46 Bacteria in Relation to Country Life 



raised by the churning action of innumerable vessels; 

 the busy life of the city, with its hurrying feet, its rattling 

 wheels, and its never-resting chimneys, all show that 

 there is ample cause for the existence of the many dust 

 particles in the air. 



Bacteria on dust particles. — The bacteria of the soil, 

 water alnd city streets, are carried along with the dust. 

 A single particle of dust, or the few salt crystals from 

 a drop of water, may carry one or more bacteria. This 

 may be easily demonstrated by exposing sterile plates 

 of gelatin to the air for brief periods of time. 

 The bacteria on the dust particles, as they fall on the 

 nutrient gelatin, begin to grow and form colonies. It 

 frequently happens that a single spot may show a 

 mixed growth of two or even three species, an indication 

 that there were, at least, that number of bacteria on a 

 single dust particle. 



The dust-laden air currents may, therefore, become 

 the means for conveying bacteria from place to place, 

 sometimes over great distances. Various disease germs 

 including those of typhoid, pneumonia, and tuberculosis, 

 may thus be carried away and become a source of in- 

 fection. Similarly, the nodule bacteria of legumes may 

 be transported from field to field in the soil-dust, may 

 affect soil-inoculation, and may thus lead gradually 

 to the establishment of new legume species in places 

 where they were previously unknown. This may, in 

 part, help to explain the appearance of some of the 

 nodules in newly established alfalfa fields. This assump- 

 tion seems to be supported by the fact that in pot-cul- 

 tures with legumes (particularly peas) nodules frequently 



