BACTERIOLOGY. 47 



The statement made a few lines above 

 Ground, that "the natural home for bacteria is 



the ground" applies to bacteria in general, 

 and as the saprophytes comprise the majority of bac- 

 teria, it is to them chiefly that reference is made. 

 Nevertheless, almost any pathogenic bacterium may 

 find in the soil a favorable environment for a longer or 

 shorter period ; and even in the event that multiplication 

 cannot take place, the conditions may be such as to 

 preserve its vitality unimpaired. Therefore, as a result 

 of the careless disposal of infectious material and 

 discharges, the soil is often contaminated by micro- 

 organisms of a dangerous character. All pathogenic 

 microbes in the soil are potentially dangerous, yet the 

 possibilities for them doing harm is not the same for 

 each, since it depends upon the species of micro-organ- 

 ism, the physical environments of the soil, and the 

 purposes for which the soil is used. It should always 

 be remembered that when a soil is contaminated with 

 infectious matter, the bacteria present in it will, in dry 

 weather, be carried into the air with dust, and in that 

 way may find their way into our lungs or contaminate 

 our foods. The most widely distributed bacterium 

 in the air, and the most 'generally dangerous, is the 

 tubercle bacillus. The typhoid bacillus is the most 

 frequent pollutor of wells and streams, although it 

 may become a most dangerous infection in the air if 

 the contamination of the soil is excessive and the 



