BACTERIA l\ EARTH. .-//A', AND ll'ATER 45 



present not merely as isolated specimens but in large numbers, is one of 

 the most important tasks bacteriology has before it. 



Methods for detecting bacteria in air, water and earth, have been 

 greatly perfected during the last few years, but only the general principles 

 can be treated of here : practical details will be found in works especially 

 devoted to bacteriological technique (23). 



Atmospheric germs may be demonstrated in a rough-and-ready way 

 merely by leaving exposed to the air nutritive culture media, such as agar 

 or gelatine. The bacteria that fall upon the moist surface soon multiply, 

 each one forming around itself a small colony visible to the naked eye. 

 For an exact determination it is necessary to allow a measured quantity of air 

 to pass slowly through a tube, the inner surface of which is coated with 

 sterilized gelatine. Upon this the bacteria are precipitated, and the resultant 

 colonies can then be counted and examined in detail. There are a number 

 of other methods in which the air is drawn through sterilized glass beads, 

 wool, sand, or sugar. The bacteria contained in it are thus filtered off, and 

 the material containing them can be mixed with gelatine or other nutritive 

 media, and the colonies allowed to develop in the usual way. The air in 

 our dwellings usually contains more bacteria than that out of doors. Ten 

 litres of air from a hospital ward, for instance, contained from thirty to 

 a hundred germs ; the same amount of outside air only from one to five, 

 half of these being the spores of fungi. Air in motion is always far more 

 laden with bacteria than when at rest, and dusting or sweeping causes their 

 number to increase enormously. That it must be so is self-evident, for dust 

 of all kinds is rich in spores and dried but still living vegetative bacterial 

 cells. They are whirled up into the air with the dust-particles which also 

 bear adherent germs, and by reason of their lightness settle down through 

 the atmosphere very slowly. 



Where there is no possibility for diseased secretions (e.g. sputum, diph- 

 theritic membrane) to become dried and pulverized, the bacteria found in 

 the atmosphere are of a harmless kind, or at most pus-cocci are present. 

 The air expired from the lungs is absolutely free from micro-organisms, so 

 that evidently the respiratory tract must act as a germ filter, the bacteria 

 we inhale being retained within the body. But the majority of them are 

 harmless, and moreover adhere to the mucous membrane of the nose, mouth, 

 and pharynx, only a very small number ever getting as far as the lungs. 

 Still, seeing that we inhale, on an average, five hundred litres of air every 

 hour and, consequently, some fifty to two hundred and fifty germs, the 

 danger of dust contaminated by disease products is very great. 



Water has always been looked upon as one of the chief agents in the 

 spread of infectious diseases, and its bacteriological analysis is therefore a 

 matter of the utmost importance. Most natural waters contain bacteria, and 

 even the distilled water of our laboratories holds sufficient nutritive substance 



