64 AGRICULTURAL BACTERIOLOGY 



the depth of the stratum from which it comes. That de- 

 rived from superficial layers will contain many bacteria, 

 while that from the deeper levels is free from them. The 

 surface waters vary widely in bacterial content, the chief 

 determining factors being the amount of organic matter 

 that they receive and the amount of soil carried by the 

 water. The bacteria are found in the greatest depths of the 

 ocean that have been examined. 



Factors that keep the number of bacteria reduced are 

 operative in both soil and water. The cycle of life in water 

 is something as follows: Many of the protozoa feed upon 

 bacteria; the protozoa in turn are used as food by the 

 Crustacea, which make up a large part of the food of many 

 fish. If it were not for these restraining factors, many of 

 our waterways into which sewage is discharged would be- 

 come so offensive as to cause a nuisance. 



Lack of food and moisture do not permit the growth of 

 bacteria in the air. They are found there, however, in 

 widely varying numbers. The soil is undoubtedly the chief 

 point of origin. The greater the dust content of the air, 

 the greater will be its content in microorganisms. The air 

 has a characteristic bacterial flora, which consists of forms 

 that are able to resist desiccation. 



The number of dust particles in the air is greater in the 

 city than in the open country. It has been shown that 

 only a few of the dust particles carry bacteria. The studies 

 of Whipple on the air of school-rooms showed an average 

 dust content of 871,000 per cubic foot, and but 113 bacteria 

 in the same volume. 



The tissues of healthy animals are free, or relatively free, 

 from bacteria; but within the alimentary tract they grow 

 profusely at all levels, except within the stomach, where 

 they are restrained by the acid of the stomach juices. In 

 the intestine conditions are especially favorable with refer- 



