PREPARATION OF MEDIA 51 



are streaked, and the characteristic reactions obtained are due principally 

 to production of lecithinase. To prepare the medium: After washing 

 shell with disinfectant, asceptially withdraw first the white and then 

 the yolk of fresh hen's egg. Dilute yolk with equal volume of 0.85 per 

 cent NaCl, and add 1 ml of yolk suspension to each 9 ml of the following 

 medium which is sterilized before addition of egg: peptone, 40 g; Na2- 

 HPO4, 5.0 g; KH2PO4, 1.0 g; NaCl, 2.0 g; MgS04, 0.1 g; glucose, 2 g; 

 agar, 25 g, 1 liter water, pH 7.6. Streak plates in such a manner as to 

 yield well-isolated colonies. Consult McClung and Toabe (1947) for 

 details of species characters displayed by Clostridia and Colmer (1948) 

 and McGaughey and Chu (1948) for information on the aerobic baciUi. 



Media for gram-negative nonsporeforming bacteria. Many differ- 

 ential media have been developed for the presumptive identification of 

 the gram-negative rods occurring in the normal and diseased intestines 

 and in samples contaminated with fecal material. Not all these media 

 can be discussed, and although those which are mentioned may be pre- 

 pared in the laboratory, readily available dehydrated products are gen- 

 erally employed because of their greater uniformity. On Endo's agar 

 other lactose nonfermenting types, such as Salmonella tijphosa, produce 

 clear, colorless colonies which do not affect the faint pink color of the 

 medium, whereas colonies of coliform organisms which ferment lactose 

 are surrounded by a dark red zone. On eosin-methylene-blue agar 

 (E.M.B.), colonies of lactose nonfermenting organisms are translucent; 

 of the lactose fermenting types, Escherichia coli colonies are small, dark 

 and have a greenish metallic sheen, while those of Aerohacter aerogenes are 

 large, moist, and gray-brown in color with a pronounced tendency to 

 coalesce. On the desoxycholate agar of Leifson (1935) growth of gram- 

 positive organisms is inhibited. Colonies of the lactose nonfermenting 

 Salmonella and Shigella are colorless, those of E. coli are red, and those of 

 Aerohacter are pale with pink centers (Paulson, 1937). 



Medium for isolation of staphylococci. Chapman (1948) suggests the 

 following medium for selective isolation of chromogenic staphylococci 

 associated with food poisoning: D-mannitol, 10 g; peptone, 10 g; gelatin, 

 30 g; yeast extract 2.0 g; NaCl, 55 g; K2HPO4, 5 g; (NH4)2S04, 75 g; 

 10 per cent NaOH, 6 ml ; water, 1 ,000 ml ; agar, 15 g. Following steriHza- 

 tion, shake medium to disperse precipitate. 



Media for Brucella. Although liver-infusion agar has been used widely 

 for cultivation and differentiation of Brucella, it is being replaced, owing 

 to batch variations, by one of the following media which are available in 

 dehydrated form: (1) Difco tryptose agar (''tryptose," 20 g; glucose, 1 g; 

 NaCl, 5 i; agar, 15 g). (2) B.B.L. trypticase soy agar (''casein peptone,'' 

 15 g; ''soy peptone," 5 g; NaCl, 5 g; agar, 15 g). Incubate cultures at 

 37°C in an atmosphere of 10 per cent CO2 and in original isolations, if 



