Morphology of endotoxin-like particles released by Vibrio cholerae non-O1.

The morphology of "free endotoxin" in cell suspensions of Vibrio cholerae non-O1 and in fractions purified from culture filtrate (Fraction 1) was examined at the beginning of the exponential phase of growth in complex medium. The observed structures were compared with those of the lipopolysaccharide obtained by phenol extraction. Blebs, vesicles, and ribbons with a trilaminar, membrane-like structure were observed by electron microscopy of ultrathin sections and negatively stained samples. Endotoxic activity was detected in these samples by the Limulus amoebocyte lysate pyrotest. The free endotoxin appeared as a heterogeneous population of particles in Fraction 1 (20-60 nm in diameter) and in the bacterial suspension (10-40 nm in diameter) while the lipopolysaccharide obtained by phenol extraction of the cells was revealed to be a uniform particle population (45 nm in diameter).