22 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



sterilised by exposure to moist steam for about twenty minutes, 

 especially if the operation is repeated in forty-eight hours. 

 In this way any spores which are specially resistant to sterilisa- 

 tion, and which may have escaped the first heating, will have 

 had time to develop, and the adult organism will be Mlled 

 by the second heating. For sterilising media or apparatus 

 in this manner, a very simple form of steriliser wiU suffice, 

 which is illustrated in Fig. i a. It consists of a large 

 semicircular tin can, with ordinary cover, and provided with a 

 perforated false bottom of tin plate about an inch from the 

 bottom of the can. About half-an-inch in depth of water 

 is placed in the bottom of the can, which can be quickly 

 boiled by the flame of a Bunsen burner beneath, the whole 

 can being thus filled with moist steam. 



Culture Media for Bacteria 



Broth or bouillon. — The basis of most media, suitable 

 for cultivating bacteria, is broth or ' bouillon.' This is made 

 by boiling up one pound of finely minced lean beef free from 

 fat or gristle with one litre of water in a large flask and strain- 

 ing through muslin ; five grammes of salt (sodium chloride) and 

 ten grammes of peptone are added, and the mixture boiled for 

 five minutes. The liquid is rendered very faintly alkaline 

 with carbonate of soda, made up to a litre if necessary with 

 fresh water, the neck of the flask plugged with cotton wool, 

 and the whole sterilised. 



Nutrient gelatine is made by dissolving 100 grammes (or 

 150 grammes if a rather high melting-point is required) of 

 gelatine in 100 c.c. of broth. The gelatine should be first 

 soaked in water to render it easily soluble and the whole 

 volume of gelatine broth made up to 1100 c.c. If necessary, 

 the solution after addition of the gelatine can be clarified by 

 warming on the water bath with the white of one egg. The 

 whole is then filtered through a pleated filter paper in a hot 



