176 EXAMINING AIR, SOIL, WATER, AND FLUIDS FOR BACTERIA 



live studies of the air can be made in the open, in a room, in a barn, 

 in a basement, etc., as the difference in the number of colonies devel- 

 oped in Petri dishes exposed simultaneously, or approximately so, to 

 air under different conditions for the same period of time, gives a 

 fairly accurate indication of the variations in the bacterial contents 

 of the air. Data as to the relation between the number of bacteria 

 and moulds present can also be obtained by this method. 



EXACT METHOD. For the exact quantitative estimation of bac- 

 teria and moulds a variety of methods have been devised. The 

 simplest of these, which is, however, only slightly more accurate than 

 the one first described, is as follows. A large Erlenmeyer flask, prefer- 

 ably one holding at least 2 liters (2000 c.c.), is filled with water, cotton 

 plugged, and thoroughly sterilized in the steam sterilizer. After 

 being removed from the latter the water is cooled and the flask taken 

 to the place where the air is to be examined. A test tube containing 

 25 to 30 c.c. of agar or gelatin is heated in a water bath and cooled 

 down to near the point of solidification. When this is ready the cotton 

 plug is removed from the flask and the sterile water poured out, 

 which is now, of course, replaced by two liters of air. The flask 

 must be energetically shaken with the mouth down in order to get 

 it as dry as possible. It is then placed on a level surface. The melted 

 culture medium poured in, the cotton plug replaced, and the medium 

 allowed to solidify. The flask must be kept perfectly quiet, either 

 at room or incubator temperature, which causes the microorganisms 

 contained in the air to fall to the bottom and develop into colonies 

 on the culture medium. Before using the latter for this purpose it 

 should have been kept in the incubator for several days, so that all 

 of the water of condensation has evaporated and the medium is 

 comparatively dry. Otherwise, the water of condensation is squeezed 

 out of the solidifying medium and running over its surface will 

 interfere with the formation of the proper number of colonies. After 

 remaining in the incubator for a few days the flask is, taken out and 

 the colonies which have developed in the medium on the bottom can 

 be counted with a magnifying glass. More exact methods for counting 

 the number of bacteria and moulds in the air are the following: 



HESSE'S METHOD. The apparatus consists of a glass tube 70 cm. 

 long and 3.5 cm. wide, the interior of which is coated with a thin 

 layer of gelatin. One end of the tube is closed by a solid rubber 

 stopper or cap, the other by a perforated rubber cork which carries 

 a small glass tube. This latter is connected by a rubber hose with a 

 suction bottle or flask which is in turn connected with a second flask 

 of the same type, each of exactly 2 liters' capacity. After the long 

 glass tube has been properly sterilized in the hot-air sterilizer and 

 coated with gelatin it is mounted in a horizontal position and one 

 of the suction flasks is connected with the small glass tube in the 

 perforated rubber stopper. This flask is filled with water and con- 

 nected with the second suction flask, which is empty, in the manner 



