Serotyping

Introduction

The cell wall of microorganisms contain a number of proteins and lipopolysaccharides. For some of the components of the cell wall such as lipopolysaccharides or proteins there are many different variations on the molecular structures that can occur. Each different structure can be called an antigen. An antigen is a molecule that reacts with an antibody. Serotyping depends on testing the organisms reaction with many different antibodies to determine which antigens are present. Based on detection of antigens a single species can be divided into hundreds of thousands of different serotypes.

Phylogentic Tree of the evolution of Salmonella species within closely related families

Salmonella serotypes

All of the Salmonella serovars belong to two species: Salmonella bongori containing eight serovars and Salmonella enterica containing the remaining 2300 serovars divided among six subspecies.

The six subspecies of S.enterica are;

S.enterica subsp. enterica (I or 1)
S.enterica subsp. salamae (II or 2)
S.enterica subsp. arizonae (IIIa or 3a)
S.enterica subsp. diarizonae (IIIb or 3b)
S.enterica subsp. houtenae (IV or 4)
S.enterica subsp. indica (VI or 6)

Nomenclature and classification of these bacteria have changed a lot in recent years. Serovar names of S.enterica are not italicised and begin with a capital letter, e.g the strain formerly known as S.typhimurium is now known as Salmonella enterica serovar Typhimurium. This can be shortened to Salmonella Typhimurium. Serotypes of other subspecies of S.enterica and  S.bongori are not named, and are designated by their antigenic formula.

Purpose of Examination:

The purpose of serotyping is to determine which of the >2500 Salmonella serovars a specific isolate belongs. This is necessary for epidemiological purposes and for looking for evidence of links between cases.

Principle of Examination:

Serotypes of Salmonella are defined based on the antigenic structure of both somatic or cell wall (O) antigens and flagellar (H) antigens. These antigens are detected using slide agglutination with commercially produced antisera, the O antigens using a suspension of growth from an agar plate while the H antigens using a suspension of broth culture. The serotype is deduced from the specific pattern of agglutination reactions using the Kauffmann-White classification scheme.

Shigella Serotyping:

Unlike Salmonella, Shigella possess no flagella and hence are non-motile. Shigellae are differentiated into four subgroups on the basis of their O (somatic) antihgens and further differentiated into serotypes.

S.dysenteriae contains 15 distinct antigenic serotypes.

S.flexneri contains 6 serotypes (1-6) that can be further divided into sub-serotypes based on their possession of group factors desinated 3,4; 4; 6; 7; and 7,8

S.boydii contains 18 distinct antigenic serotypes.

S.sonnei contains only 1 serotype.

Listeria monocytogenes Serotyping:

L.monocytogenes strains are serotyped according to variation in their O (somatic) and H (flagellar) antigens. Although more than 14 serotypes have been described, only 3 (1/2a, 1/2b and 4b) cause the vast majority of human infections. Serotype 1/2a is the most frequently isolated serotype from food, while serotype 4b causes the majority of human epidemics. It is therefore likely that serotype designation is associated with virulence potential.

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