Oral Immunization of Rats Against Pneumococcus Types 2 and 3.

Abstract

We have previously reported on the successful immunization of white rats against pneumococcus Type 1 by oral administration of the living 1 or acid killed germ. 2 We wished to see whether our results with Type 1 could be duplicated with Types 2 and 3. The organisms were grown on glucose meat extract media but no HCl acid was added to kill the growth. After centrifuging, the germs were mixed with cracker meal and water, and growth from 50 ccm. was fed to each rat daily for 18 to 21 days. Tests were made by injecting intraperitoneally in volume of 0.20 ccm.

When the first feeding experiment with Type 2 was completed it was found that the treated animals tolerated large doses, but that the controls did the same. On reexamining virulence tests which had been made earlier, and, on extending these tests, it was learned that small untreated rats (up to ca. 85 to 95 grammes) were relatively as susceptible (with exceptions) to Type 2 as to Type 1, 10⁻⁷ and 10⁻⁶ cc. being fatal doses. However, rats, from this range of weight and above, showed a distinct tendency to rapidly increasing resistance. The age or weight at which this relative immunity begins to develop cannot be given more definitely, since individuals in the same group as well as whole groups of animals vary in this respect. The increased resistance is entirely out of proportion to the increased weight, and at its maximum is of the magnitude obtained to Type 1 when this germ is fed. It may be that the tolerance of the larger animals to Type 2 follows, with changing diet, the

appearance among the intestinall flora of an organism closely related immunologically to this germ.