VetBact

Salmonella spp., have, like other members of the Enterobacterioceae family, three main types of antigens that can be used in serological diagnostics and for identification. These antigens are especially useful for identifying bacteria within the following taxa: Escherichia coli, Klebsiella spp., Salmonella spp. and Shigella spp. For epidemiological studies, Salmonella enterica subsp. enterica is divided into different serovars (= serovarians) depending on which set of antigens the strain in question is possessing. The word serotype is also used synonymously with serovar. The following three main types of antigens have been defined:

1. Somatic antigens (= O antigens or cell wall antigens). These antigens consist of different oligosaccharides in the heat-stable part of the cell wall and are also called lipopolysaccharide (= LPS). There are more than 60 different O antigens in Salmonella spp.
2. Surface antigen (= K antigens). These antigens are heat sensitive and consist of the capsular polysaccharides, which thus surround the cell wall and may cover O antigens. The Vi antigen is present in a few serovars.
3. Flagell antigen (= H antigens). These antigens, which consist of proteins and are heat sensitive, are only found in flagella of motile members of the Enterobacteriaceae family. Motile Salmonella types produce two types of H antigen (termed phase 1 or phase 2 antigen.

The Kauffman-White's classification scheme is based on O and H antigens of Salmonella spp. More than 2500 serovars (serotypes) based on O and H antigens have been identified in Salmonella spp. For epidemiological studies, Salmonella isolates are first tested with a polyvalent O antiserum targeting all different serotypes. If the isolate is positive, it is further tested with monovalent O antisera targeting specific serotypes. If the isolate is negative with the polyvalent antiserum, it must then be heat treated to destroy any K antigen which can mask O antigens. Thereafter the test is repeated with the polyvalent antiserum and if the isolate is now postive, it has to be tested with the monovalent antisera as well. Similarly,  the strain in question is also tested for different types of H antigens. The O antigen determines which serogroup a certain Salmonella isolate represents and in combination with the H antigen, the serovar (= serotype) can be defined.

Example: A salmonella type called Salmonella enterica subsp. enterica Serovar Dublin has the antigen set: 1.9.12:g,p:no phase 2, which means that it has:

•      O antigens: 1, 9 and 12
•      H antigens of type Phase 1: g and p
•      H-antigens of type Phase 2: none

The antibodies used in the test are linked to very small latex beads, so that the latex spheres should form visible aggregates with the bacteria and this is what is called agglutination. A bacterium, which is positive in agglutination with polyvalent antisera against both the O and H antigen, is likely to represent a Salmonella isolate, but in order to do epidemiological studies (trace the infection) one has to determine which serovar the isolate belongs to.

Reference: Read more about the salmonella nomenclature on the bacterial pages of Salmonella spp. and Salmonella enterica on VetBact and about different serovars in the Term List of VetBact.

Updated: 2023-02-22.