150 



BACTERIA IN AIR 



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 remaining 



S'ug, /, uppermost. The latter is removed and the air sucked through, 

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

 nitration 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. 30) 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 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. 



It must be admitted that comparatively little 

 information bearing on the harmlessness or 

 harmfulness of the air is obtainable by the mere 

 enumeration of the living organisms present, for 

 under certain conditions 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 the spores, 

 the organisms from which they are derived 

 often consist of felted masses of threads, from 

 which are thrust into the air special filaments, and in con- 

 nection 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 



FIG. 48. Petri's 

 sand filter. 



