64 A MANUAL OF BACTERIOLOGY 



Standard Nutrient Media 



Slight variations in the composition of the nutrient 

 media have a marked influence upon the characters of 

 the growths of micro-organisms developing upon them. 

 In order to obtain more uniformity for descriptive pur- 

 poses, etc., a committee of the American Public Health 

 Association drew up a scheme for the preparation of 

 nutrient media of approximately constant composition 

 and reaction. Eyre 1 has devoted considerable attention 

 to this subject, and the following descriptions are based 

 largely upon his papers. 



(1) Preparation of acid beef -broth.- 1000 c.c. of distilled 

 water are introduced into a large flask, 500 grm. of finely 

 minced fresh lean beef added, and the mixture is heated 

 in a water-bath at 40-45 C. for twenty minutes with 

 frequent agitation. It is then boiled for ten minutes, 

 strained, and filtered through paper. To the filtrate 

 sufficient distilled water is added to make up to 1000 c.c. 



(2) Standardisation. This may be most simply described 

 in the case of acid broth. A 100 c.c. Erlenmeyer flask is 

 rinsed out with boiling distilled water, 25 c.c. of the acid 

 beef-broth are introduced into it, and 0-5 c.c. of phenol- 

 phthalein solution is added (0-5 per cent, phenolphthalein in 

 50 per cent, alcohol). This is kept boiling and decinormal 

 caustic soda solution 2 is run in from a 25 c.c. burette, 

 divided into tenths, until a faint pink tinge appears in the 

 boiling fluid. From the amount of soda solution used the 



1 Brit. Med. Journ., 1900, vol. ii, p. 921 ; 1901, vol. ii, p. 788. 



2 By a " normal " solution is meant the equivalent weight in grammes 

 of a substance dissolved in (i.e. made up to) a litre of water ; a " deci- 

 normal " solution contains one tenth of, a deka-normal ten times, this 

 amount. A normal solution of caustic soda contains 40 grm. of pure 

 NaOH (NaOH = 40), of sulphuric acid 49 grm. of pure H 2 S0 4 



