PREPARATION OF MEDIA 49 



noDpathogenic Escherichia and Aerobacter include nutrient broth plus 

 1 per cent of lactose. Gas production gives presumptive evidence of 

 the group, and the necessary confirmation is accomplished by use of one 

 of a variety of media listed under ''Differential Media." Similarly the 

 pathogenic Salmonella may be enriched from feces, urine, water, sewage, 

 contaminated foodstuffs, etc., by use of tetrathionate and selenite (Leifson, 

 1936) broths. These media are available in dehydrated form. 



For obligately anaerobic bacteria, a primary requirement is the initial 

 attainment of a low 0/R potential, either by eliminating oxygen from 

 the nutritional environment or by counteracting its effect by the addition 

 of chemicals. The material in Chap. VI outlines methods for the cultiva- 

 tion of members of the genus Clostridium. For the enrichment of the 

 sporeforming anaerobic bacteria a variety of media are available, and 

 choice among them will depend upon the species desired and the sample 

 material. For the pathogenic species the thiogly collate medium^ or beef 

 heart infusion listed under "Cultivation and Storage Media" is suitable. 

 For certain soil types (organisms producing butyric acid, or butyl 

 alcohol, and some others) the liver infusion medium (same section), 

 corn liver medium, or potato infusion is useful. In the last two, the 

 fermentation of starch may be observed, and some bacterial types give a 

 characteristic ''head" (a slimy mass of unfermented cellulosic material). 

 The starch-fermenting types usually sporulate readily on these natural 

 infusions. The corn liver medium is prepared as follows: Add 50 g of 

 ground (white or yellow) corn meal and 10 g of dried liver powder to 1 liter 

 of tap water. Heat in flowing steam for 1 hr with occasional stirring. 

 Remove from steam, cool almost to room temperature, and dispense in 

 tubes, flasks, or bottles. Sterilize 45 min at 121°C. The resulting 

 medium, on cooling, should be semisolid, with the coarser particles of 

 corn settling to the bottom leaving a 2- to 3-cm layer of starchy material 

 at the top. For determination of pigment production, omit the liver 

 powder. The potato infusion is prepared as follows: White potatoes, 

 200 g; glucose, 5 g; (NH4)2S04, 1.0 g; CaCOs, 3 g; tap water to make 1 

 liter. Peel potatoes, and add water. Steam for 1 hr, or boil slowly until 

 soft, and put through potato ricer. Add other ingredients, and bring 

 up to original volume. Cool, and tube, with stirring, so as to obtain an 

 even distribution of the potato particles. 



As an example of the enrichment of an anaerobic bacterium, which has 

 been isolated (Baker and Taha, 1942; Bornstein and Barker, 1948) by 

 use of a medium constituting a simple and chemically defined nutritional 

 environment, Clostridium kluyveri may be isolated from black mud using 

 a medium containing the following components: ethanol, 8 g; sodium 

 acetate hydrate, 8 g; KH2P04-Na2HP04 buffer, 1 M, pH 7.0, 25 ml; 

 (NH4)2S04, 0.5 g; NaaCOa, 0.1 g; MgS04-7H20, 0.2 g; CaS04-2H20, 



