The ultrastructural morphology of endotoxins and lipopolysaccharides

Endotoxins and LPS are constituents unique to the outer surface of gram-negative bacteria. Cell-associated endotoxins are now readily observable on the cell outer membrane with labelled monoclonal antibodies. These probes are not only more specific than those used in the past, but also easier to see. Interest in free endotoxin as a method to generate outer membrane proteins without contamination with other cell constituents is also increasing (Gamazo and Moriyon, 1987). The morphologic identification and characterization of LPS by electron microscopy has been facilitated recently by advances in chemical extraction and purification techniques. LPS, originally thought to be heterogenous, exists in forms that are dependent upon (1) the method of its extraction, (2) its chemical composition, and (3) the physical or chemical conditions of its environment. New models were proposed on the arrangement of LPS molecules in molecular aggregates (i.e. discs, vesicles or ribbons) and a schematic was presented on the dissociation from one morphologic type to another. Morphologic studies on endotoxins and LPS will continue in the future. Using molecular biological techniques, carbohydrate epitopes of LPS from one bacterial species will be expressed with increasing frequency in other bacterial species (Manning et al., 1986; Stein et al., 1988). Electron microscopy will help visualize the distribution of the 'new' LPS on the recipient cell surface. Labelled monoclonal antibodies will also differentiate host cell LPS from the recombinant LPS. As molecular model programming becomes more complex, new schematics will help visualize the arrangement of LPS in membranes to explain recombinant LPS structure as well as other characteristics (i.e. membrane permeability to various antibiotics).