Lipid A and O-chain modifications cause Rhizobium lipopolysaccharides to become hydrophobic during bacteroid development

Modifications to the lipopolysaccharide (LPS) structure caused by three dif ferent growth conditions were investigated in the pea-nodulating strain Rhi zobium leguminosarum 3841. The LPSs extracted by hot phenol-water from cult ured cells fractionated into hydrophilic water and/or hydrophobic phenol ph ases. Most of the LPSs from cells grown under standard conditions extracted into the water phase, but a greater proportion of LPSs were extracted into the phenol phase from cells grown under acidic or reduced-oxygen condition s, or when isolated from root nodules as bacteroids. Compared with the wate r-extracted LPSs, the phenol-extracted LPSs contained greater degrees of gl ycosyl methylation and O-acetylation, increased levels of xylose, glucose a nd mannose and increased amounts of long-chain fatty acids attached to the lipid A moiety. The water- and phenol-phase LPSs also differed in their rea ctivity with monoclonal antibodies and in their polyacrylamide gel electrop horetic banding patterns. Phenol-extracted LPSs from rhizobia grown under r educed-oxygen conditions closely resembled the bulk of LPSs isolated from p ea nodule bacteria (i.e. mainly bacteroids) in their chemical properties, r eactivities with monoclonal antibodies and extraction behaviour. This findi ng suggests that, during symbiotic bacteroid development, reduced oxygen te nsion induces structural modifications in LPSs that cause a switch from pre dominantly hydrophilic to predominantly hydrophobic molecular forms. Increa sed hydrophobicity of LPSs was also positively correlated with an increase in the surface hydrophobicity of whole cells, as shown by the high degree o f adhesion to hydrocarbons of bacterial cells isolated from nodules or from cultures grown under low-oxygen conditions. The implications of these LPS modifications are discussed for rhizobial survival and function in differen t soil and in planta habitats.