Mice were injected intraperitoneally with one of the following bacterial products having endotoxin activity: pertussis vaccine, a suspension of heat-killed cells of Klebsiella pneumoniae (type C), or a purified lipopolysaccharide prepared from cultures of Salmonella typhosa.

Following treatment with either one of these materials, the animals were infected intravenously with virulent cultures of coagulase-positive staphylococci, with bovine tubercle bacilli, or Friedländer bacilli.

The effect of treatment with endotoxin materials on resistance to Friedländer bacilli, staphylococci, or tubercle bacilli was estimated by observing the mortality rates in infected animals, and by determining quantitatively the numbers of living bacteria in the organs at different periods of time after infection.

It was found that mice receiving the infective dose of virulent culture a few hours after treatment with the endotoxin material, were usually more susceptible to infection than were untreated animals. In contrast, mice infected at a later period proved far more resistant to infection than did untreated animals.

The duration of the negative and of the positive phase of resistance was affected by the amount of endotoxin injected. Marked increase in resistance of mice to infection with staphylococci or tubercle bacilli was still evident several weeks after treatment with pertussis vaccine or with purified lipopolysaccharide extracted from typhoid bacilli.

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