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 
sealing the cans others are prevented from entering. Alcohol, 
formaldehyde, carbolic 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 
shown in Figure 298. The 
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 Fic. 298. — Bacteria of fermenta- 
re tion. u, b, and c, vinegar Bacteria; 
alcohol and carbon dioxide, d, Bacteria that ferment milk; e¢ 
while others attack the alco- butyric acid Bacteria. > 1000. Be 
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. The 
advantage to the Bacteria is that they secure energy in this 
way from these compounds, while the advantage to the soil is 
