BACTERIA OF NITRIFICATION AND NITROGEN FIXATION 345 



vent the soil from becoming depleted of plant nutrients. Of 

 course they attack meats, canned fruits, and many other things 

 which we do not wish to have decomposed, but the good they do 

 more than compensates the harm. Methods, such as cold stor- 

 age, applications of salt and other preservatives, and canning, are 

 employed in checking or preventing the activity of Bacteria in 

 foods. In cold storage the temperature is too low for them to be 

 active. Salt solutions keep them dormant by extracting water 

 from them. In canning those present are killed by heat, and by 

 seaHng the cans others are prevented from entering. Alcohol, 

 formaldehyde, car])olic acid, etc., are useful in preventing bac- 

 terial action in materials not intended for food. 



Bacteria of Fermentation. — These Bacteria attack carbohy- 

 drates and break them into simpler substances, such as alcohol, 

 lactic acid, acetic acid, butyric 



acid, etc. A few forms are c:^^^^ '^'^cscs 



shown in Figure 298. The _a^^^^^ c^'?^^^'^ 

 product produced depends 

 upon the substance attacked 

 and the kind of Bacteria at 

 work. For example in the 

 fermenting of cider, some Bac- 

 teria break the sugar into . ^ig. 298. — Bacteria of fermenta- 



alcohol and carbon dioxide, j°"„ "' .' ^^\ c, vinegar ac eria, 



' d. Bacteria that ferment milk; e, 

 while others attack the alco- butyric acid Bacteria. X 1000. Re- 

 hol, changing it into acetic drawn from Fisher. 

 acid. All the forms working 



together change the cider into vinegar. After the vinegar Bac- 

 teria become inactive, due to the exhaustion of the food supply 

 or the accumulation of the fermented products, they form the 

 well-known mother of vinegar, which consists of the Bacteria 

 held together in a gelatinous matrix. In milk, certain kinds of 

 Bacteria attack the milk sugar and change it into lactic acid. 

 Another kind produces butyric acid in butter, turning it rancid. 

 Bacteria of Nitrification and Nitrogen Fixation. — In the soil 

 there are some kinds of Bacteria that change certain nitrog- 

 enous compounds of manure and other organic matter into 

 nitrates in which form the nitrogen is available for crops. Tlie 

 advantage to the Bacteria is that they secure energy in tliis 

 way from these compounds, while the advantage to the soil is 



