Peptidoglycan-bound Polysaccharide Associated with Resistance of Rhizobium loti Strain NZP2037 to Lotus pedunculatus Root Flavolan

SUMMARY: Resistance of Rhizobium loti strain NZP2037 to the prodelphinidin-rich flavolan (condensed tannin) present in the roots of Lotus pedunculatus was associated with the presence of a peptidoglycan-bound flavolan-binding polysaccharide (FBP) in the outer cell membrane of this bacterium. The outer cell membrane of a flavolan-sensitive strain, NZP2213, did not contain this polysaccharide. FBP was extractable in highest concentration from NZP2037 cells during exponential growth of the bacteria. This correlated with exponential-phase cells of NZP2037 showing maximum resistance to flavolan. When added to cultures of NZP2213, FBP afforded a concentration-dependent protection of this strain to the bactericidal effect of the flavolan. ELISA and immunogold labelling of Rhizobium cells using FBP-specific antibodies confirmed the presence of FBP on the outer cell surface of NZP2037 cells and its absence from NZP2213 cells. Co-inoculation of L. pedunculatus with antibiotic-resistant mutants of NZP2037 and NZP2213 resulted in the formation of N2-fixing nodules containing both strains.