SUMMARY: An evaluation of the ELISA technique was made in order to obtain a very specific and sensitive method for strain identification of Antisera against an strain were produced by using as antigen either whole cells or purified lipopolysaccharides from the cell wall. The use of whole cells as antigen gave rise to antibodies which were more sensitive and strain specific than those produced from purified lipopolysaccharides. A further evaluation of the ELISA method was the use of a new type of conjugate consisting of a purified antibody linked to β-galactosidase by using as coupling reagent a hetero-bifunctional reagent, -succinimidyl-3-(2-pyridyldithio)propionate (SPDP), which contains two reactive groups directed towards different functional groups. Substrate for this type of conjugate consisted of -nitrophenyl-β--galactopyranoside. This improved technique makes it possible to detect bacteria in samples containing 1 × 10 cells ml whilst retaining strain specificity. Furthermore, preparations of nodules formed by different strains containing 1 × 10 cells ml could be analysed whilst retaining strain specificity. The technique presented thus offers advantages in higher sensitivity and reliability than conventional ELISA techniques.


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