144 



BACTERIA IN AIR 



/ 



e 



J 



by cotton plugs. The whole is sterilised by dry heat. One plug is 

 removed, and a sterile rubber cork, c, inserted, through which a tube, 

 d, passes to an exhausting apparatus. The tube is then clamped in an 

 upright position in the atmosphere to be examined, with the remaiuing 

 plug,/, uppermost. The latter is removed and the air sucked through. 

 Difficulty may be experienced from the resistance of the sand if quick 

 filtration be attempted. The best means to adopt is to use an air-pump 

 — the amount of air drawn per stroke of which is accurately known — and 

 to have a manometer (as in Fig. 26) interposed between the tube and the 

 pump. Between each two strokes of the air-pump the mercury is allowed 

 to return to zero. After the required amount of air has passed, the sand 

 a is removed, and is distributed among a number 1 of sterile gelatin tubes 

 which are well shaken ; plate cultures are then made, 

 and when growth has occurred the colonies are 

 enumerated ; the sand b is similarly treated, and acts 

 as a control. 



When it is necessary to examine air for particular 

 organisms, special methods must often be adopted. 

 Thus in the case of the suspected presence of tubercle 

 bacilli a given quantity of air is drawn through a 

 small quantity of bouillon and then injected into a 

 guinea-pig. 



Comparatively little information bearing on 

 the ha'rmlessness or harmfulness of the air is 

 obtainable by the mere enumeration of the 

 living organisms present, for under certain con- 

 ditions the number may be increased by the 

 presence of many bacteria of a purely non- 

 pathogenic character. The organisms found in 

 the air belong to two groups — firstly, a great 

 variety of bacteria ; secondly, yeasts and the 

 spores of moulds and of the lower fungi. 

 With regard to moulds, the organisms often 

 consist of felted masses of threads, from which 

 are thrust into the air special filaments, and in 

 connection with these the spores are formed. By currents of 

 air these latter can easily be detached, and may float about 

 in a free condition. With the bacteria, on the other hand, 

 the case is different. Usually these are growing together in 

 little masses on organic materials, or in fluids, and it is very 

 much by the detachment of minute particles of the sub- 

 stratum that the organisms become free. The entrance of 

 bacteria into the air, therefore, is associated with conditions 

 which favour the presence of dust, minute droplets of fluid, etc. 

 The presence of dust, in particular, would specially favour a large 

 number of bacteria being observed, and this is the case with the 

 air in many industrial conditions, where the bacteria, though 

 numerous, may be quite innocuous. Great numbers of bacteria 



Fig. 43.— Petri's 

 sand filter. 



