PATHOGENIC PROPERTY OF MICRO-ORGANISMS 203 



air. Pneumococci and streptococci also attenuate rapidly in 

 artificial culture. Even those bacteria which retain then- 

 virulence in ordinary cultures become attenuated when grown at 

 unusually high temperatures (42 C.) or in the presence of 

 antiseptics, both of which methods have been employed in 

 attenuating the anthrax bacillus. Attenuation also results 

 sometimes from parasitism in hosts of another species. Variola 

 and vaccinia present a conspicuous example of this. Mere 

 dessication of a virus seems to attenuate it in some instances 

 (rabies) but this is somewhat doubtful. Many pathogenic 

 agents become somewhat attenuated upon long residence in the 

 same host in chronic infections. Exaltation of a virus, on the 

 other hand, is accomplished by rapid passage through susceptible 

 animals in series. When the organism is too attenuated to 

 produce an infection alone, it may be aided by the admixture of 

 other organisms (mixed infection) or by the presence of irritating 

 foreign bodies (splinters, stone dust ) or by mechanical protection 

 in collodion capsules. 



Microbic Poisons. The weapons which the pathogenic 

 agent employs to injure its host are various. The physical 

 mass of the invaders may be injurious, more particularly by 

 obstructing blood-vessels, as in estivo-autumnal malaria in man 

 and anthrax in the mouse. Usually, however, the offensive 

 weapons are chiefly chemical poisons. The soluble toxins, or 

 true toxins are substances of unknown chemical composition 

 produced inside bacterial cells and passed out to their surround- 

 ings. These so-called extracellular toxins include the most 

 poisonous substances known. Brieger and Cohn obtained a 

 toxin, still impure, from tetanus bacilli, of which five one hundred 

 million ths of a grams (.00000005 gram) killed a mouse weighing 

 15 grams. At this rate .00023 of a gram would kill a man weigh- 

 ing 70 Kilos. 1 The soluble toxins elaborated by the diphtheria 

 and tetanus bacilli have been studied most, and many of our 

 ideas concerning toxins in general have been derived from these 



1 Vaughan and Novy, Cellular Toxins, Phila., 1902, p. 62. 



