THE BACTERIA AND VIRUSES 351 



rapid consumption of the available food and the accumulation of injurious 

 excretory products slows down the growth rate progressively until a stationary 

 state is reached. When this occurs there is a rapid natural death, the number 

 of survivors falling logarithmically, so that after seventy-two hours less than 

 I per cent, of the organisms may be left alive. A few resistant individuals, 

 however, may persist for weeks or even months. 



Certain Bacteria, namely members of the family Bacillaceae, have the 

 power of forming spores (Fig. 339), sometimes called endospores because 

 they form inside the body of the organism and are liberated by its dissolution. 



9 



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• 



s 



Fig. 339. — Clostridium sporogeties. Rods showing 

 development of spores. Gram stained. 



These spores possess an exceedingly impervious coat and show 

 astonishing tenacity of life. They can withstand the action of many powerful 

 killing agents, such as absolute alcohol or phenol, and they can remain alive 

 for years in a dry state. They are also very resistant to extremes of tempera- 

 ture. Some, such as the spores of the anthrax bacillus, can withstand more 

 than an hour's exposure to boiling w^ater, while others have been found to 

 germinate even after having been immersed in liquid Helium ( - 269° C.) 

 for several hours. 



These properties make it very difficult to kill them, and it is indeed 

 fortunate for us that very few disease-producing Bacteria form spores. 

 The most important exception is Bacillus tetani, the cause of lock-jaw, 

 which, however, does not enter the blood but remains at the site of the 

 wound which gave it entrance to the body. 



Bacteria and Disease 



The first bacterium identified as the cause of a disease was the anthrax 

 bacillus, first observed by Rayer in 1850 and recognized as a micro-organism 



