THE DISTRIBUTION OF MICRO-ORGANISMS 1 83 



ferment sugars, starches and fats, and there are several known 

 species capable of dissolving cellulose.^ 



Pathogenic Soil Bacteria.— Certain pathogenic bacteria are 

 of common occurrence in the soil. Whether this is their normal 

 habitat or whether they gain entrance to the soil with animal 

 excrement, may be questioned. At any rate the pathogenic an- 

 aerobes, B. edematis, B. tetani, and B. welchii, are likely to occur^ 

 in garden soil, and it seems probable that they actually multiply 

 there to some extent. Bact. antkracis also occurs in the soil of 

 fields where the disease has prevailed, and it is not improbable 

 that this organism multiplies in the ground at times. Other 

 pathogenic bacteria, such as those of typhoid and cholera, seem 

 to be rather quickly eliminated in the struggle for existence under 

 the conditions found in surface soils. 



Micro-organisms of the Air. — Micro-organisms exist in the air 

 only as floating particles of dust, or as passengers on small drop- 

 lets of moist spray,, or as parasites on or in winged aerial creatures. 

 Those floating as dust are derived from the earth's surface, and 

 most of the living germs usually found in this condition are the 

 spores of molds. Living tubercle bacilli are unquestionably 

 suspended in the air as dust, especially in the dry sweeping of 

 floors where careless consumptives have lived. The spores of 

 anthrax bacilli may also be suspended in the air where hides or 

 wool of anthrax animals are handled. Other pathogenic bacteria 

 may at times float as dust, but their presence in the air in this 

 condition is apparently rather uncommon, and should be expected 

 only in the fairly recent environment of cases of the disease. The 

 moist droplets, expelled from the mouth and nose in speaking, 

 in coughing and especially in sneezing, may remain suspended in 

 the air for many minutes and be distributed to considerable dis- 

 tances. After drying the solicf material may still float as dust. 

 Pathogenic micro-oganisms may readily be transmitted from 

 person to person in this way. 



' For a discussion of the microbiology of the soil, see Monograph by Lipman in 

 Marshall's Microbiology- 1920. 



