Oral Immunization Experiments with Disrupted,Dissolved and Avirulent Pneumococci. Time of Appearance of Immunity. ∗

Earlier experiments 1 have demonstrated that when dead pneumococci are fed to white rats, the animals become immune to an injection of virulent germs of the same type. The present report deals with the observations made when mechanically disrupted, bile dissolved and avirulent organisms are fed. It also includes a report of experiments in which rats were fed the pneumococcus grown in whole milk.† The mechanically opened cells and the bile-dissolved ones were fed in order to learn whether the intact cell is necessary in this mode of conferring immunity, or whether the contents alone will serve as well. In one of the latter experiments some of the animals were examined a few days following the first feeding, instead of much later as was the custom in the earlier part of this work. The purpose was to observe how quickly the protection is created. Feeding with avirulent pneumococcus was carried out with a group of rats to determine whether virulence is an essential factor. Type I pneumococcus was used throughout. The resistance of the rats was tested by injecting the organism intraperitoneally in a volume of 0.20 cc.

Effect of Feeding Mechanically Disrupted Germs. The organisms were grown in glucose meat extract broth, centrifuged and desiccated in partial vacuum. The scaly material was then pulverized in a pebble mill for about 18 hours. Microscopic examination failed to detect any intact pneumococci. Each rat received the equivalent of 50 cc. of culture per day for 16 days. The powdered material was mixed with milk and placed in the cage. The animals were well protected by this method, surviving 1000 to 10,000 fatal doses. Doubtless, some of the germs escaped disruption in the mill, but it is believed that these were too few in number to have been responsible for the pronounced effect observed.