A study has been made of the time variations in the susceptibility of albino mice to experimental infections with Klebsiella pneumoniae, Staphylococcus aureus, and Mycobacterium tuberculosis.

Susceptibility to infection was determined by two criteria: (a) mortality rates following intravenous injection of a known infective dose; (b) numbers of bacterial colonies that could be recovered from the tissues of the infected animals at various times after infection. When measured in terms of either one of these two criteria, susceptibility was consistently modified by temporary deprivation of food, and by various changes in the composition of the diet.

Increase in susceptibility to infection could be detected within a few hours to a few days depending upon the nature and intensity of the nutritional disturbance imposed upon the animal. Under the proper conditions, return to a normal state of resistance could also occur within a very short time. There was commonly observed an explosive bacterial multiplication in mice receiving the infective dose shortly after being subjected to nutritional disturbances. In the case of infection with Klebsiella pneumoniae, however, this phase of increased susceptibility was often followed a few hours later by one during which the numbers of living bacteria that could be recovered from the various organs fell to a very low level. Many of the animals which had exhibited severe but transient bacteriemia recovered rapidly and survived the infection.

Mice rendered more susceptible to infection by being fed deficient diets progressively recovered their normal resistance while being kept on the same inadequate regimens. This adaptation occurred even though the weight of the animals on these regimens remained much lower than the weight of the animals fed a complete diet ad lib.

The weight of the animal at the time of infection appeared to have little bearing on susceptibility. However, susceptibility consistently increased during the periods while the animals were losing weight—whatever the cause of the weight loss.

The facts disclosed in the present study, as well as findings reported by other investigators, make clear that profound changes in susceptibility to infection can be brought about by varied non-specific procedures. These changes can occur within very short periods of time and are often rapidly reversible.

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