H. W. SCHOENLEIN 283 



sole source of nitrogen is the nitrogen gas of the atmosphere. Such synthetic media 

 are generally used for the cultivation of Azotobacter and Rhizobium. 



Synthetic media to which nitrogen compounds are added may be divided into (a) 

 those in which the nitrogen is supplied as ammonia, or some closely related amine, 

 amino-acid, etc., from which ammonia is produced by hydrolysis; (b) those contain- 

 ing nitrites or nitrates; and (c) those containing other nitrogenous compounds such 

 as cyanogen, ring compounds, etc. Ability to utilize these various forms of nitrogen 

 constitutes an excellent criterion for the differentiation of micro-organisms. Most 

 micro-organisms are able to utilize ammonia; a smaller proportion nitrates; and fewer 

 yet, other compounds. 



DETERMINATION OF THE OPTIMUM MEDIUM 



It is sometimes advisable to determine the concentration of each of the various 

 ingredients which will produce a medium optimum for the growth of a particular 

 micro-organism. Such media have been frequently elaborated by plant physiologists 

 for the cultivation of plant seedlings. Several investigators have used a somewhat 

 similar technique for the development of a medium for bacteria. 



It should be emphasized that there can scarcely be such a thing as an optimum 

 medium in the broad sense for any organism. The addition of one ingredient may 

 alter the optimum concentration of another. For certain materials the optimum con- 

 centration is a function of the temperature; in other cases it varies with hydrogen-ion 

 concentration. 



It is evident that the term "optimum" as applied to a medium can mean only 

 those concentrations of a given number of ingredients which will give maximum 

 growth with other environmental influences fixed. In the development of such an 

 optimum medium it is usually advisable to determine the materials which must be 

 present for growth, and vary their concentrations one at a time. Such a method, for 

 example, has been used by Fulmer, Nelson, and Sherwood,' in the development of 

 their Medium E for the growth of a yeast. It was found that cane sugar, K2HPO4, 

 NH4CI, CaCL, and CaC03 were necessary for growth. The exact concentration of 

 each which would give a maximum rate of growth per cell during the logarithmic 

 growth period was determined. It is of interest to note that the optimum concen- 

 tration of certain of the ingredients named varied little or not at all with temperature. 

 This was not true of the concentration of the ammonium salts; this varied with the 

 temperature, that is, for each temperature there was a concentration of ammonia 

 which gave maximum growth. 



Many studies have been made for the selection of optimum media by other meth- 

 ods. A notable example of such is the effort of Krasnow, Rivkin, and Rosenberg-' who 

 studied the suitability of some 671 different synthetic media to support continuous 

 growth of streptococci without success. 



Much interest has been manifested since the work of Wildiers' on the study of the 

 continued growth of organisms in synthetic media. This author stated that yeasts 



• Fulmer E. I., Nelson V. E., and Sherwood, F. F.: /. Am. Chem. Soc, 43, 191-99. 1921. 

 ' Krasnow, F., Rivkin, H., and Rosenberg, M. L.: /. Bact., 12, 385. 1926. 

 3 Wildiers, E.: Cellule, 18,313. 1901. 



