Serotype

A serotype or serovar is a distinct variation within a

Serotyping often plays an essential role in determining species and subspecies. The Salmonella genus of bacteria, for example, has been determined to have over 2600 serotypes. Vibrio cholerae, the species of bacteria that causes cholera, has over 200 serotypes, based on cell antigens. Only two of them have been observed to produce the potent enterotoxin that results in cholera: O1 and O139.^{[citation needed]}

Serotypes were discovered in hemolytic

The binding between a surface antigen and the antiserum can be experimentally observed in many forms. A number of bacteria species, including Streptococcus pneumoniae, display the Quellung reaction visible under a microscope.^[7] Others such as Shigella (and E. coli) and Salmonella are traditionally detected using a slide agglutination test.^[6][8] HLA types are originally determined with the complement fixation test.^[9] Newer procedures include the latex fixation test and various other immunoassays.

"Molecular serotyping" refers to methods that replace the antibody-based test with a test based on the nucleic acid sequence – therefore actually a kind of genotyping. By analyzing which surface antigen-defining allele(s) are present, these methods can produce faster results. However, their results may not always agree with traditional serotyping, as they can fail to account for factors that affect the expression of antigen-determining genes.^[10][11]

Role in organ transplantation

The immune system is capable of discerning a cell as being 'self' or 'non-self' according to that cell's serotype. In humans, that serotype is largely determined by human leukocyte antigen (HLA), the human version of the major histocompatibility complex. Cells determined to be non-self are usually recognized by the immune system as foreign, causing an immune response, such as hemagglutination. Serotypes differ widely between individuals; therefore, if cells from one human (or animal) are introduced into another random human, those cells are often determined to be non-self because they do not match the self-serotype. For this reason, transplants between genetically non-identical humans often induce a problematic immune response in the recipient, leading to transplant rejection. In some situations, this effect can be reduced by serotyping both recipient and potential donors to determine the closest HLA match.^[12]

Human leukocyte antigens

Bacteria

Most bacteria produce antigenic substances on the outer surface that can be distinguished by serotyping.

• Almost all species of Gram-negative bacteria produce a layer of lipopolysaccharide on the outer membrane. The outermost portion of the LPS accessible to antibodies is the O antigen. Variation in the O antigen can be caused by genetic differences in the biosynthetic pathway or the tranporter used to move the building-blocks to the outside of the cell.^[13]
• The
  flagella on motile bacteria is called the H antigen in serotyping. Minute genetic differences in the components of the flagella lead to variations detectable by antibodies.^[14]
• Some bacteria produce a
  polysaccharide capsule, called the K antigen in serotyping.^[15]

Some antigens are invariant among a taxonomic group. Presence of these antigens would not be useful for classification lower than the species level, but may inform identification. One example is the enterobacterial common antigen (ECA), universal to all Enterobacterales.^[16]

E. coli

E. coli have 187 possible O antigens (6 later removed from list, 3 actually producing no LPS),^[17] 53 H antigens,^[18] and at least 72 K antigens.^[19] Among these three, the O antigen has the best correlation with lineages; as a result, the O antigen is used to define the "serogroup" and is also used to define strains in taxonomy and epidemiology.^[17]

Shigella

Shigella are only classified by their O antigen, as they are non-motile and produce no flagella. Across the four "species", there are 15 + 11 + 20 + 2 = 48 serotypes.^[6] Some of these O antigens have equivalents in E. coli, which also cladistically include Shigella.^[20]

Salmonella

The

Streptococcus pneumoniae has 93 capsular serotypes. 91 of these serotypes use the Wzy enzyme pathway. The Wzy pathway is used by almost all gram-positive bacteria, by lactococci and streptococci (exopolysacchide), and is also responsible for group 1 and 4 Gram-negative capsules.[24]

Viruses

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Other organisms

Many other organisms can be classified using recognition by antibodies.

• The malaria pathogen Plasmodium falciparum is notorious for its many surface antigen variants.^[25] A certain vaccine candidate is designed to cover all of these serotypes.^[26]
• Toxoplasma gondii can be classified into serotypes.^[27]
• Trypanosoma cruzi, which causes Chagas disease, can be serotyped using whole parasites.^[28]

See also

• Biovar
• Morphovar

References