A serotype or serovar is a distinct variation within a species of bacteria or virus or among immune cells of different individuals. These microorganisms, viruses, or cells are classified together based on their surface antigens, allowing the epidemiologic classification of organisms to a level below the species.[1][2][3][clarification needed] A group of serovars with common antigens is called a serogroup or sometimes serocomplex.[clarification needed]
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 streptococci by the American microbiologist Rebecca Lancefield in 1933.[4]