284 BACTERIOLOGICAL EXAMINATION OF WATER 



PREPARATION OF CULTURE MEDIA. 



Nutrient Broth (Peptone Bouillon). One pound of beef free 

 from fat must be finely minced, then infused in a litre of cold 

 distilled water and allowed to stand in a cold place for twenty- 

 four hours. The whole mass is then strained through a cloth 

 and distilled water added to the filtrate so as to make up the 

 volume of fluid to one litre. Ten grammes of peptone and five 

 grammes of common salt are now added to the litre of fluid, 

 which is then boiled in the steam steriliser for one hour. The 

 medium is next made slightly alkaline with a 1 percent, solution 

 of sodium carbonate, litmus being used as the indicator, and 

 again boiled in the steamer for half an hour. Coagulated 

 material is then removed by filtration through moistened 

 " Chardin " paper. The medium is finally sterilised in the 

 steamer on three successive days for ten to fifteen minutes each 

 day. Sometimes it is impossible to obtain good beef; Liebig's 

 extract may then be employed with advantage. The broth 

 obtained is perfectly reliable for ordinary purposes, and though 

 perhaps not so nourishing as that made with the best beef, it 

 has the advantage of being more uniform in quality. Five 

 grammes of Liebig's extract are added to a litre of tap-water 

 containing ten grammes of Witters peptone and five grammes 

 of common salt. The mixture is then boiled in the steam 

 steriliser or over a Bunsen flame, made faintly alkaline with 

 sodium carbonate and filtered into a sterile flask. The flask is- 

 placed in the steam steriliser for three-quarters of an hour and 

 coagulated material removed by filtration through moistened 

 " Chardin " paper. The medium may then be used to prepare 

 nutrient gelatine or agar-agar ; if not required for this purpose, 

 it is poured into a series of sterile test-tubes, 10 c.c. in each tube,, 

 which are then sterilised on three successive days. 



Another method which is largely used in some laboratories is- 

 as follows : Take one pound of beef free irom fat, mince finely, 

 and diffuse in a litre of tap-w r ater, boil for twenty minutes and 

 allow to stand all night in an ice chest. Next day strain the 

 fluid, add ten grammes of Witters peptone, five grammes of 

 salt, and make up the bulk to one litre, dissolve by heat and 

 then carefully neutralise with a 28'6 per cent, solution of sodium 



