DEHYDRATED CULTURE MEDIA 33 



Infusions of meat were first generally employed together with peptone as 

 nutriments in culture media. Later it was found that for many routine procedures 

 beef extract gave fully as good results and had the decided advantages of greater 

 ease of preparation, greater uniformity, and economy, A simple medium com- 

 posed of beef extract, peptone, and agar has been one of the most generally used 

 media in bacteriological procedures. It is used for the ordinary routine examina- 

 tions of water, sewage, and food products; for the carrying of stock cultures; for 

 the preliminary cultivation of samples submitted for bacteriological examination; 

 and for isolating organisms in pure culture. 



Bacto-Nutrient Agar was originally prepared to duplicate an extract agar of 

 approved and standard formula. The American Public Health Association in its 

 earliest reports on methods for water analysis emphasized the necessity of the 

 universal use of a standard medium, and since the Third Edition of "Standard 

 Methods of Water Analysis" in 191 7^ has recommended the use of beef extract 

 rather than infusion of meat in the preparation of Nutrient Agar. In the Fifth 

 Edition of "Standard Methods of Water Analysis," 1923,^ and the Fourth Edition 

 of "Standard Methods of Milk Analysis," 1923,* the use of dehydrated media of 

 "Standard Methods" composition has been permitted for the bacteriological ex- 

 amination of water and milk. In the Fifth Edition of "Standard Methods of Milk 

 Analysis," 1927,^ the use of Bacto dehydrated media was approved as being on 

 a par with laboratory-made media for the bacteriological plate count of milk, 

 and in this connection the work of Norton and Seymour^ was cited. Bacto- 

 Nutrient Agar is prepared in accordance with the formula specified in "Standard 

 Methods of Water Analysis"'^ and since 1924 our label has stated "Conforms to 

 Standard Methods Formula." The "Standard Methods for the Examination of 

 Dairy Products" Ninth Edition^ specifies a medium known as Tryptone Glucose 

 Extract Milk Agar, as discussed on page 57, to take the place of Nutrient Agar 

 in the bacteriological examination of dairy products. 



To rehydrate the medium, suspend 23 grams of Bacto-Nutrient Agar in 1000 

 ml. of cold distilled water and heat to boiling to dissolve the medium completely. 

 In preparing large volumes of the medium the heating period required to effect 

 complete solution may be reduced by boiling about three-fourths of the distilled 

 water over a free flame and suspending the dehydrated medium in the remaining 

 cold distilled water, taking care that all particles of the medium are thoroughly 

 wetted and evenly suspended. The suspension is then slowly added to the boiling 

 water and boiling continued for a minute or two to complete solution. The 

 medium is distributed in tubes or flasks and sterilized in the autoclave for 15 

 minutes at 15 pounds pressure (121°C.). The final reaction of the medium will 

 be pH 6.8. 



One pound of Bacto-Nutrient Agar will make 19.7 liters of medium. 



1 Standard Methods for the Examination ^ Standard Methods of Milk Analysis, 



of Water and Sewage, gth Edition: 186:1946. 5th Edition: 7:1927 



2 Standard Methods of Water Analysis. « Am. J. Pub. Health, 16:35:1926. 

 3rd Edition: 93:191 7- 'Standard Methods of Water Analysis, 



3 Standard Methods 0' '*' ' ' ' « . .. . 

 5th Edition: 97:192 



* Standard Methods c 

 4th Edition: 4:1923 



itandard Methods of Water Analysis, 8th Edition: 201:1936. 



th Edition: 97:1923- s Standard Methods for the Examination 



tandard Methods of Milk Analysis, of Dairy Products, gth Edition: 93: 1948. 



BACTO 



TRYPTONE GLUCOSE EXTRACT AGAR 



DEHYDRATED 



The option of using Bacto-Tryptone Glucose Extract Agar as discussed on 

 page 57 for plate counts of water is permissible in following "Standard Methods" 

 procedures in laboratories where both milk and water counts are made, so that 



