Antigens and toxins

ANTIGENS AND ITS VARIATIONS

Antigens

Flagellar antigen

  • It is heat labile protein when mixed with H antisera, agglutinates rapidly producing large, loose fluffy clumps.
  • The H antigen is strongly immunogenic. They have the unique character of diphasic variation.

Somatic antigen

  • It is a heat stable phospholipid protein polysachharide complex, which forms an integral part of the cell wall.
  • It is identical with endotoxin.
  • Boivin is extracted from the bacterial cell by treating with trichlor acetic acid hence, it is called as Boivin antigen.
  • When mixed with O antiserum, it forms compact chalky granular clumps. The O antigen is less immunogenic than H antigen.

Vi antigen

  • Many strains of S.typhi, S.paratyphi and S.dublin fail to agglutinate with O antiserum due to the presence of Vi antigen enveloping the O antigen.
  • The Vi antigen is related to virulence and may be lost by serial subculture. The Vi antigen is poorly immunogenic.
  • In addition to endotoxin, enterotoxins similar to heat labile entero toxin of E.coli and cholera toxin are produced. The cytotoxin similar to shigella cytotoxin is also produced.

Antigenic variations

  • The antigens of salmonellae frequently undergo phenotypic and genotypic variations

H-O variation

  • This variation is associated with loss of flagella. When Salmonella are grown in agar containing phenol (1 in 800), flagella are inhibited. This change is phenotypic and temporary.
  • To attain a population of Salmonella rich in H antigen Craigie’s tube or U tube on soft agar can be employed.

Phase variation

  • Phase 1 antigen is either specific for a species or shared by a few species only. Hence termed as specific phase.
  • Phase 2 antigens are widely shared and hence termed as nonspecific or group phase.
  • Phase 1 antigens are designated as a, b, c…Z and after Z as Z1, Z2, Z3…etc. Phase 2 is designated as 1,2…etc.
  • The flagellar antigens of salmonellae occur in 1 or 2 phases.
  • Strains that posses both phases are called diphasic and strains having 1 phase are called as monophasic.

V-W variation

  • Fresh isolates of S. typhi carry a surface Vi antigen that completely masks the O antigen.
  • Such bacillus does not agglutinate with O antiserum but agglutinable with Vi antiserum. This is called as V form.
  • After repeated sub culturing the Vi antigen is completely lost and does not agglutinate with Vi antiserum but agglutinate with O antiserum.
  • This is called as W form. Intermediate stages which agglutinate with both Vi and O antisera is called VW form.

S-R variation

  • The smooth to rough variation is associated with the change in colony morphology, loss of O antigen and virulence.
  • The colonies become large, rough and irregular.
  • R forms occur by conversion into mutation.
  • It can be prevented by maintaining the cultures by lyophilisation.

Variation in O antigens

  • Changes in the structure of O antigens may be induced by lysogenisation with certain phages resulting in alteration of serotypes.
Last modified: Monday, 4 June 2012, 5:17 AM