SYNTHETIC MEDIA. 51 



15. Tubes of 10 cc. slant agar with 10, 20, and 30 grams of grape-sugar. 



1 6. The same, with the same amounts of cane-sugar. 



17. Gelatin with cane-sugar, varying amounts. 



18. Gelatin with malic acid. (17 and 18 may be combined.) 



19. Gelatin plates with soluble starch and i per cent potassium iodide and with 

 or without i per cent potassium nitrate. Try a mixture of the pear -blight organism 

 and B. coli. Can the colonies be distinguished in this way using the nitrate? 



20. Agar plates with various sugars and the addition of calcium carbonate, or 

 zinc carbonate, for detection of acid-forming colonies. ('91, Beyerinck, Bibliog., XX.) 



21. Silicate-jelly. See p. 36. Known also as silica-jelly, 



22. Nitrate bouillon (+ 15 bouillon with i per cent potassium nitrate). 



23. Triple-distilled water and nutrient mineral substances free from nitrogen. 

 The same, with addition of potassium nitrate. The same, with other nitrogen foods, 

 e.g., sodium asparaginate. 



24. Bouillon with lead acetate. 



25. Bouillon with neutral red. 



26. Salt bouillon, i. e., + 15 bouillon with varying amounts of c. p. sodium 

 chloride (i to 5 percent). 



27. Standard peptonized bouillon with varying amounts of sodium hydrate (from 

 -f- 40 to 40) for determining the optimum reaction and the tolerated range of acidity 

 and alkalinity. 



Synthetic media may be varied indefinitely to fit special cases and are often 

 extremely useful as differential tests. They have frequently been condemned because 

 some particular organism has not grown well in them. The very fact of feeble 

 growth or of no growth is, however, a matter of interest, and not infrequently a 

 means of distinguishing organisms which resemble each other in many particulars. 

 The value of such media becomes apparent at once when a number of organisms 

 are compared. Synthetic media afford more exact methods of research than do the 

 common media, and their value must increase rather than diminish as time goes 

 on. (Consult Grimbert in Archives de Parasitologie, T. I, pp. 191-216.) It does not 

 follow, however, that the common media should be at once abandoned. Festina 

 lente is a good rule. The formulae for some synthetic media are given under 

 " Formulae." For others see various text-books and the papers cited in the Bibli- 

 ography under XVI, XVII, XVIII, XXV, etc. 



RELATION TO FREE OXYGEN. 



(/) Surface and deep grmvths. Note the behavior of deep stabs in tubes of 

 recently steamed gelatin and agar, or of the colonies in shake-cultures of gelatin 

 and agar which are protected from the free action of air by pouring into the tubes 

 as soon as solidified another tube of gelatin or agar in the surface layers of which, 

 as an additional precaution, some active aerobe may be grown, e. g., Bacillus sub- 

 tilts. Observe also the relative rate of growth of buried and surface colonies in plate 

 cultures, growth under sterile mica plates, etc. Of course, whether an organism 

 will or will not grow under the conditions mentioned depends often to a large extent 

 on the composition of the culture medium. It might be able to respire in the pres- 

 ence of grape-sugar or cane-sugar, but not when milk-sugar or glycerole is substi- 



