, *S.* London. Other than that, it was also named according to its clinical conditions or host specificity, such as *S. typhimurium, S. enteritidis, S. typhi,*  'S. gallinarum', 'S. abortusovis' or *S. choleraesuis*. However, it was soon realised that these so-called species were ubiquitous [16].

A great number of serovars have been described as a result of O and H antigens analysis initiated by White in 1926 and continued by Kauffmann later in 1941. The species that were defined by Kauffman as 'a group of related sero-fermentative phage types' created more than 2000 serovar names according to the species. However, the concept of one serovar one species was discovered to be unsuitable since biochemical test still could not separate most of the serovars. Although the terms serotype and serovar are interchangeable, according to the Bacteriological Code, serovar is now recommended for scientific communication (1990 Revision).

In the early years, Kauffman identified *Salmonella* serovars in 1966 based on its antigenic composition, and there were multiple species within its genus. He found *Salmonella* serovars and a variety of species within the genus using the antigenic formula. Some clinically important *Salmonella* strains were found before 1966, and the majority of serovars were named after the illness and/or the host, such as *S. typhi* and *S. typhimurium,* or by the geographical area or origin of the species that were first isolated.

The epidemiologic classification of *Salmonella* is based on the preferences of the hosts. *S.* Typhi was the first of the host-restricted serotypes that only infect humans. The second group includes host-adapted serotypes associated with one host species but can cause disease in other hosts. An example of a host-adapted serotype was *S.* Pullorum, which was discovered in an avian. The remaining serotypes are in the third group. Each year, the three most common serotypes recovered from humans are *Salmonella* Enteritidis, *Salmonella* Typhimurium, and *Salmonella* Heidelberg.

Given the complexities of the various *Salmonella* species, it was proposed that the genus *Salmonella* was separated into three species: *S. choleraesuis,* '*S*. typhosa' (*S. typhi*) and '*S*. kauffmannii'. '*S*. kauffmannii' contains entirely additional serovars. *S. enterica*, according to Kauffmann and Edwards (1952), should include all *salmonellae. S. enterica* subsp. *enterica (S. enterica* subsp. I*)* are the most frequent *Salmonella* serovar among the approximately >2600 *Salmonella* serovars that have been found to date [17]. Human and warm-blooded animal infections with *Salmonella* infections are virtually always caused by strains belonging to the O-antigen serogroups (bacteria's surface of their outermost layer), A, B, C1, C2, D, and E [18]. The oligosaccharides associated with lipopolysaccharide determine the O antigen.

*Salmonella* infections that are typically isolated from cold-blooded animals and the natural environment but uncommonly isolated from human are caused by serovars in the *S. enterica* subspecies *salamae (S. enterica* subsp. II*)*, *arizonae (S. enterica* subsp. IIIa*)*, *diarizonae (S. enterica*subsp. IIIb), *houtenae (S. enterica* subsp. IV), *indica (S. enterica* subsp. VI), and *S. bongori.*

The White-Kauffmann-Le Minor scheme, previously designated as Kauffman-White scheme, described *Salmonella's* characterisation based on antibody recognition with antigens on *Salmonella's* surface. Three major antigenic determinants are used

to classify *Salmonella* into three groups in the scheme: flagellar H antigens, somatic O antigens and virulence (Vi) capsular K antigens. This scheme is an established document that lists all identified serovars [19, 20]. The document has been updated by the World Health Organisation (WHO) Collaborating Centre for Reference and Research on *Salmonella* at the Pasteur Institute, Paris, France, and every newly identified serovar is reported in the journal *Research in Microbiology* yearly. The WHO Collaborating Centre for Reference and Research on *Salmonella*, in the Pasteur Institute in Paris, has updated the document. As a result, every newly discovered *Salmonella* serovar as well as other relevant microorganism is reported in the journal *Research in Microbiology* every year [21].

On the recommendation of the WHO collaborating centre, the current nomenclature used by the Centers for Disease Control and Prevention (CDC), Department of Health and Human Services, USA, is widely recognised. It is based on a two-species system (*S. enterica* and *S. bongori*), with multiple serovars in each species [22]. Microbiologists in clinical and public health have praised the system for meeting their needs [23]. *Salmonella* nomenclature is currently divided into two species, *S. enterica*  and *S. bongori*, which has six subspecies and one subspecies, respectively. The nomenclature is summarised in **Table 1**. In addition, the relationship of phylogenetic tree among *Salmonella* subspecies is demonstrated in **Figure 2**.

Serovar names should not be printed in italics because they are no longer considered species names. For example, *S. enteritidis* becomes *S. enterica* subsp. *enterica* serovar Enteritidis, or simply written as *Salmonella* serovar Enteritidis and can be shortened to *S*. Enteritidis. Only serovars of *S. enterica* subsp. *enterica* are given names associated with disease syndrome or host habitat, while others represent the geographical origin of the first isolate found. On the other hand, other subspecies' serovars are identified by their antigenic formula O:H.

Serovars designated by antigenic formulae include the following: (i) subspecies designation (subspecies I through VI); (ii) O (somatic) antigens separated by a comma if needed, followed by colon: (iii) H (flagellar) antigens (phase 1) separated by a colon and (iv) H antigens (phase 2, if present) (for example, *Salmonella* serotype II 39:z10:-) [25].


**Table 1.** Salmonella *nomenclature.* Salmonella*: The Critical Enteric Foodborne Pathogen DOI: http://dx.doi.org/10.5772/intechopen.103900*

**Figure 2.**

*Summary of relationship of phylogenetic tree among* Salmonella *subspecies and other bacterial species (adapted from reference [1]).*
