Serotyping often plays an essential role in determining species and subspecies. TheSalmonellagenus of bacteria, for example, has been determined to have over 2600 serotypes.Vibrio cholerae, the species of bacteria that causescholera, has over 200 serotypes, based on cell antigens. Only two of them have been observed to produce the potententerotoxin that results in cholera: O1 and O139.[citation needed]
Serotyping is the process of determining theserotype of an organism, using preparedantisera that bind to a set of known antigens. Some antisera detect multiple known antigens and are known aspolyvalent orbroad; others aremonovalent. For example, what was once described asHLA-A9 is now subdivided into two more specific serotypes ("split antigens"),HLA-A23 andHLA-A24. As a result, A9 is now known as a "broad" serotype.[5] For organisms with many possible serotypes, first obtaining a polyvalent match can reduce the number of tests required.[6]
The binding between a surface antigen and the antiserum can be experimentally observed in many forms. A number of bacteria species, includingStreptococcus pneumoniae, display theQuellung reaction visible under a microscope.[7] Others such asShigella (andE. coli) andSalmonella are traditionally detected using a slide agglutination test.[6][8] HLA types are originally determined with thecomplement fixation test.[9] Newer procedures include thelatex fixation test and various otherimmunoassays.
"Molecular serotyping" refers to methods that replace the antibody-based test with a test based on thenucleic acid sequence – therefore actually a kind ofgenotyping. 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]
Theimmune 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 byhuman leukocyte antigen (HLA), the human version of themajor histocompatibility complex. Cells determined to be non-self are usually recognized by the immune system as foreign, causing an immune response, such ashemagglutination. 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 totransplant rejection. In some situations, this effect can be reduced by serotyping both recipient and potential donors to determine the closest HLA match.[12]
Most bacteria produce antigenic substances on the outer surface that can be distinguished by serotyping.
Almost all species ofGram-negative bacteria produce a layer oflipopolysaccharide on the outer membrane. The outermost portion of the LPS accessible to antibodies is theO antigen. Variation in the O antigen can be caused by genetic differences in the biosynthetic pathway or the transporter used to move the building-blocks to the outside of the cell.[13]
Theflagella on motile bacteria is called theH antigen in serotyping. Minute genetic differences in the components of the flagella lead to variations detectable by antibodies.[14]
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 theenterobacterial common antigen (ECA), universal to allEnterobacterales.[16]
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 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 inE. coli, which also cladistically includeShigella.[20]
TheKauffman–White classification scheme is the basis for naming the manifold serovars ofSalmonella. To date, more than 2600 different serotypes have been identified.[21] ASalmonella serotype is determined by the unique combination of reactions of cell surfaceantigens. ForSalmonella, the O and H antigens are used.[22]There are two species ofSalmonella:Salmonella bongori andSalmonella enterica.Salmonella enterica can be subdivided into six subspecies.The process to identify the serovar of the bacterium consists of finding the formula of surface antigens which represent the variations of the bacteria. The traditional method for determining the antigen formula isagglutination reactions onslides. The agglutination between the antigen and theantibody is made with a specificantisera, which reacts with the antigen to produce a mass. The antigen O is tested with a bacterial suspension from anagar plate, whereas the antigen H is tested with a bacterial suspension from a broth culture. The scheme classifies the serovar depending on its antigen formula obtained via the agglutination reactions.[8] Additional serotyping methods and alternative subtyping methodologies have been reviewed by Wattiau et al.[23]
Streptococcus pneumoniae has 93 capsular serotypes. 91 of these serotypes use theWzy enzyme pathway. TheWzy 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]
Many other organisms can be classified using recognition by antibodies.
Themalaria pathogenPlasmodium falciparum is notorious for its many surface antigen variants.[25] A certain vaccine candidate is designed to cover all of theseserotypes.[26]
^Baron EJ (1996). Baron S; et al. (eds.).Classification.In: Baron's Medical Microbiology (4th ed.). Univ of Texas Medical Branch.ISBN978-0-9631172-1-2.(via NCBI Bookshelf).
^Ryan KJ, Ray CG, Sherris JC, eds. (2004).Sherris Medical Microbiology (4th ed.). McGraw Hill.ISBN978-0-8385-8529-0.
^"Serovar".The American Heritage Medical Dictionary. Houghton Mifflin Company. 2007.
^abDanan C, Fremy S, Moury F, Bohnert ML, Brisabois A (2009). "Determining the serotype of isolated Salmonella strains in the veterinary sector using the rapid slide agglutination test".J. Reference.2:13–8.
^Frohn C, Fricke L, Puchta JC, Kirchner H (February 2001). "The effect of HLA-C matching on acute renal transplant rejection".Nephrology, Dialysis, Transplantation.16 (2):355–60.doi:10.1093/ndt/16.2.355.PMID11158412.
^Wang, L; Wang, Q; Reeves, PR (2010). "The Variation of O Antigens in Gram-Negative Bacteria".Endotoxins: Structure, Function and Recognition. Subcellular Biochemistry. Vol. 53. pp. 123–52.doi:10.1007/978-90-481-9078-2_6.ISBN978-90-481-9077-5.PMID20593265.
^Yother, J (2011). "Capsules of Streptococcus pneumoniae and other bacteria: paradigms for polysaccharide biosynthesis and regulation".Annual Review of Microbiology.65:563–81.doi:10.1146/annurev.micro.62.081307.162944.PMID21721938.
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