74 



BACTERIA IN THE AIR 



of the medium should be employed for each Petri dish in order to ensure an even 

 surface and sufficient depth of medium all over the plate. After exposure the 

 plates are, under ordinary circumstances, best left at room temperature during the 

 development of the colonies, but if it is desired to examine the bacteria alone it will 

 be found well to favour the growth of these at the expense of the moulds, by first 

 incubating the dish at a temperature of 37 C. for, say, eighteen hours. In any 

 case the plate should be shielded from light, or otherwise many of the chromogeriic 

 organisms will not assume their typical coloration. Should it be desired to photo- 

 graph the plates in order to obtain a permanent record, the growth should be 

 arrested, and the organisms killed about the third or fourth day of incubation. The 

 best method of doing this is to reverse the dish, and to pour upon a piece of blotting 

 paper placed on the inner surface of the lid, which will now be undermost, a 

 sufficient quantity of Formalin to saturate it. The results of this method of 

 examination may be expressed per square foot per minute, the area of the Petri dish 



/ 22 \ 



being calculated f = (radius) 2 x J. 



2. The Flask Method of Mquel. Pasteur was the first to analyse air by the culture 



method, and he adopted a plan which, in 

 principle, is ivashing the air in some fluid 

 culture medium which will retain all the 

 particulate matter, which may then be 

 cultured directly or sub-cultured into any 

 favourable medium. Miquel has con- 

 trived a simple piece of apparatus for the 

 carrying out of this principle. It consists 

 of a flask with a central tube through its 

 own neck for the entrance of the air. On 

 one side of the flask is a tube to be con- 

 nected with the aspirator, on the other 

 side of the flask a tube through which to 

 pour off the contained fluid at the end of 

 the process. In the flask are placed 30 c.c. 

 of sterilised water (or, indeed, if it be pre- 

 ferred, sterilised broth). The entrance 

 tube is now unplugged, and the aspirator 

 draws through a fair sample of the air in 

 the room (say ten litres). This air perforce 

 passes through the water, and by the exit 

 tube to the aspirator, and is thereby 

 washed, leaving behind in the water its 

 bacteria. The aspiration is then stopped, 

 and the entrance tube closed. The water 

 (plus bacteria) is now poured out into 

 test-tubes of media or plated out on 

 Petri's dishes. Provided that the appar- 

 atus has been absolutely sterilised, and 



that only sterilised water is used, any colonies developing upon the Petri dish are 



composed of micro-organisms from the air examined. 



3. The Method of Hesse. This method is somewhat akin to Pouchet's aeroscope, 

 but is in addition a culture method. Hesse's tube is 50-70 cm. long and 3-5 cm. 

 bore throughout. At one end is an indiarubber stopper bored for a glass tube to 

 the aspirator. The other end is open. Before using, the tube is sterilised, and 40 

 or 50 c.c. of sterilised gelatine are placed in it. The tube is now rapidly rotated 

 in a groove on a block of ice or under a cold-water tap, and by this simple means 

 the gelatine becomes fixed and forms a layer inside the tube throughout. We have 

 therefore, so to speak, a tube of glass with a tube of gelatine inside it. The 

 apparatus is now ready for use. It is fixed on the 1 tripod, and 10-20 litres of air 

 are drawn through, and the tube is properly plugged and incubated at room 

 temperature. In two or three days the colonies appear upon the gelatine. They 

 are most numerous generally in the first part of the tube. The disadvantages of 



FIG. 11. Miquel's Flask. 



