164 PHYSIOLOGY OF BACTERIA 



different for every species of bacterium, and while some 

 general statements may be made concerning the necessity 

 of K, P, Ca, S, also probably of Fe, Mg and Na, and 

 possibly of Mn and eventually even of Zn and Cu, the 

 organic requirements are much more complex. 



The minerals are usually resorbed as ions, eventually 

 as salts, and it makes little difference in what form they 

 are given as long as they are at all soluble. Carbon and 

 nitrogen on the other hand are capable of forming 

 innumerable different compounds which do not ionize. 

 A good deal of attention has been given to the finding 

 of good, synthetic media, i.e., media of which every 

 component is chemically well defined and known. While 

 good growth has been obtained with some bacteria in 

 certain media containing amino acids and sugars, the 

 media containing peptone, meat extract, or milk are 

 still the only standard ones. 



It is probable that the reason for this is not the unfitness of the 

 food as such, but the lack of something else. It is difficult to see 

 why bacteria should not grow in synthetic media to the same large 

 numbers as in peptone media because we can be certain of providing 

 the same source of energy. A systematic study considering hydrogen 

 donators and reduction potentials (see p. 82) might lead to better 

 results now than the efforts of 20 years ago. 



(c) SOURCES OF CARBON 



The element carbon may be used by one species of 

 bacterium, according to Potter (1908). Carbon mon- 

 oxide is a source of carbon for the Carboxidomonas; it 

 oxidizes CO to CO2, and can grow in the absence of any 

 organic matter. 



Carbon dioxide can be changed to cell material by 

 all prototrophic bacteria. This group of bacteria, 

 simpler in their metabolism than any other known 



