Visualization of lipopolysaccharide aggregates by freeze-fracture and negative staining

The morphology of Escherichia coli 0111:B4 lipopolysaccharide (LPS) in aqueous medium was studied by freeze-fracture and negative staining. Samples processed by freeze-fracture showed LPS aggregates that were mainly spherical or elliptical and of rather homogeneous size. Negative staining, however, showed a more heterogeneous population, although globular structures revealed by both procedures had a similar size. Considering the mechanisms involved in the processing of the samples, we suggest that the genuine shape of LPS aggregates is more likely to be globular, thus being artefactual those forms visualized by negative staining, traditionally associated with the organization of LPS in aqueous suspension.