Production of an extracellular toxic complex by various strains of Klebsiella pneumoniae.

Six isolates of Klebsiella pneumoniae (two serotype 1 isolates and a capsular variant of one of these, and two serotype 2 isolates and a capsular variant of one of these) possessing various degrees of virulence in rats and mice were examined for their in vitro production of an extracellular toxic complex (ETC). The ETC has been shown to be lethal for and produce extensive lung pathology in mice. This compound has been shown to be composed of capsular polysaccharide, lipopolysaccharide, and a small amount of protein. All six isolates produced the ETC. Immunization of experimental animals with sublethal doses of the ETC was protective against both homologous and heterologous strains, and this protection was due to antibody production. An examination of the various phases of growth of K. pneumoniae showed that there was extracellular release of the component parts of the ETC occurring during all phases of growth. The presence of the ETC in the supernatant fluids was due to actual release of this material as opposed to cell lysis. Antibodies to the lipopolysaccharide portion (which has been shown to possess the observed toxicity) of the ETC were protective against the homologous bacterium.