46 MANUAL OF MICROBIOLOGICAL METHODS 



allowing approximately half -maximal growth, stock cultures should be 

 stored at refrigerator temperature between transfer periods, which may 

 vary from two to several weeks according to the longevity of the types 

 under study. Screw-capped tubes or small bottles may be used to 

 minimize contamination and drying of medium during the storage period. 

 Some workers (see Mc Clung, 1949) prefer to cover the growth with a 

 layer of mineral oil (sterilized in shallow layers for 2 hr at 160°C with dry 

 heat on three successive days). In some laboratories, stock cultures 

 are prepared as agar slant cultures, but usually the stab technique is 

 employed. Spore-producing types may be kept in stock condition in the 

 form of spore suspensions ; often these are dried on sterile soil. McClung 

 (1949) gives references on this technic and on details for lyophilization of 

 all types. 



For those organisms which will grow on nutrient agar, peptone (1-2 per 

 cent) agar, or yeast-infusion-glucose (0.5 per cent) agar, these media are 

 generally used. For maintenance of Acetobacter species, Vaughn (1942) 

 recommends agar containing yeast infusion, glucose, calcium carbonate, 

 or liver-infusion broth prepared as above but diluted with an equal vol- 

 ume of water before the addition of peptone and phosphate. For aerobic 

 nitrogen-fixing types {Rhizohium, Azotohacter) , the y east-extra ct-man- 

 nitol agar of Fred, Baldwin, and McCoy (1932) is prepared as follows: 

 mannitol, 10.0 g; K2HPO4, 0.5 g; MgS04, 0.2 g; NaCl, 0.2 g; CaCls, 3.0 g; 

 yeast infusion (pH 6.8), 100 ml; distilled water, 900 ml; agar, 15.0 g. 

 The following medium is recommended for Neisseria by Vera (1948) and 

 can be used for a variety of genera: peptone (pancreatic digest of casein), 

 20.0 g; cystine, 0.5 g; NasSOs, 0.5 g; NaCl, 3.0 g; agar, 3.5 g; distilled 

 water, 1,000 ml; pH 7.3. 



In addition to the above, many formulas will be found in the literature 

 which are of special value for particular types; dehydrated media are 

 available for this purpose also. 



ENRICHMENT MEDIA 



The enrichment culture technic constitutes a means for the isolation of 

 a wide variety of bacteria by adjusting the nutritional environment in 

 such a manner as to enhance selectively the growth of a certain bacterial 

 type within a given mixed inoculum. Use of this simple methodological 

 approach, so ably exploited by Beijerinck (1921-1940), has assured the 

 ready isolation of nearly all types of bacteria (van Niel, 1949) and con- 

 stitutes a powerful tool for the bacteriologist in the isolation and identi- 

 fication of pure cultures from an initially mixed population. Even 

 fastidious pathogenic bacteria can be isolated in this manner, using for 

 this purpose the animal body as a selective medium. In addition, selec- 



