INFECTION AND INTOXICATION. 67 



organism is isolated and grown experimentally the 

 unnatural environment begins to tell upon it, and its 

 biological characteristics begin to change. As it is 

 transplanted again and again, artificial selection comes 

 into play to modify the species. To illustrate this, I will 

 mention the case of a streptococcus that is growing in the 

 laboratory at the time of this writing. This micro- 

 organism was isolated from the blood of a patient dead 

 of puerperal sepsis. It at first grew very scantily upon 

 glycerin agar-agar and blood-serum, and was fatal to rab- 

 bits in intravenous injections of y 1 ^ c.c. of a 24-hour-old 

 bouillon culture. It has now been cultivated for a year 

 or more, during which time its virulence has apparently 

 entirely disappeared, so that at present 5 c.c. fail to pro- 

 duce any symptoms when injected into the ear vein of a 

 rabbit. It has also greatly altered its cultural appear- 

 ances, so that it now grows luxuriously upon glycerin 

 agar-agar and blood-serum. The attenuation of this 

 streptococcus probably depends, in large measure, Upon 

 the fact that but few of its individual cocci were capable 

 of growing upon artificial media, and in transplanting it 

 these vegetative cocci and their progeny have been 

 gradually picked out, until they have entirely outlived 

 the less vegetative but more pathogenic members of the 

 colonies. 



If this coccus had been treated differently, it might have 

 become as virulent as that with which Marmorek experi- 

 mented. He kept his experiment organism constantly 

 passing from rabbit to rabbit, using in the intervals a 

 culture-medium consisting of bouillon containing asses' 

 serum, human serum, ascitic fluid, etc. In this manner 

 he was able to produce a coccus of very moderate vegeta- 

 tive capacity, but so extraordinary in virulence that one 

 one-hundred millionth of a cubic centimeter was suf- 

 ficient to kill a rabbit. 



Many bacteria attenuate when kept in the laboratory. 

 A few species, like the Bacillus anthracis, are so persist- 

 ently virulent that when for experimental purposes it is 



