Fluorescent-antibody approach to study of rhizobia in soil.

Application of fluorescent-antibody (FA) techniques to the study of rhizobia as free-living soil bacteria was explored. Antiserum to a particular strain of Rhizobium japonicum proved specific in both agglutination and FA tests. Within the R. japonicum group, 2 of 12 strains were stained by the conjugate and these fluoresced brightly; all others were entirely negative. FA tests were negative for 7 strains of R. meliloti, 9 strains of R. leguminosarum, 9 strains of R. trifolii, 6 strains of R. phaseoli, and 65 unidentified bacteria isolated from 12 soils. R. japonicum grew in autoclaved soil and was readily detectable by FA examination of contact slides. The FA technique also detected antibody-reacting bacteria in a field soil whose rhizobial content was unknown. Fluorescent cells were probably R. japonicum, since nodules developed on soybean plants grown in the same soil sample and FA preparations of the crushed nodules proved positive. Autofluorescence was not a problem, but nonspecific adsorption of conjugate restricted observations to microscopic fields free from soil particles. Nonspecific adsorption was substantial, irrespective of the soil used.