SUMMARY: The ability to protect mice against intracerebral infection and to elicit, after one dose, complement-mediated bactericidal antibody capable of killing a mouse-virulent phase I strain , was tested in seven cultures of : three typical phase I strains; two strains grown in the presence of 0.5 mg nicotinic acid/ml; one phase IV strain; and the atypical strain 134. The typical strains showed both activities: the nicotinic acid-grown and phase IV strains elicited high antibody titres, but did not confer protection, whereas strain 134 protected, but did not elicit bactericidal antibody.

Pyrogenic material extracted from dried organisms by hot phenol and water (lipopolysaccharide) was not antigenic in mice; however, with all strains, except 134, multiple doses of this material coupled to stromata or conjugated protein elicited bactericidal and precipitating antisera in mice, although the mice were not protected.

Strain 134 contained approximately one quarter of the normal amount of endotoxin, as assayed in actinomycin D-sensitized mice, but was as effective as normal strains as an adjuvant for haemolysin production.


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