Covalently linked hopanoid-lipid A improves outer-membrane resistance of a Bradyrhizobium symbiont of legumes

Lipopolysaccharides (LPSs) are major components of the outer membrane of Gram-negative bacteria where they act as a structural barrier and play an important role in the interaction with eukaryotic hosts. The work reported in the article demonstrates that a photosynthetic Bradyrhizobium strain, symbiont of Aeschynomene legumes, synthesizes several hopanoids among which one is covalently attached to lipid A thus forming a unique LPS which reinforces the stability and rigidity of the outer membrane.

A hopanoid-deficient strain, lacking a squalene hopene cyclase, displays increased sensitivity to stressful conditions and reduced ability to survive intra-cellularly in the host plant. This unusual combination of hopanoid and LPS molecules may represent an adaptation to optimize bacterial survival in both free-living and symbiotic states.