H. W. SCHOENLEIN 281 



COMPOUNDS OF ELEMENTS ESSENTIAL FOR GROWTH 



It is necessary in a synthetic medium not only that the essential elements be 

 present, but that they be present in types of compounds which may be utilized by the 

 cell. Of special importance are the compounds which contain the elements phos- 

 phorus, carbon, or nitrogen. 



Apparently phosphates constitute the source of jihosphorus for practically all 

 micro-organisms. Salts of phosphoric acid are therefore almost universally added to 

 synthetic media. They are useful not only as nutrients but also as buffers. 



Synthetic culture media may be sharply differentiated into two groups based upon 

 the form in which carbon is supplied. Certain types of organisms are capable of 

 synthesizing organic compounds from carbon dioxide or the carbonates, while others 

 require carbon in "organic" form. 



Those micro-organisms which are capable of utilizing carbon dioxide, or the 

 carbonates, must in the synthesis of cell-carbon compounds reduce the carbon partial- 

 ly. Those bacteria which bring about this change have been termed "oligocarbo- 

 philous" (Beijerinck) or "autotrophic." For their cultivation, synthetic media are 

 commonly employed. 



The reduction of carbon dioxide and the synthesis of organic carbon compounds 

 are endothermic processes, and require available energy. Apparently two sources of 

 such energy have been utilized by different t^-pes of autotrophic organisms. The 

 radiant energy of sunlight is absorbed by the pigments of certain types of cells, and is 

 used as an energy source. Such organisms are said to be "photosynthetic." Among 

 the bacteria most of the autotrophic forms are "chemosynthetic." These secure the 

 needed energy by bringing about chemical changes (oxidations) in elements or their 

 compounds. 



Synthetic media for the growth of photosynthetic organisms have been found 

 suitable for cer ain of the green algae (Chlorophyceae), blue-green algae (Schizo- 

 phyceae), and attempts (largely unsuccessful) have been made to secure such a 

 medium for certain of the true sulphur bacteria containing bacteriopurpurin. For the 

 group last named a suitable synthetic medium is highly desirable, as the metabolism 

 is poorly understood. For such would be necessary a medium relatively rich in carbon 

 dioxide or carbonates and a sufficient concentration of hydrogen sulphide to stabilize 

 the oxidation-reduction potential at a point suitable for growth of these micro- 

 aerophiles. All of these strictly photosynthetic forms require exposure to light. 



Synthetic media for the cultivation of chemosynthetic micro-organisms have been 

 elaborated for each of the important known groups. The first were those developed 

 by Winogradsky' for the culture of the so-called "nitrifying bacteria." Essential to 

 the growth of the organisms which oxidize ammonia to nitrites (Nitrosomonas) was 

 found to be the presence of a suitable concentration of an ammonium salt (usually 

 ammonium sulphate), carbon dioxide or carbonates, a high buffer content to prevent 

 the solution from becoming acid,^ and a sufficient supply of free oxygen. Buffer and 

 carbonates are usually supplied by adding an excess of magnesium carbonate. Simi- 



' Winogradsky, S.: Attn. deVInst. Pasteur, 4, 213; 4, 577. 1890. 



'If the reaction be written as (NH^), 804+40, = 2HN03-fH,S044-2H,0, it is evident that 

 from each molecule of the neutral salt there are developed three molecules of acid. The necessity for 

 the presence of an ample base reserve is evident. 



