BACTERIA 19 



Another group of soil bacteria that are of great importance, 

 particularly to the farmer, are' the nitrifying bacteria. The 

 decay-producing bacteria, as we have seen, decompose the 

 remains of plants and animals into simpler substances, but 

 not for the most part into materials which higher plants, like 

 the squash and the cucumber, can use for food. Among 

 the substances formed by decay are compounds containing 

 ammonia. The nitrifying bacteria change these ammonia 

 compounds first into a class of compounds called nitrites, 

 and then into nitrates, and the nitrates are substances which 

 the cucumber and other garden and farm plants can absorb 

 through their root hairs and can use in the building up of their 

 own bodies. It is not sufficient, therefore, that the soil be 

 fertilized by the addition of organic substances, such as 

 manures ; it must contain also both the decay-producing and 

 the nitrifying bacteria to change these substances into a form 

 suitable for plant food. 



28. Bacteria in Milk. One of the means used to measure 

 the cleanliness of milk is to count the bacteria contained in a 

 certain amount. The number of bacteria present varies 

 enormously with the care taken to secure clean milk and to 

 prevent the entrance of bacteria while handling and trans- 

 porting it. If extreme care is taken, the number may be kept 

 down to a few hundred per cubic centimeter. In certified 

 milk the number of bacteria is frequently limited to 10,000 

 per cubic centimeter, and in some cities milk cannot be sold 

 that contains more than 250,000 bacteria per cubic centi- 

 meter. On the other hand, the bacterial content of dirty 

 milk offered for sale has been found to run in some cases to 

 more than 15,000,000 per cubic centimeter. Of the many 

 kinds of bacteria found in milk, some are harmless and pro- 

 duce no particular effect ; some cause disease in persons who 

 drink the milk, and these, of course, are the forms that it is 

 especially important to keep out ; and some produce fermen- 

 tations of various kinds, most of which make the milk unfit 



