Using 131I-labelled rabbit antiflagellar antibody and a strain of Salmonella typhimurium fixed in the specific phase, we have measured the number of antibody molecules per bacterium that are absorbed from antibody in a concentration that immobilizes 50 per cent of an initially fully motile suspension of organisms.
For intact IgG antibody the 50 per cent immobilizing dose varied from 132 to 367 molecules attached per bacterium (and was usually under 200). In contrast, Fab′ univalent antibody fragments, prepared from IgG antibody, had no perceptible effect on motility, even when 1.37 × 105 or more molecules were attached to each bacterium.
We conclude that antibody must be at least bivalent to immobilize and that it acts by linking flagella together and not by interfering with the propagation of wave motion along individual flagella.