BACTERIA 21 



familiar process due to bacteria is the fermentation that 

 produces vinegar. 



31. Ptomains. — We have seen that bacteria decompose 

 organic substances into a great number of simpler compounds. 

 Some of these simpler compounds are useful to the bacteria 

 as food ; but many of them are merely b}'-products. Among 

 the latter are a class of substances called ptomains. Some 

 of the ptomains, produced for example in decaying meat, 

 cheese, or milk, are extremely poisonous to human beings. 

 Many deaths are caused by eating parth- spoiled foods, 

 especiallj' canned meats, in which ptomains have been formed 

 by the action of bacteria. 



32. Disease-producing Bacteria. — Although only a small 

 proportion of the known species of bacteria are parasitic, the 



A 



A ^ C ^ 



Fig. q. — Disease-producing bacteria. .4, the tuberculosis bacterium; 

 B, the diphtheria bacterium; C, the bacillus of typhoid fever; D, the 

 bacillus that causes lockjaw ; some of the cells are forming spores. 



parasitic forms include those that cause most of the con- 

 tagious and infectious diseases of man and animals. Bac- 

 teria are not responsible for all these diseases ; some, such as 

 malaria and the African sleeping sickness, are produced by 

 one-celled animals; and the organisms which cause some 

 common diseases (for example, smallpox) are unknown. 

 Many important plant diseases, too, are due to bacteria, 

 though the number of such diseases thus far known is smaller 

 than the ntunber of bacterial diseases in animals. Some 



