624 BACTERIOLOGY OF THE SOIL, WATER, AND AIR 



and an absence of liquefaction in gelatin. These reactions are 

 regarded as satisfactory to establish the identity of Bacillus coli. 



In some laboratories a direct plating of the sample of water in lactose 

 litmus agar or upon Endo medium is practiced, but this procedure is 

 considerably less sensitive than the fermentation enrichment method 

 outlined above. 



Colon bacilli may occasionally be isolated from considerable volumes 

 of water 10 or 100 c.c. when they cannot be detected with regularity 

 in 1 c.c. or less. Very little significance attaches to such results, 

 because experience has shown that even springs in uninhabited regions 

 may occasionally contain a few colon bacilli, derived probably from 

 chance contamination with the feces of wild animals. If, on the 

 contrary, colon bacilli are regularly present in a water supply to such 

 an extent that a cubic centimeter of the water gives a positive culture 

 in a decided majority of attempts, that water is viewed with suspicion. 

 If the organism is regularly present in one-tenth of a cubic centimeter, 

 the water is judged unfit or dangerous for human consumption until 

 it is purified. 



Other organisms have from time to time been proposed as indi- 

 cators of pollution thus streptococci and gas bacilli have been studied 

 in this connection but up to the present time they have not been 

 accepted as authoritative criteria for evaluating the potability of 

 water supplies. 



BACTERIA OF THE AIR. 



Bacteria when dried and attached to dust particles may be wafted 

 into the air and remain suspended there for considerable periods of 

 time. Even the gentlest air currents suffice to prevent their settling 

 out. At high altitudes and over large bodies of water the bacterial 

 population of the air is very small indeed; over large cities and cul- 

 tivated land the number of organisms in the air is frequently much 

 greater. Heavy rains and snow tend to remove bacteria from the 

 atmosphere, while dry windy weather increases the aerial contamina- 

 tion. 



Usually the more hardy organisms alone are found in the air, but in 

 houses and hospitals pathogenic bacteria may be detected occasionally; 

 probably the extrusion of minute droplets of sputum 1 containing these 

 organisms is a most potent factor in air contamination by bacteria. 



1 See Droplet Infection, p. 91. 



