Since Salmonella typically exhibit phase variation between two motile phenotypes,[3] different "H" antigens may be expressed.
A common set of working antisera is shown below: Laboratories that are likely to investigate typhoid also carry antiserum raised against the Vi antigen.
A set of "Rapid Diagnostic Sera" is also held and is used for determination of common specific H-antigens except i-H. After obtaining a positive agglutination with the polyvalent-H specific and non-specific antiserum, the three RDS antisera are used to identify the H antigen present.
Depending on the pattern of positive and negative reactions with the RDS antisera, the specific H antigen may be identified: E = polyvalent for eh, enx, etc.
This use of the O and H symbols is based on the historic observations of Edmund Weil (1879–1922) and Arthur Felix (1887–1956) of a thin surface film produced by agar-grown flagellated Proteus strains, a film that resembled the mist produced by breath on a glass.