GENERAL BACTERIOLOGY 87 



The presence of streptococci is determined by mak- 

 ing Gram stains from each of the fermentation tubes 

 after two or three days. The number of streptococci 

 can be determined approximately by the same method 

 of calculation as for B. coli, by substituting the number 

 of tubes containing streptococci for N. 



Isolation of B. coli and streptococci is accomplished 

 by plating from the fermentation tubes in lactose 

 litmus agar. 



EXERCISE 2. THE BACTERIOLOGICAI, EXAMINATION 

 OF AIR 



For precise methods see: 



"Report of the Committee on Standard Methods for the 

 Examination of Air," Am. Jour. Public Hygiene, 1910, 20, p. 

 346- 



Rettger, Jour. Exp. Med., 1910, 22, p. 461. 



An approximate determination of the number of 

 bacteria in the air can be made by the following simple 

 method: Place 50 c.c. of broth in an Erlenmeyer flask 

 (Fig. 29, a). This flask is provided with a rubber 

 stopper (6) with two holes, through which two glass 

 tubes (c with a wide opening and d) lead. Cotton 

 plugs are then inserted at c and d, and the apparatus 

 is sterilized in the autoclave. A large bottle (/), con- 

 taining 5 liters of water, is then provided with a rubber 

 stopper, and also with two glass tubes (g and h); his 

 connected with a short piece of rubber hose and a 

 pinchcock {i). When the Erlenmeyer flask and con- 

 tents are sterilized, the tube d is connected, by means 

 of the rubber hose e, with g, and the plug at c is removed. 

 By opening the pinchcock i, 5 Kters of air are aspirated 



