O-antigenic determinants in Salmonella species of serogroup C1 are expressed in distinct immunochemical populations of chains.

The O-antigenic specificities found among Salmonellae of serogroup C1 are O:6(1),7, O:6(2),7, O:6(1),6(2),7 and O:6,7,14, as defined by classical serology. Factor O:7 is the group-wide determinant while factors O:6(1), O:6(2) and O:14 are found in some strains but not others. Strains of the O:6(2),7 specificity are subject to lysogenic conversion by phages 6(1) and 14 to the O:6(1),7 and O:6,7,14 specificities, respectively. To further delineate antigenic complexity and serological relationships among strains of this serogroup monoclonal antibodies (mAbs) were generated against the O:6(1),6(2),7 polysaccharide of Salmonella thompson. Five mAbs of either the O:6(1) or O:6(2) specificities did not bind O:6,7,14 strains or LPS, showing that the O:6 determinant in these strains is neither O:6(1) nor O:6(2). Thus antigenic conversion of O:6(2),7 strains by phage 14 is accompanied by addition of O:14 as well as loss of O:6(2). Three mAbs which demonstrated group-wide reactivity, and were thus specific for O:7, recognized clearly by separable epitopes hereby defined as sub-specificities, O:7(1), O:7(2) and O:7(3). Immunoblotting of mAbs against electrophoretically resolved LPS showed that factors O:6(1) and O:6(2) are expressed only in LPS molecules of high molecular mass whereas O:7(2) and O:7(3) are expressed only in relatively low-molecular-mass chains. These results are consistent with the expression of different antigenic determinants in structurally distinct subpopulations of O chains. The implications of the existence of distinct subpopulation of chains is that the published structure of the O:6,7 repeat unit is not fully representative of the O-antigenic structure of this group.