CH. X] THE BACTERIA 405 



(Fig. 277) appears always in hay which has been boiled and 

 left standing, the resting spores being resistant to boiling 

 water ; it is the principal agent in the subsequent decay, or 

 rotting, of the hay. The chief of the putrefactive Bacteria 

 is Bacillus vulgaris (Fig. 278), which produces decay in meat 

 with release of very offensive gases ; and this form, likewise, 

 is able to exist in a variety of forms under diverse condi- 

 tions. The poisonous ptomaines, found at times in im- 

 perfectly preserved foods, are a product of decay Bacteria. 

 Certain definite forms, notably Spirochcete dentium, are 

 associated with the decay of the teeth. And many others 

 occur, in all kinds of substances. The decay of complex 

 materials, like flesh, involves several kinds of Bacteria, one 

 kind reducing the substance to simpler and less highly ener- 

 gized materials, others reducing these to still simpler, and 

 so on, until in successive stages even the most complicated 

 materials become reduced back to the original carbon di- 

 oxide, water, nitrogen, and mineral substances from which 

 they were originally formed. Such action is obviously 

 beneficial in organic nature as a whole, since it prevents a 

 continuous accumulation of the remains of plants and 

 animals, and restores their substance to the general circu- 

 lation for renewed use. Conversely, when the activity ^of 

 decay Bacteria is checked, organic substances do accumulate, 

 as illustrated by the growth of peat bogs, which contain 

 bactericidal substances. Our methods of preserving fruits 

 and meat depend upon inhibition of bacterial activity. 



Fermentation Bacteria. These produce several forms of 

 fermentation, though not the alcoholic kind, which is effected 

 mainly by Yeasts (page 169). The fermentation Bacteria 

 oxidize various substances to organic acids, thus producing 

 a souring. Bacterium aceti (Fig. 278) produces acetic acid 

 from alcohol, thereby converting fermented cider to vinegar. 

 Another produces the lactic acid which makes milk sour. 

 Another produces butyric acid in butter, turning it rancid. 

 Others produce in like manner special flavors and odors 



