Immunity to pneumococcus afforded rats by feeding them the germ.

Abstract

In December it was reported 1 that rats became resistant to pneumococcus injections if fed on the tissues of other rats which had been killed by the same organism. It was shown that such animals tolerated 1000 or more times the dose of pneumococci that proved fatal for control rats. It was suggested that the living germs present in the tissue being fed might be the real cause of this increased resistance. Attention was also called to the fact that experiments had been done in which pneumococci were fed with the object of seeing whether a similar immunity could be built up in this manner. Our experiments at that time indicated that feeding the cocci from 50 cm. of culture per day to a rat produced a decided degree of immunity. It is our intention in the present report to give a representative experiment (our latest), in tabular form and to show that an immunity almost as good as, if not quite equal to that produced in the tissue feeding experiment, can be produced by feeding live pneumococci. Tests have also been made in which the dead germs have been fed, and are included in the table. The immunity built up by this latter method, is not so good as that obtained with living organisms, when fed in equal quantities.

Each of the rats listed in the accompanying table received the germs from approximately 50 cm. of a 24 hour culture of pneumococcus Type I, daily, for about 20 days. The organisms were grown in meat infusion media. The culture was centrifuged, the germs suspended in 0.1 per cent gelatin solution, stirred, and cracker meal added and mixed. This was spread out on a shallow dish and given to the rats several hours after the last meal.