BACTERIA IN AIR, SOIL, WATER, AND FOOD 173 



proofs of the greater frequency of " contact " in- 

 fection. 



In Soil. Besides the saprophytes which grow and 

 multiply in the soil, and are necessary to the soil and 

 to the higher plants, there are several pathogenic 

 bacteria which find favorable resting places in the 

 soil, notably tetanus, and typhoid-fever bacilli, 

 and the spirilla of Asiatic cholera. 



Cultivated garden soil, which is frequently ferti- 

 lized, shows the greatest number of bacteria, owing to 

 the presence of dead organic material, and sandy soil 

 the least. In favorable soils bacteria may be found 

 to the depth of four or five feet in the soil. 



Wherever filth of any kind, as manure, sewage, and 

 decaying vegetation, is deposited in or upon the soil, 

 pathogenic bacteria are usually found. Their growth 

 and multiplication in the soil is still a disputed ques- 

 tion, although typhoid bacilli and the spores of 

 tetanus may survive and retain their virulence for 

 many months in soil containing large quantities of 

 organic matter, such as would be found in a garden. 

 Experiments have proven, however, that in time 

 the soil bacteria will overcome the disease-producing 

 organisms, or in other words the soil purifies itself. 



In Water. --Water outside of the laboratory is 

 never free from bacteria, although most of them are 



