Agglutination of avirulent strains of Pseudomonas sofanacearum by potato lectin

Abstract

Fifty-five virulent isolates and 34 avirulent variants of Pseudomonas solanacearum from different geographic regions and representing all major races and biotypes were tested for their ability to bind to potato lectin. The lectin was extracted from “Katahdin” potato tubers and purified to homogeneity. All avirulent isolates agglutinated strongly with the lectin, whereas virulent isolates either failed to agglutinate or agglutinated only weakly and at much higher lectin concentrations. Failure of the virulent cells to bind to the lectin was correlated with the presence of extracellular polysaccharide (EPS) which is formed by virulent, but not avirulent, cells. Virulent cells were agglutinated strongly by potato lectin after most of the EPS was removed by repeated washing and centrifugation. Agglutination of avirulent cells or washed virulent cells could be totally prevented by the addition of EPS to the cells prior to addition of lectin. The weak agglutination exhibited by some unwashed virulent cells in suspension could also be prevented by addition of EPS. This indicates that the amount of EPS formed in culture and released in water suspensions determines the degree of agglutination obtained when lectin is added. Potato lectin, conjugated with fluorescein isothiocyanate, bound to rabbit erythrocytes and to avirulent, but not to virulent, cells of P. solanacearum . Moreover, binding of lectin to avirulent cells was hapten-specific and could be inhibited by chitin oligomers. Purified lipopolysaccharide (LPS) from P. solanacearum K60-B1 cells precipitated when mixed with purified potato lectin. Initial evidence indicates that binding sites for the lectin are present in the lipid A portion of the LPS.