The ultrastructural distribution and immunological accessibility of the variable surface proteins VspA, VspB, VspC and VspD were determined by immunoelectron microscopy on the surface of negatively stained cells of PG45 and 18 subclones, expressing either one or two of the Vsps. The variable proteins VspA, VspB, VspC and VspD, recognized by two monoclonal antibodies (mAb 1E5 and mAb 87-2) and visualized by goat-anti-murine-lgM labelled with gold particles, showed identical distribution patterns on the surfaces of the cells of all clones investigated. Gold particles were distributed over the whole cell surface, arranged in clusters. The cell form seemed not to have an influence on the decoration pattern. Gold particles were also observed in irregular distributions around the cells. All clones showed unlabelled cells as well as strongly and weakly labelled cells. There were in general, however, no significant differences in the percentages of unlabelled, weakly labelled and strongly labelled cells, either between clones expressing different Vsps or between individual clones. No correlations were found between the numbers of labelled cells in immunoelectron microscopy and the numbers of labelled colonies in immunobinding assay (IBA) originating from the same broth cultures. The percentage of positive colonies in IBA was generally much higher than the percentage of positive cells in immunoelectron microscopy. The results show that the cells of the clones are not identical, but differ in their surface antigens, and reveal the high variable potential of this species.


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