Abstract
The Department of Bacteriology and Experimental Pathology of the University of California possesses a strain of hemolytic streptococcus which has been maintained constantly for a period of years by culture upon blood agar in test tube and by passage in vivo through rabbits. Intrapleural injection is used and the organism has been stored in the pleural exudate produced. This fluid exudate between passages is kept at icebox temperature. Injections are made from the previous pleural fluid after three to five weeks. To date this in vivo strain has passed through sixty-five animals or “generations”.
The virulence of the passage portion of this strain is such that 1/100,000 to 1/1,000,000 cc. of this fluid is lethal to a 3,500 gm. rabbit. On the other hand, one cubic centimeter of the portion which has been propagated in test tube culture does not induce death in an animal of similar weight. The numbers of organisms per cubic centimeter in each inoculated fluid are comparable.
An interesting difference in heat resistance has developed between these two portions of this strain of S. hemolyticus. Using exposure to heat in a water bath and with all conditions as nearly similar as are experimentally possible, including numbers of organisms in the inoculum, the diluting medium and with properly controlled thermometers it has been determined that the more virulent organisms are more susceptible to the lethal effects of heat than are those the virulence of which is decreased.
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