Relationship between Particle Size and Biological Activity of E. coli Boivin Endotoxin*

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Abstract
The relationship between particle size and biological activity of Escherichia coli 0:113 endo-toxin (Boivin) labeled with C14 or Cr51 was studied within a density gradient containing sucrose in concentrations ranging from 10 to 34%. Approximately one-half of the Boivin extract collected near the top of the gradient as a relatively nontoxic substance capable of precipitating antiserum in agar but showing little if any capacity to stimulate antibody formation. The remainder of the endotixin complex was uniformly distributed throughout the gradient with peak toxicity in the midportion of the gradient. Radioactivity from chromate-labeled endotoxin seemed intimately associated with toxicity, in contrast to that from C14, which was uniformly distributed throughout the endotoxin complex and produced a high peak at the top of the gradient. Dodecyl sulfate reduced the endotoxin into small monodisperse particles that appeared near the top of the gradient with a sharp reduction in ability to stimulate antibody formation but no demonstrable loss of toxicity or chromate label. The results indicate that toxicity is not a function of particle size, despite an apparent association between size and toxicity in endotoxin prepared by the Boivin technique. This association of toxicity with large particles suggests that the toxic subunits are assembled into large aggregates during synthesis of the bacterial cell wall.