32 BIOLOGY AND TECHNIQUE 



potassium. The phosphates seem to be necessary constituents of cul- 

 ture media, while chlorides, on the other hand, according to Pros- 

 kauer 13 and Beck, are not absolutely essential. Sodium salts, as a 

 rule, seem to be more advantageous for purposes of bacterial culti- 

 vation than potassium salts. 



The uncombined sulphur, which is often a constituent of bacteria, 

 is usually supplied by soluble sulphates. In the case of the thio- 

 bacteria of Winogradsky, however, free H 2 S is necessary for its 

 formation. 14 



The relative quantities of various nutrients in culture media are 

 important in so far as too high concentrations may inhibit growth. 

 In this respect, however, separate species vary. The development of 

 bacteria is far oftener arrested by the accumulation of waste prod- 

 ucts than by an exhaustion of nutrient materials. 



Substances of Unknown Composition. Although the necessity 

 for supplying bacteria with all the elements of protoplasm is obvi- 

 ous, many forms, particularly among the pathogenic bacteria, will 

 fail to grow on media composed of amino-acids or peptones together 

 with carbohydrates and mineral material. So far as is known, no 

 chemical element necessary for growth is lacking in such a medium, 

 and the failure of bacteria to multiply must indicate either that car- 

 bon and nitrogen are not present in suitable combination, or that 

 some special accessory chemical grouping is wanting. For many 

 practical purposes such media may be rendered suitable by the addi- 

 tion of meat extract or meat infusion. The components of meat in- 

 fusion which the bacteria utilize in their growth have not been 

 isolated. They seem to be somewhat labile, chemically, and the 

 infusion loses its power to promote growth under relatively simple 

 chemical manipulation. That there is more than one necessary com- 

 ponent is indicated by recent work 15 in which it has been shown 

 that boiling the infusion with charcoal renders it unsuitable for the 

 growth of certain bacteria, while the addition of an acid hydrolysate 

 of casein and of some other proteins renders it again satisfactory, 

 although the reactivating substance is probably not one of the known 

 amino acids, and will not itself produce growth of the bacteria with- 

 out the decolorized meat infusion. 



13 Proslcauer and Beck, Zeit. f . Hyg., xviii, 1895. 



14 Voges, Cent. f. Bakt., I, xviii, 1893. 



15 Mueller, Proc. Soc. Exp. Bi6l. and Med., 1920-21. 



