PREPARATION OF MEDIA 47 



tion of the optimum temperature and optimum pH for growth aids in the 

 cultivation of certain bacterial types, as does the proper adjustment with 

 regard to oxygen relationship or 0/R potential. 



In this discussion, a few examples will be presented of the media used 

 foi enrichment cultures; detailed information concerning many other 

 media similarly employed will be found in the literature deaUng with 

 specific groups. It is to be noted that certain of the formulas given in 

 the section on ''Cultivation and Storage Media'' and on ''Differential 

 Media" are of media which also may be used as enrichment media. In 

 contrast, many of the media listed in the latter section contain com- 

 pounds which inhibit the growth of certain types which might be expected 

 in the initial sample in addition to the organism sought. In other 

 instances, the composition of the medium allows the demonstration of 

 the growth characteristics of the desired organism to make presumptive 

 identification possible. 



The sulfur oxidizing bacteria. The chemoautotrophic growth of 

 Thiohacillus thiooxidans is accomplished by inoculating a shallow layer 

 of the following medium with about 1 g of mud or soil and incubating at 

 30°C: powdered sulfur, 10 g (or NasSsOa-SHsO, 5 g); (NH4)2S04, 0.4 g; 

 KH2PO4, 4 g; CaCh, 0.25 g; MgS04-7H20, 0.5 g; FeS04, 0.01 g; water, 

 1 liter (Starkey, 1935). When good development has occurred, a transfer 

 is made to fresh medium of the same composition. From the latter, iso- 

 lations are made on the following solid medium: Na2S203-5H20, 5 g; 

 K2HPO4, 0.1 g; NaHC03, 0.2 g; agar, 20 g; water, 1 liter. Thiohacillus 

 colonies are recognized by the deposition of free sulfur: 2Na2S203 +O2 — > 

 2Na2S04 + 2S. 



The nonsulfur photosynthetic bacteria (Athiorhodaceae) utilize organic 

 compounds as H-donors and require growth factors for their develop- 

 ment. Some are strict anaerobes, growing only photosynthetically, but 

 others are facultative aerobes and can grow well heterotrophically in the 

 dark under aerobic conditions, obtaining energy by the oxidation of 

 organic substrates. Here, general enrichment media for cultivation 

 under anaerobic conditions will be described, the source of energy being 

 light (van Niel, 1944). The basal medium consists of (NH4)2S04, 1 g; 

 K2HPO4, 0.5 g; MgS04, 0.2 g; NaCl, 2 g; NaHCOs, 5 g; water, 1 liter. 

 This medium is supplemented by the addition of a single organic sub- 

 stance (ethanol, glycerol, mannitol, formate, acetate, succinate, malate, 

 alanine, asparagine) in a final concentration of 0.15-0.2 per cent, after 

 which the reaction is adjusted to pH 7.0 with H3PO4. The various 

 media are dispensed into glass-stoppered bottles, inoculated with a small 

 amount of surface water or mud, and the completely filled and stoppered 

 bottles incubated in a light cabinet at 25-30°C under continuous illumi- 

 nation with electric bulbs (25-40 watts). For more rapid and abundant 



