2IO GENERAL BIOLOGY OF MICRO-ORGANISMS 



of air. Pneumococci and streptococci also attenuate rapidly in 

 artificial /culture. Even those bacteria which retain their viru- 

 lence in ordinary cultures become attenuated when grown at 

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

 septics, both of which methods have been employed in attenuat- 

 ing 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 some- 

 what attenuated upon long residence in the same host in chronic 

 infections. Exaltation of a virus, on the other hand, is accom- 

 plished 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 

 millionths of a gram (.00000005 gram) killed a mouse weighing 

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

 ing 70 Kilos. 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 

 studies. These poisons are rapidly destroyed by heat, resembling 



